Upset in Response to a Sibling’s Partner’s Infidelity

Introduction

During the past decades, sex differences in the nature of jealousy have been widely studied by evolutionary psychologists. Although both sexes experience high levels of upset in response to a long-term partner’s real or hypothetical infidelity, there is a well-documented sex difference in the psychological weighting of the type of infidelity or the cues that trigger jealousy (see Becker, Sagarin, Guadagno, Millevoi, & Nicastle, 2004; Confer & Cloud, 2011; DeSouza, Verderane, Taira, & Otta, 2006). Specifically, men are more likely than women to report greater upset in response to a partner’s sexual (vs. emotional) infidelity, whereas women are more likely than men to report greater upset in response to a partner’s emotional (vs. sexual) infidelity (Buss, Larsen, Westen, & Semmelroth, 1992; Shackelford, Michalski, & Schmitt, 2004).

From an evolutionary perspective, this sex difference in jealousy may be explained in relation to the different adaptive problems that men and women recurrently faced. According to the theory of parental investment (Trivers, 1972), the sex that invests more in offspring—namely, women in humans, with a minimum obligatory 9-month investment—should be more fastidious about mates (Buss, 2006, 2007). Thