Self-expansion motivation and inclusion of others in self: An updated review

Positionality

This review was prepared by a team of close relationships scholars, including three social psychologists, a positive developmental psychologist, and a clinician. We are all white and North American, but do differ by age and gender. We all have some experience working on research on samples that vary across cultures and other demographics.

Method

• Built on a 10-year-old review.

Overview

The self-expansion model proposes that individuals seek to enhance their abilities and increase their competence and resources in order to better accomplish their goals (Aron & Aron, 1986; for reviews see Aron & Aron, 1996; Aron et al., 2013). Close relationships are a key way for individuals to increase their competence and resources, which enhances their sense of self-efficacy (Mattingly & Lewandowski, 2013a). Though such focus on the self may seem highly individualistic arising from Western thought, the basic self-expansion model was largely inspired by Eastern thinking and philosophy. In particular, self-expansion draws on the Vedic traditions of viewing life’s major goal being to expand maximally the self’s identity and that loving connections are an essential and natural step in that process (Aron & Aron, 1986). This model proposes that close connection with others promotes individual competence and resources. The model also has an evolutionary foundation, with self-expansion being a fundamental human motivation, along with survival, and that a main opportunity for self-expansion is through close relationships (marriage, children, etc.). Thus, at its heart, self-expansion is thought to be universal (although the details of its functioning may well differ across demographics and societies).

Differences in self-expansion motivation

Because we each want to have positive experiences that enhance the self and help increase the chances of future success, the self-expansion model considers self-expansion to be universally appealing to all individuals and consequently a fundamental human motivation. However recent research does show individual differences in their preferences for the degree of self-expansion (e.g., Hughes et al., 2020). Using a WEIRD sample, researchers identified variations in the desire for self-expansion, with some preferring a greater degree of stability or self-conservation and others preferring less (i.e., greater expansion). Using a similar sample, researchers found that individuals may also emphasize self-growth differently depending on their inclination toward approach or avoidance motivations (Mattingly et al., 2012). In particular, those high in approach motivation seek out partners who provide ample opportunities for self-expansion and find those who offer fewer opportunities less appealing. Self-expansion was unrelated to avoidance motivation.

Similarly, other individual characteristics may affect self-expansion, such as individuals’ attachment style (Aron & Aron, 2006). Those who are securely attached believe their partner will support them during self-expansion, while avoidant and anxious-ambivalent individuals are less certain, resulting in diminished benefits from potentially self-expanding experiences. Another set of studies using WEIRD samples from Mechanical Turk and the community surrounding Austin, Texas, found that individuals who believe relationships have the potential to grow and improve tend to experience more self-expansion than those who score low on these beliefs (Mattingly et al., 2019). In other words, those who hold growth beliefs experience self-expansion differently in their relationship and ultimately benefit from it more than those with destiny beliefs.

Similarly, an individual’s self-esteem may also impact self-expansion motivation. Two studies of male and female college students at a Pennsylvania university supported the hypothesis that when selecting a romantic partner, those with high self-esteem would adopt the positive attributes of the partner, while lower self-esteem people would self-expand by adopting negative attributes (Slotter & Kolarova, 2020). This dynamic corresponds with the need for people to self-verify, or have others view the self in ways that match one’s own self-views.

Taken together, these studies demonstrate that individuals may have different self-expansion preferences (Hughes et al., 2020) based on their motivation (Mattingly et al., 2012), and that a person’s beliefs impact how a person interprets potentially self-expanding experiences (Mattingly et al., 2019). Importantly, although the self-expansion model makes no explicit predictions about differences based on race, ethnicity, social class, sexual orientation, gender identity, age, or geographical location, given that these sociodemographic variables affect the way individuals experience the world, it introduces the possibility that different populations may on average have varying degrees of self-expansion motivation or experience it differently.

Though individuals may seek self-growth to different degrees and in different ways, the experience of actually expanding the self should have broad appeal. There are two key ways for individuals to experience self-expansion: (a) anticipating or directly obtaining new identities, perspectives, and resources, especially when experienced in a rapid or intense way; and (b) engaging in novel/interesting/challenging/exciting activities (provided they are not overly stressful or overwhelming), circumstances deeply associated with rapid expansion. Expansion in close relationships usually starts from initial attraction.

Initial attraction and entering relationships

A major way that individuals self-expand is by obtaining new identities, perspectives, and resources through their close relationships. That process typically begins when partners become attracted to each other. A potential partner who offers greater novelty should be particularly enticing because those new qualities provide special opportunities for self-expansion. Researchers tested this in a study with U.S. college students focusing on friendship, describing to each partner a potential same-gender stranger (Aron et al., 2006). The researchers then told the participants a friendship with their potential partner was either likely or unlikely. When participants (especially men) believed a friendship would likely form, then a more novel partner, that is one who possessed more differences in interests, was particularly appealing. A similar U.S. college-sample study found that when pairing two strangers, participants found partners who they believed would provide greater self-expansion more attractive (Sprecher et al., 2015). Finally, other researchers manipulated participants’ desire for self-expansion by providing false feedback on a personality test (Wright et al., 2004). They found that White students in Canada who were led implicitly to want more expansion (based on feedback from a bogus personality test indicating that they were in a bit of a “rut.”) were more likely to want to work with fellow students of a different ethnicity.

Falling in love

When individuals prefer dissimilarity in new relationship partners, it appears to be because it provides a greater chance to expand. Actually forming a new relationship also provides an opportunity for self-expansion. In particular, falling in love should facilitate adding new self-concept content, especially if it occurs in a rapid or intense way. Consistent with this, a classic study asked U.S. college students to complete a measure every 2 weeks over a semester of their self-concept (open-ended response to “Who are you today?”), and also report in another part of the survey whether they had fallen in love over the last 2 weeks, plus complete a set of self-concept measures (Aron et al., 1995). As the self-expansion model predicts, participants who feel in love showed significant increases to questionnaire measures of self-esteem and self-efficacy, and increases in the number of categories included in their who-are-you-today responses. These increases occurred both when compared with other time periods when they did not fall in love, and when compared with other participants who did not report the experience of falling in love. That is, for these students, falling in love appeared to literally expand the self.

Self-expansion is considered to be especially linked with passionate love continuing after first falling in love, a type of love associated with more intense feelings of attraction and longing for the partner. For example, a study of 500 adults from the Midwestern U.S. found that those who reported greater self-expansion in their own relationship also reported more passionate love, while companionate love was not associated with self-expansion (Sheets, 2014). Similarly, research on a representative sample of U.S. married couples found that although only 40% of long-term married couples reported being “very intensely in love,” those who did, also reported doing more self-expanding activities (i.e., new and challenging) together (O’Leary et al., 2012).

Research on romantic love’s neural correlates further solidifies self-expansion’s link with passionate love. The model suggests that the opportunity for and experience of self-expansion (especially rapid) is a powerful motivator that should activate the brain’s dopamine reward, motivation, and learning system (Aron et al., 2005). Several functional magnetic resonance imaging (fMRI) studies have confirmed that this brain system shows s greater activation when participants view a facial photo or subliminally encounter the name of a person with whom they recently fell in love, compared to when they are reminded of other familiar acquaintances (see Acevedo & Aron, 2014, for a review). Research has also found similar brain system activation among U.S. undergraduates who remain intensely in love following romantic rejection (Fisher et al., 2010) and for those who report intense passionate love within long-term (mean 21 years) marriages (Acevedo et al., 2012). These fMRI studies were conducted with heterosexuals, mainly in the U.S., United Kingdom, and Switzerland, but in this case researchers have replicated these studies cross-culturally in a sample of female college students in China (e.g., Xu et al., 2011), as well as across genders and sexual orientation (Zeki & Romaya, 2010).

Self-expansion in ongoing relationships

As the studies of passionate love suggest (e.g., Acevedo et al., 2012; O’Leary et al., 2012), couples experience self-expansion to varying degrees throughout their relationship. The Self-Expansion Questionnaire (SEQ) measures individual experiences of self-expansion in the context of a close relationship (Lewandowski & Aron, 2004). To gauge the extent of self-expansion in the relationship, the SEQ (developed with a U.S. college student sample) assesses perceptions of the relationship’s ability to introduce new perspectives, provide new experiences, develop new abilities, facilitate learning new things, and help the individual become a better person. Sample items include, “How much do you see your partner as a way to expand your own capabilities?” “How much does your partner increase your ability to accomplish new things?” and “How much does your partner help to expand your sense of the kind of person you are?”

The scale’s reliability has been replicated several times (e.g., Lewandowski & Ackerman, 2006). In terms of correlates with other relationship variables, those who report more self-expansion in their relationship on the SEQ also report being happier and more committed to their relationship (Lewandowski & Aron, 2004; Nardone et al., 2008). Similarly, in the context of the two-dimensional model of self-concept change in relationships (which explores how relationships add or subtract positive and negative elements to the self), and using a shorter version of the SEQ, those who reported greater self-expansion in their relationship also reported more forgiveness, accommodation, and willingness to sacrifice, as well as greater commitment and satisfaction over time (McIntyre et al., 2015).

Unlike much of the self-expansion research, these results have been tested cross-culturally (Rajabi et al., 2017). A sample of students from several Iranian universities (Tehran University, Shahid Beheshti University, Allameh Tabatabai University, and Alzahra University) who had been married for at least 1 year completed a Persian version of the SEQ along with measures of need fulfillment and commitment. Consistent with findings from U.S. samples, the Iranian sample revealed that self-expansion was positively correlated with relationship outcomes such that those who reported greater expansion also reported greater commitment, and that self-expansion played a larger role in commitment than need fulfillment. (Unless we note otherwise, studies of couples, including this one, included entirely or mainly mixed-sex couples, and there were no substantial differences between women and men. Also, due to the ambiguity associated with how previous studies reported couple samples’ sexuality, except where they explicitly have used the term “heterosexual,” we have defaulted to describing partnerships of women and men as mixed-sex rather than heterosexual.)

Potential for self-expansion

Relationships are dynamic changing experiences, and the amount of self-expansion a relationship provides currently may differ from what one expects it to provide in the future. Using a community sample of cohabiting couples from Canada, researchers conducted a daily diary study that assessed participants’ (including mixed-sex, lesbian, gay, and non-binary couples) daily experience of self-expansion as well as their perceived potential for self-expansion (Dobson et al., 2021). Potential for self-expansion (Lewandowski & Ackerman, 2006) gauges individuals’ beliefs about their relationship’s ability to give them opportunities for future growth (e.g., “I feel that if this relationship with my partner were to continue I would be able to gain more insights, experiences, and/or knowledge from my partner”). Results indicated that those who believed their relationship would offer more future self-expansion engaged in more daily self-expanding activities (e.g., “Did something spontaneous with your partner”). Although daily self-expansion and potential for self-expansion both predicted relationship commitment and satisfaction, potential for future expansion was a better predictor.

Potential for self-expansion’s role in relationship quality is consistent with previous research on college students showing that greater perceived potential expansion positively correlated with attraction (Sprecher et al., 2015). Similarly, those who believed their relationship had less potential for future expansion reported a greater likelihood of committing infidelity (Lewandowski & Ackerman, 2006). Further, the role of anticipated future expansion is also consistent with evidence from a study of middle-aged (M age = 37) North American couples from CrowdFlower (an online recruitment site) showing that couples' expectations about the stability and quality of the relationship were higher when both partners expected similar changes in themselves (Cloutier & Peetz, 2017).

Seeing one’s relationship as offering greater potential for self-expansion also has benefits for physical health. A study of U.S. individuals (ages 25–71) found that perceiving greater potential for self-expansion from a relationship was associated with greater self-reported physical health, largely due to perceived decreases in negative affect and increases in positive affect (Stanton et al., 2020). Similarly, a follow-up study of U.S. couples found that when both partners believed in their relationship’s potential for future self-expansion, the reported greater positive affect for themselves, as well as better physical health.

Self-expanding activities

The self-expansion that individuals report in their relationship can result from inclusion of the other in the self (more on that in the second half of this paper) or from engaging in activities that generate feelings associated with self-expansion. Activities that are likely to promote that feeling are ones that are novel, interesting, and challenging, and often contain elements of excitement (Aron et al., 2000). Research on several different samples of U.S. college students demonstrates that novel, interesting, and challenging activities produce expansion of individuals’ self-concepts as well as increasing their willingness to devote effort to cognitive and behavioral tasks (Mattingly & Lewandowski, 2013b). Others have suggested that self-expansion relates to the flow experience, which is characterized as a mental state of energized focus, feeling fully immersed in a challenging and enjoyable activity that meets a person’s skill level (Graham, 2008).

As predicted, new and interesting activities can benefit relationships as well. In a randomized controlled trial of U.S. couples, those who did exciting self-expanding activities reported greater relationship satisfaction after 10 weeks compared to couples who did either pleasant (but not exciting) activities together or simply a neutral control condition (Reissman et al., 1993). A longitudinal study of 101 Australian couples found that those randomly assigned to 1.5 hours per week of engaging in self-expanding activities had greater positive affect, relationship excitement, and relationship satisfaction compared to the waitlist control group over the 4-week study, and 4 months after the intervention concluded (Coulter & Malouff, 2013). In another study, U.S. couples’ daily reports over 1 week showed that when they experienced more activation (i.e., alertness, involvement, excitement), they also reported greater relationship quality (Graham, 2008). In an experiment, U.S. couples randomly assigned to engage in a novel, interesting, and challenging task (i.e., crawling together on a mat while transporting a foam cylinder with their heads and bodies) reported greater subsequent relationship satisfaction and passionate love from pretest to posttest compared to a merely pleasant activity (Aron et al., 2000).

Expansion doesn’t have to be exciting

Although arousal resulting from an exciting activity can certainly be associated with self-expansion, research has demonstrated that arousal is distinct from self-expansion, and that expansion is possible in the absence of arousal such as when couples attend a cooking class together or individuals read a new and interesting book together (Mattingly & Lewandowski, 2013a; Tomlinson et al., 2019). This is consistent with research finding that individuals can sometimes experience self-expansion in the context of ordinary tasks such as taking care of children or housework (Graham, 2008). The nature of the activity is important because another study of individuals and couples from throughout the United States found that engaging in challenging activities benefits the relationship, provided the activity was not too challenging or stressful (Graham & Harf, 2015). In those cases where an activity surpasses an individual’s skill level, these data suggest that challenging activities can undermine relationship quality. However, when couples skilled in the challenging activities did the challenging activities, they experienced positive affect that benefited their relationship quality.

Much as flow is associated with self-expanding activities, other interventions that may produce expansion can benefit relationships. For example, a randomized controlled trial of U.S. couples in their late 30’s found that couples assigned to engage in mindfulness exercises reported less relationship distress and greater relationship satisfaction (Carson et al., 2007). Importantly, mindfulness’ association with the relationship outcomes was largely due to couples feeling as though the mindfulness intervention they did together was exciting and self-expanding.

Relationship goals/approach

A person’s relationship goals can also impact how self-expansion influences their relationship. Research on middle-aged couples in Canada found that individuals who focused on positive relationship goals such as growth and greater intimacy were more likely to engage in self-expanding activities (i.e., novel and exciting) with their partner and report more relationship self-expansion (Harasymchuk et al., 2020). Further, those who reported more expansion also reported greater relationship satisfaction. Other research with a U.S. sample from Amazon’s Mechanical Turk found that those with more approach goals were more likely to achieve greater closeness from the dates they planned with their partner, partly due to their ability to plan more self-expanding (i.e., novel and exciting) dates (Harasymchuk et al., 2021).

Reducing boredom

Experiencing self-expansion in a relationship not only helps promote positive experiences, but can also help counteract negative experiences. Relationship boredom is a problem. For example, in a large longitudinal study in the U.S., with one-third black and two-thirds white participants, couples who reported more current boredom in their relationship at year 7, 9 years later (year 16) reported less relationship satisfaction, with no significant differences in effect sizes between races (Tsapelas et al., 2009). When relationships lack sufficient self-expansion, boredom can result. Research with dating college students and married couples from Canada found that participants believed boredom was due to insufficient self-expansion (i.e., low levels of novelty and relationship stimulation) and was associated with low pleasure and arousal (Harasymchuk & Fehr, 2010). To address their boredom, participants sought strategies such as talking to their partner and trying new things. Similarly, a U.S. community sample from Amazon’s Mechanical Turk found that participants confronting boredom believed they should engage in growth-enhancing novel activities with their partner rather than more familiar security-restorative activities (Harasymchuk et al., 2017). Participants in this study who underwent a boredom induction (i.e., imagining a time when the relationship was no longer exciting) had greater intentions to engage in more new activities and reduce familiar ones.

Sexual desire

As we learn more about our relationship partners, it’s easier to experience boredom. Boredom can affect sexual desire too. Using samples from the U.S. and Canada, in three studies researchers found that those who did more self-expanding activities with their partner also reported greater sexual desire for the partner, were more likely to have sex, were happier with their sexual experiences, and reported greater relationship satisfaction, with the benefits to relationship satisfaction persisting over time (Muise et al., 2019). A separate study focused on the sexual desire of women in mixed-sex couples found that women experiencing greater self-expansion in the relationship also reported greater sexual desire, more affection, as well as greater sexual and relationship satisfaction (Raposo et al., 2020). When the women’s partners reported more self-expansion in the relationship, the women also reported lower sexual distress, greater sexual satisfaction, less relationship conflict, and higher relationship satisfaction.

However, not all sexual relationships are monogamous, with some deriving self-expansion from non-monogamy. A middle-aged sample (M = 34.54, range 19–82 years) from the U.S. and Canada of 540 individuals involved in consensual non-monogamous relationships, wrote about their motivations for entering a multi-partnered relationship (Wood et al., 2021). When the researchers conducted a thematic analysis of the qualitative data, they found that growth and expansion was a major motivation for consensual non-monogamous relationships. In particular, responses to open-ended questions emphasized the importance of having new experiences as well as personal and relationship growth. For example, on the theme of personal growth, a 28-year-old pansexual women explained her motivation as “Mostly curiosity and self-discovery. I wanted to know if I could handle it, and what it would teach me.” Speaking of relationship growth, a 51-year-old mostly heterosexual man said, “It feels right and allows us both to grow,” while a 48-year-old mostly heterosexual women commented, “It’s about experiencing things we can’t as a couple...” In other words, consensual non-monogamy can also fulfill the motivation for self-expansion through novel sexual experiences, which helps participants with having fun, excitement, and greater personal and relational well-being. In addition, consensual non-monogamy can also provide non-sexual opportunities for growth and self-expansion through exploring new interests and worldviews, as well as learning hobbies and doing new activities from different partners

Infidelity and attention to alternatives

Because individuals appear to experience their relationships with others as a primary way to expand the self, those who do not experience sufficient self-expansion from their relationship should be more motivated to pay attention to relationship alternatives. In a series of studies with U.S. college undergraduates (VanderDrift et al., 2010), participants with less self-reported self-expansion in their own relationship enjoyed interacting with a potential alternative (actually a computer program) who was portrayed as more self-expanding compared to a non-expanding partner. A follow-up study found that participants currently in a relationship signed up for more “getting to know you” opportunities with attractive single partners and asked more questions of them during the interaction when their own relationship provided less self-expansion.

Similarly, a study of mostly women U.S. college students found that participants randomly assigned to feel a greater need for self-expansion (using the same priming procedure from Wright et al., 2004) were better able to remember details about potential alternative partners who had positive traits their own partner did not (Tsapelas et al., 2020). However, participants in the sufficient self-expansion condition had better recall for alternative partners’ traits that their current partner already possessed. A follow-up study using a sample of couples from a U.S. campus and community looked at whether there were differences in how the brain processes information about alternatives when memories of self-expansion in the relationship are primed. To test that, participants and their partner provided information about three types of time together: time when they were having self-expanding experiences together, times they felt strong love toward each other, and times they did everyday mundane activities together. The researchers then created videos of partners describing the three types of activities. While being scanned in an fMRI machine, participants viewed each of the three (expansion, love, mundane) videos, followed by a series of attractive faces. The fMRI data revealed that participants’ brain scans showed less brain activation (i.e., blood flow) in regions responsible for perceiving attractive faces when they had just watched the video describing self-expanding activities with their partner compared to when they had watched the love or mundane activities video. This suggests that experiencing self-expansion with one’s partner can discourage a person from noticing or paying attention to potential attractive alternative partners.

Paying greater attention to alternatives and having better memory for them are potentially problematic (especially in monogamous relationships) but may not necessarily lead to infidelity. However, a sample of U.S. undergraduates in long-term exclusive relationships revealed that those who indicated their relationship’s current ability to provide self-expansion was deficient also reported greater susceptibility to committing infidelity (Lewandowski & Ackerman, 2006). Later research replicated this finding and extended it by demonstrating that attention to alternatives mediated the association between self-expansion and susceptibility to infidelity (VanderDrift et al., 2010). Finally, another set of studies involving undergraduates found that those reporting less self-expansion in their relationship were more likely to report being unfaithful, including having been unfaithful over a 4-week winter break (Le et al., 2009).

Relationship dissolution

Insufficient self-expansion can not only threaten commitment to the relationship, but result in breakup. Deciding whether to stay in a relationship or leave is a difficult decision with many factors for individuals to consider. In a study of those factors (Joel et al., 2018) key categories considered included “improvement of the self” and “hindering self-improvement.” When a partner is a bad influence or fails to nurture growth, dissolution consideration is greater. In another study (Mattingly et al., 2019), using a sample of emerging adults recruited from around Austin, TX, researchers measured individuals’ relationship self-expansion at Time 1 and found that it predicted relationship dissolution 9 months later, such that those with less self-expansion were more likely to break up. In fact, self-expansion was a better predictor of dissolution than either inclusion of other in the self or boredom.

How much self-expansion a relationship provides while intact also has implications for individuals’ post-dissolution experience. Research finds that when U.S. college students’ relationships provide sufficient novelty, interest, challenge, and opportunities for growth, dissolution results in loss of self, or self-contraction, in which individuals become less certain about who they are (Lewandowski et al., 2006). However, in another U.S. sample, when the former relationship failed to provide sufficient levels of self-expansion, ending the relationship had benefits for the self (Lewandowski & Bizzoco, 2007). For example, losing a relationship that provided low self-expansion could be perceived as providing new opportunities that the previous relationship prevented. This seemed to be the implication of the data, that when impoverished relationships break up, participants reported more positive emotions, less loss of self, and a greater sense of personal growth compared to those who ended relationships that provided sufficient self-expansion.

Individual self-expansion and its implications for relationships

Though relationships may be a primary source of self-expansion, the model suggests that expanding one’s own knowledge, skills, abilities, and resources in any context is a fundamental motivation (Aron et al., 2013) and doing so will have positive effects, as found in a review of studies (Mattingly & Lewandowski, 2014). For example, as the self-expansion model would predict, when U.S. college students completed a novel, interesting, and challenging task (i.e., carrying items with chopsticks), they reported greater self-expansion than those who did a more mundane task (Mattingly & Lewandowski, 2013b). When students felt a greater sense of self-expansion, they exerted more effort on subsequent tasks. Similarly, other research with U.S. college students found that those who reported a more expanded self also felt more capable (i.e., had higher self-efficacy) regarding future tasks (Mattingly & Lewandowski, 2013a).

Individual self-expansion fosters other behavioral changes as well, for example enhanced physical health (Xu, 2020), including cessation of smoking (Xu et al., 2010) and weight loss (Xu et al., 2017). It’s also worth noting that there has been much more cross-cultural empirical attention on nonrelational self-expansion than expansion in the context of relationships. For example, a Chinese study found that when adolescents reported more self-expansion via a smartphone it was positively correlated with smartphone addiction (Liu et al., 2020). A French study of Millennials found that self-expansion through experiences with luxury brands strengthened the perceived relationship with that brand and enhanced consumer-brand identification (de Kerviler et al., 2019). Finally, a Polish study found that attending a music festival provided a sense of self-expansion that increased self-awareness and facilitated connections with others (Lawendowski & Besta, 2020).

When individuals experience new and challenging experiences on their own it can also benefit the relationship. A study of Canadian couples in long-term cohabiting relationships found that when individuals reported greater personal/nonrelational self-expansion, they also reported more positive emotions along with greater intimacy and passion in their relationship as well as more time spent with their partner (Carswell et al., 2021). Importantly, when individuals did not discuss or share their individual expansion experience in some way with their partner, that was associated with decreased connection in general between the partners, as well as decreased passion. This is consistent with other research that suggests that nonrelational self-expansion is more helpful to the relationship when the partner is supportive (Fivecoat et al., 2015). Using a sample of ethnically diverse New York undergraduate dating couples, individuals engaged in either expanding (e.g., learning new photography skills) or stressful tasks (e.g., having to teach others about photography techniques while being videotaped) while receiving either active (e.g., “I bet you’ll be really good at that”) or passive (e.g., “I’ll talk to you in a little while”) support from their partner. When participants in relatively long-term relationships (average 16 years) received active support from their partner during their individual self-expansion activity, it increased relationship satisfaction. Similarly, a study of retired married couples from the United States, mostly in their 60’s, examined how partner support for individual self-expansion predicted health and satisfaction with retirement a year later (Tomlinson et al., 2020). Consistent with previous research, individuals who felt more supported and encouraged in their nonrelational self-expansion reported being happier and healthier in retirement, in part because those individuals also reported experiencing more self-expansion.

What does it mean to include others in the self?

Although the motivation to expand the self focuses on the basic tendency to seek out novel resources, perspectives, and identities, inclusion refers to the incorporation of those aspects of another person into the self. This is a key point that is sometimes overlooked in the literature: evidence for actual inclusion of another is not the same as the perhaps universal motivation to include for the sake of self-expansion. That is, the motivation for self-expansion leads to a desire to include others in the self in order to expand the self; the actual expansion of the self occurs when a relationship develops (and, as noted above, can occur in other situations as well).

The foundational early research on the including of another in the self supports the fundamental principle that when another person becomes included in the self, cognitive construction of the self shares activation potentials with--or overlaps with--the cognitive construction of the other (Aron et al., 1991; Smith et al., 1999; for reviews, see Aron et al., 2004; Aron et al., 2013; Branand et al., 2019; for a related view, see Agnew et al., 1998). That is, the line between self and other blurs as we include others in the self, taking on their resources, perspectives, identities, and outcomes. Other people, especially close others, inform who we are, enhance the tools we believe we have at our disposal, shape how we see the world, and affect the costs and benefits we perceive ourselves to incur. Although (as noted in much research below) this inclusion is very strongly related to closeness, and the data suggest a strong indicator of closeness, they are not entirely identical constructs. (For example, one can in some cases include the other in the self to some extent, such as a celebrity or fictional figure, and not feel close in the sense of actually having behavioral or bidirectional interaction.)

The following discussion reviews three categories of IOS literature: measurement, predictors of IOS, and outcomes of IOS. In each sub-section we review foundational work and highlight recent studies that have expanded and enhanced the understanding of IOS.

Measuring inclusion of other in self

The Inclusion of Other in Self (IOS) Scale (Aron et al., 1992), designed to measure an individual’s perception of connection with a specific other, is the most widely used measure of inclusion. It presents a series of seven pairs of overlapping circles, with each pair overlapping slightly more, with a slightly larger diameter than the preceding pair. Serving as a visual metaphor for this idea of overlapping or interconnected selves, respondents select the pair of circles that best portrays their relationship. Data from the original scale validation studies suggest that, at least among the U.S. college students who participated in the studies, the IOS Scale captures a large percentage of the variance in both feeling close and behaving close, correlating strongly with complex, multi-item measures of closeness and intimacy (Aron et al., 1992). The simple visual metaphor of overlapping selves seems also to increase the accessibility of this idea across a range of populations, languages, and cultures.

The IOS Scale’s incredible flexibility has resulted in its adaptation and adoption across a wide range of contexts. In addition to being effectively deployed to study diverse categories of personal relationships (e.g., romantic relationships, friendships, social groups, connection to nature, and God) across cultures (e.g., the US, the Netherlands, Japan, Turkey; e.g., Dalsky et al., 2008; Dincer et al., 2018; Uleman et al., 2000), alternate formats have also been created. For example, Le et al. (2007) offered a Web-based IOS Scale that invites users to drag the circles closer or further apart, introducing “distance between the centers” as a new variable of interest. More recently, Hernandez et al. (2019) modified the IOS to include just three options, making the case that older adults, including those with dementia, may find too many response options confusing. Their findings suggest that this adaptation made sense to the older adults in the sample and that participants provided reasonably consistent explanations about how they understood the tool. And adding to the IOS literature overall, the most recent comprehensive evaluation of the IOS Scale found it to be a psychologically relevant and reliable measure of the subjective closeness of relationships within an online sample of U.S. adults (average age of 34 years old) recruited through Amazon’s Mechanical Turk (Gächter et al., 2015).

The standard pictorial IOS Scale has the virtue of being minimally reliant on language, may avoid some biases triggered by verbal self-report items, and has little correlation with social desirability measures (Aron et al., 1992). Nevertheless, it is susceptible to response bias inherent in all self-report measures. When possible, researchers can also deploy nonobvious measures such as: response time in making me/not-me decisions (Aron et al., 1991, Study 3); cognitive confusions made when processing information about self versus others (Mashek et al., 2003); plural pronoun use in descriptions of one’s relationships (Agnew et al., 1998); and a trait-rating attribution task (based on Sande et al., 1988).

In addition to these long-time existing measures, recently Ketay et al. (2019) offered a clever extension of the “me/not-me” task. They evaluated whether reaction times for judging photographs as “me/not-me” varied as a function of the degree the photograph had been morphed (e.g., one’s own image with that of a close friend) and the other person with whom the self’s photo had been morphed (e.g., a close friend vs. a famous person). Their results, based on samples of traditional U.S. college-aged women who identified as white, Asian, and Latina, suggest that self-other overlap extends to visual processing.

And research by Quintard et al. (2020) suggests self-other overlap actually extends to the bodily level. Using the established Joint Simon Task paradigm in which a spatial compatibility task is given to two people so that each participant took care of one of two responses (i.e., using a computer mouse to click when either a blue or yellow circle appears on the screen) Sebanz et al. (2003) demonstrated in a sample of European college-aged individuals who were involved in mixed-sex romantic relationships that there was more self-other confusion when a joint task was done with a romantic partner versus with a friend, further demonstrating overlap in self-other representations.

Evidence suggests overlap in Facebook profiles can be used as another objective indicator of IOS (Castañeda et al., 2015). Analysis of the Facebook profiles of 46 couples (primarily white, European-American participants in mixed-sex relationships ranging from 1 month to 15 years) found positive associations between profile overlap (i.e., shared pictures, friends, similar “likes”) and self-reported IOS. Further, Facebook overlap was positively associated with relationship investment and commitment in ways comparable to self-reported IOS.

In yet another recent development, Helgeson and Van Vleet (2019) adapted the IOS Scale to measure communal coping within U.S. couples where one person has type 2 diabetes. These couples were primarily mixed-sex married people in their 50s; the sample was 51.5% White, 41% Black, and 7.5% mixed race. Two key findings stand out. First, the IOS can be meaningfully modified as a very brief, easily deployed assessment of communal coping. Second, and particularly relevant to questions about the generalizability of close relationship theory and assessment, it is being found that the IOS may not be construed consistently across demographic groups. For example, when Helgeson and Van Vleet asked their respondents, in their fifties and half non-white, to explain why they had selected their response on the IOS Scale, only a third generated responses that would later be coded as “interpersonal connection; ” the interpersonal connection theme emerged twice as frequently in the original validation studies with college student participants. Other interpretations of the IOS Scale in Helgeson and Van Vleet’s community sample included teamwork, thinking alike, and time together, which were similar to the categories of “similarities” and “behaving close” identified in Aron et al.’s (1992) study.

In addition to the alternate format and non-self-report assessments of inclusion mentioned above, the IOS Scale has also been modified to tap neighboring constructs. For example, Mashek and Sherman (2004) explored the idea of feeling “too close” to a romantic partner by asking U.S. college-aged dating participants to indicate which pair of overlapping circles represented their desired relationship and which pair represented their current relationship. Based on data collection from approximately 1200 dating undergraduates from the Northeastern U.S., Mashek and Sherman estimated that 7%–19% of undergraduates felt too close to their partner at any given time. Mashek and Sherman proposed threat to personal control as one reason why a partner may desire less closeness in a relationship.

In another theoretical innovation, Tomlinson and Aron (2013) asked U.S. college students in relationships to answer as if they were their partner. This “predict what your partner would say” approach captures a new construct—perceived inclusion of other in the self by the partner (IOS-perceived), which extended the IOS model to incorporate one’s perception of the extent to which the partner includes oneself in his or her self-concept. This model posits that perceived partner satisfaction (i.e., one’s belief regarding how satisfied one’s partner is in the relationship) leads to perceptions of partner closeness (i.e., how close their partner feels to them, or IOS-perceived), which affects one’s own closeness to the partner (IOS). Across two studies, researchers found strong support for the proposed mediational model, demonstrating the importance of measuring specific perceptions of the partner’s feelings about closeness and satisfaction.

Predictors of including others in the self

A number of studies (typically involving U.S. and Canadian samples) included in a 2013 review paper of self-expansion (Aron et al., 2013) demonstrated several predictors of IOS: mutual, escalating self-disclosure was found to be a driver of including others in self among both individual strangers (Aron et al., 1997) and between two couples who had previously never met (Schwartz & Slater, 1991); sharing exciting activities in marriage (Tsapelas et al., 2009); inducing positive affect (Waugh & Fredrickson, 2006); short-term “we-ness” therapy (Reid et al., 2006, Study 2); and humor (e.g., Treger et al., 2013). Recent insights regarding mechanisms that create or enhance IOS are introduced below.

Humor, the subject of previous relationship research, is a common interpersonal tool that has been shown to positively influence the trajectories of IOS (Storey, 2003; Fraley & Aron, 2004; Wilbur & Campbell, 2011). Two social interaction experiments utilizing undergraduates at a midwestern U.S. university investigated the association between humor, liking, and closeness as measured by the IOS Scale. The use of humor was positively associated with liking and closeness in both studies and these associations were mediated by perceived reciprocal liking and enjoyment of the interaction, suggesting humor may be employed to establish connections with others (Treger et al., 2013).

Bernstein et al. (2015) found, among a sample of U.S., mainly Caucasian, college-aged mixed-sex couples, that a brief perspective-taking induction preceding couples’ unresolved conflict discussions interacted with individual differences in attachment avoidance to impact post-conflict ratings of self-partner overlap, as measured by the IOS Scale.

The perspective-taking intervention involved instructions to both members of the couple to approach the conflict discussion by first imagining themselves as their partners, experiencing the conflict from their viewpoint. The short perspective-taking intervention appeared to buffer the negative effect of partner attachment avoidance, but not actor avoidance, on an individual’s perception of post-conflict self–partner overlap. The authors noted that individuals high in attachment avoidance tend to use preemptive regulatory strategies (e.g., distancing) during conflict situations, which elucidates why higher partner avoidance would lead to a decreased sense of overlap with the partner during conflict. However, the current findings demonstrate that when in the perspective-taking condition, a partner’s attachment avoidance no longer leads to this decreased sense of closeness. These findings suggest that perspective-taking interventions might reduce attachment avoidance-related relationship challenges.

In a clever technology-focused study, researchers explored Snapchat use and closeness in friendships and romantic relationships among primarily U.S. college-aged female students (Kahlow et al., 2020). Relational closeness (assessed using the IOS Scale) was positively associated with the Snapchat use frequency, and Snapchat use on one day predicted relational closeness on the following day. Snapchat use increased reported social presence and closeness to one’s partner, and was found to be more strongly associated with relational closeness than multiple monomodal media (i.e., media that doesn’t combine text, image, video, and audio). These findings suggest that some communication technologies are more conducive to the maintenance of closeness in friendships and romantic relationships than others.

Working with a community sample of married older adults in the Southeast U.S., Burke et al. (2019) sought to explore satisfied couples’ experiences of both desired and actual closeness over a 1-year time period, as measured by the IOS Scale. Participants were first asked to identify the set of overlapping circles that best represented their actual relationship with their spouse, and then were asked to choose the set of overlapping circles that depicted their desired relationship with their spouse. Findings revealed that older spouses reported fairly stable marital closeness—enjoyed and desired—within their marriages over time; however, this finding was sensitive to both gender and health. Feelings of, and desire for, closeness were more stable for husbands than wives over time, and husbands’ desires for closeness appeared to be more sensitive to wives’ earlier desires, whereas wives’ desires for closeness were not predicted by husbands’ desires. However, the stability of husbands’ desired and achieved closeness over time was stronger when wives reported better health. In contrast, when wives reported being in poorer health, husbands’ desires were linked to the wives’ initial desires for closeness; when their wives were in poorer health, husbands appeared to be guided by what their wives wanted. Together, these findings suggest that gender differences may become greater as couples age, and that wives’ initial desire for closeness and the state of her health may be especially important for both spouses’ experience of closeness.

Gender differences were again found in an examination of closeness and cardiovascular reactivity in response to emotional spousal support for health concerns in a community sample of approximately 100 married couples over the age of 50 in the Northeastern U.S. (Monin et al., 2019). When husbands received support from wives, husbands’ blood pressure and distress decreased and both partners’ closeness (as assessed by the IOS Scale) increased. When wives received support, husbands and wives felt closer, but both partners’ heart rate remained elevated and wives felt more distressed. These findings suggest that support interventions for couples coping with health conditions should take into account that while both partners feel closer upon receiving support for health concerns, husbands may receive greater benefits from spousal support than wives.

In a mostly female, German sample of individuals in dyadic relationships, Gesell et al. (2020) tested conflict resolution styles and IOS as possible mediating factors in the association of mindfulness and romantic relationship outcomes such as partnership equality and satisfaction. Mindfulness was conceptualized in this study as the ability to be present in the moment and act with awareness, thus enabling partners to maintain close contact with one another both in everyday situations and in intimate interactions. They proposed that trait mindfulness would increase the use of constructive conflict resolution styles (positive problem solving), and promote feelings of closeness (measured by the IOS Scale) between partners, which in turn would predict these positive relationship outcomes. They found a significant mediation effect of both IOS and positive problem solving between mindfulness and partnership quality metrics, providing two potential mechanisms for the positive association between mindfulness and fulfilling romantic relationships.

In another direction, Rossignac-Milon et al. (2021) proposed that developing a sense of dyadic, generalized shared reality, or “the subjective experience of sharing a set of inner states (e.g., thoughts, feelings, or beliefs) in common with a particular interaction partner about the world in general, including the world external to the relationship” is essential to interpersonal relationships. Across nine, mixed-methods studies utilizing either U.S. university-based or Mechanical Turk samples, evidence suggested that shared reality with a close partner can fluctuate within-person from day to day. Further, on days when people felt a greater sense of shared reality with a partner relative to their average level of shared reality with that person, they felt a greater sense of closeness, as measured by the IOS Scale, and this effect was not due to residual effects from the previous day. People with greater shared reality tended to be closer and more committed to their romantic partners, and more likely to have experienced the feeling of having “merged minds.” Further, results indicated that in the face of feedback at odds with their sense of shared reality, dyads exhibited behaviors to restore their sense of shared reality (e.g., vocalizing agreement, finishing each other’s sentences). Together, these findings suggest shared reality as a distinctive predictor of relational closeness that partners are motivated to uphold in the face of a threat. The authors posit that the beneficial effects of shared activities for close partners are in part due to co-attending to the world outside the relationship, which allows them to create a shared experience of the world and sense of shared reality.

The recent studies highlighted in this section suggest several interesting possible avenues to establish and enhance IOS. For example, demographic differences were shown to impact IOS functions as IOS showed more benefit for males than females in older adult couples. Also, building upon earlier research that found sharing experiences increased IOS, recent work demonstrated that actually sharing perceived reality of the world (e.g., thoughts, feelings, or beliefs) through experiences predicted increased IOS. Additionally, the studies suggested the manner in which one relates to another can influence the likelihood of IOS. For example, utilizing humor, defusing attachment avoidance, using multimedia interactions, or being mindful all lead to increases in IOS. Thus, the IOS literature has been advanced in several directions and dimensions with these recent studies.

Outcomes of including others in the self

A number of studies (mostly involving the usual U.S. WEIRD college student samples) included in the Aron et al. (2013) review paper demonstrated several outcomes of IOS:

• Multiple studies (e.g., Slotter & Gardner, 2009) using versions of the “me/not-me” paradigm (originally from Aron et al., 1991) illustrated that when another person is included in the self, one’s ability to process information about the self on a particular trait is slowed to the extent that the other is dissimilar on that trait.

This section will examine the advantages and disadvantages of IOS as demonstrated in recent literature. Scientific understanding of the advantages of including others in the self continues to grow. For example, Weinstein et al. (2016) applied principles from self-determination theory to examine whether individual differences in self-determined motivation (i.e., the need for autonomy, relatedness, and competence) moderated the impact of higher self-other overlap on partner outcomes. Participants were college-student and community dyads of either best friends or romantic partners from the U.S. and U.K. If self-determined individuals (i.e., those whose needs for autonomy, relatedness, and competence had been fulfilled) reported greater self-other overlap (assessed using the IOS Scale), their partners reported enhanced commitment and receiving more positive motivational support. Conversely, when individuals were low in self-determination, their partners did not benefit from increased self-other overlap. These results indicate that the benefits of closeness in a romantic relationship may depend on one’s partner approaching the relationship authentically, fully, and from their own values.

A recent study examined “identity fusion” and its impact on handling conflicts in romantic relationships. Data collected from newlywed couples, who had been recruited both on Facebook and in Texas, suggest that individuals who perceived greater fusion with their partner (i.e., perceived an equal blending of the personal and partner’s self in creating a unique couple identity) showed reduced vigilance for relationship threats and utilized more constructive coping responses to relationship conflict. On the other hand, individuals who perceived an imbalanced couple identity (i.e., perceived either their own or their partner’s identity as dominant in the couple identity) demonstrated fewer of these pro-relationship behaviors. This research provides an important expansion to the IOS literature by not just focusing on the amount of overlap between partners, but rather the various ways selves can be integrated (Walsh & Neff, 2018)—equally or one contributing more.

Although the experience of closeness in romantic relationships has been found to be associated with increased levels of mental health and relational well-being (Holt-Lunstad et al., 2010; Reis & Aron, 2008; Reis et al., 2000), individuals differ in the level of their desire for closeness in a relationship (Mashek & Sherman, 2004). To further examine that variance, a longitudinal survey of partnered individuals in Canada and the United States recorded participants’ actual and ideal IOS across three time points. Results revealed that optimal levels of mental health and relational well-being existed when individuals had minimal discrepancies between actual and ideal IOS over time, regardless of their actual levels of IOS (Frost & Forrester, 2013). Closeness regulation may be an important mechanism to improve mental health and relational well-being in addition to promoting closeness itself. (This is an area in which different cultures may emphasize more or less closeness in different relationship types, such as family vs. marriage.)

Another study sought to determine whether individuals respond to comparisons involving romantic partners as they would to comparisons involving the self. Results from a large online sample recruited through Amazon’s Mechanical Turk suggested that when reminded of their partner’s inferiority in a domain, high self-other overlap participants kept positive global partner perceptions, whereas low-overlap participants’ global perceptions were negatively impacted. These results indicate that perceptions of partners remain robust when we feel a high degree of overlap with them, even when faced with specific evidence that our partners may not be perfect (Thai & Lockwood, 2015).

Other recent work, however, found potential downsides of increased self-other overlap (i.e., IOS). For example, a 2013 study tested the hypothesis that greater daily knee pain would be associated with poorer sleep for the spouse, and that the spouse’s sleep quality would be worse in couples who have a closer relationship as measured by the IOS Scale. Participants were all at least 50 years of age and were recruited from the Pittsburgh, PA area. Results showed that greater knee pain at the end of the day was associated with spouses’ deficient overall sleep quality that night, controlling for disturbances in patient sleep; this effect was increased in couples with a high level of closeness, as measured by the IOS Scale (Martire et al., 2013). Notably, the authors did not control for whether or not a couple shared a bed; however, the distinction was suggested for future research.

In a more recent neurological study of empathy for partner pain, López-Solà et al. (2020) examined the role of closeness in activating an area of the brain responsible for vicarious pain, the Vicarious Pain Signature (VPS). In a sample of 30 females in the Western U.S., greater interpersonal closeness between partners as measured by the IOS Scale was associated with greater unpleasantness and VPS fMRI responses when observing painful heat stimulation to their partner’s forearm. This study provides an important look at the neural markers of empathy in close relationships.

Another downside of self-other inclusion that may be relevant to certain cultures is its effect when there is endorsement of a “culture of honor,” where male partners’ or family members’ reputations can be tarnished by the acts of the females in the family. Male honor is tied to female obedience across a spectrum of moral values such that women perceived as dishonoring their male mates may be more violently punished. For this study, male participants at a large southwestern U.S. university completed a measure of cultural honor and closeness to their wife or partner as measured by the IOS. Participants with high levels of both closeness and honor were most aggressive regarding a hypothetical moral violation committed by the wife. The study suggested that within a culture of honor, the closer honor-endorsing men are to their partners, the more perceived violations by women are met with higher levels of aggression (Benavidez et al., 2016).

In another vein, Slotter et al. (2014) explored the effect of relationship dissolution on attributes garnered through the inclusion of the other in the self. Results from a Mechanical Turk sample of U.S. individuals who were in a mixed-sex relationship indicated that individuals preserve in their self-concepts aspects they had garnered from a former partner if they had invested more versus less psychological, mental, or physical effort to maintain those attributes. However, when these attributes conflicted with their own previously held beliefs and attitudes, there seemed to be reduced self-concept clarity if and when the relationship dissolves. This research indicates that the more one works to include another’s attributes in one’s self-concept, the more vulnerable one’s self-concept may be should the relationship dissolve.

In an examination of individual differences that may moderate the association between IOS and relationship outcomes, Park et al. (2019) explored the role of attachment styles in the relationship between relational closeness (assessed using the IOS Scale) and commitment. Across two large online Mechanical Turk samples of U.S. individuals in romantic relationships, the authors found a significant interaction effect such that for individuals who exhibited both high avoidance and high anxiety (i.e., fearful avoidant attachment style), the positive association between IOS and commitment was weaker than for those with only fearful or only avoidant (i.e., dismissive styles). This finding suggests that even in the presence of the potential rewards of closeness, fearful avoidants are likely to exhibit ambivalence toward maintaining the relationship due to strong internal conflicts between seeking and avoiding closeness, which hinders their ability to commit. However, for dismissive avoidants, strong closeness may alleviate their distrust of others without triggering any fears of rejection, and thus result in stronger commitment.

And finally, Parise et al. (2019) questioned whether and how self-concept clarity, “the extent to which self-beliefs are clearly and confidently defined” (Campbell et al., 1996, p. 141), is linked with relationship satisfaction at the dyadic level. To answer this question, they examined two mechanisms: couple identity and dyadic coping dynamics, both positive and negative. The authors operationalized couple identity as the degree to which one saw one’s partner as being part of one’s self-concept, as measured by the IOS scale. Using a community sample of dating and married adults in northern Italy, they tested and supported the hypotheses that higher self-concept clarity predicts relationship satisfaction through stronger couple identity and that self-concept clarity predicts longitudinal change in their own and their partner’s relationship satisfaction because of positive dyadic coping behaviors. More generally, the study’s findings also align with the proposition that a solid sense of personal identity provides a base for the ability to establish close intimate connections.

Taken together, the recent studies in this outcomes section suggest there are both clear benefits and challenges associated with self-other inclusion. For example, partners who feel more connected are shown to practice constructive responses to relationship conflict; to have enhanced commitment to a self-determined, authentic partner; to have positive global perceptions of partners; and to experience increased relationship satisfaction in dyads. Yet, connection comes with costs, including poorer sleep when one’s partner experiences chronic pain, increased risk of aggression within a culture of honor, and more difficulty reconciling self-concept when a relationship ends.

Conclusion to this review of the inclusion of other in the self aspect of the self-expansion model

The inclusion of other in the self aspect of the self-expansion model highlights how one’s partner’s identities, perspectives, skills, and resources become to some extent “included in the self” as one’s own. In this review we focused on research (noting briefly the foundational work and then expanding on the more recent work) on the meaning of including other in the self. There are multiple ways of indicating overlap of self and other, using basic and newly developed measures of the construct to function as predictors or as measures of outcomes (mostly positive, but some negative). As with the motivational aspect of the model theory (and as with most relationship research) studies were largely conducted in WEIRD cultures with college students and with mixed-sex couples, with just a few interesting exceptions. However, the initial evidence suggests that basic processes of the inclusion of the other in the self principle are likely found across cultures, but may vary in strength as a function of variables that vary in their averages across cultures and other demographic factors.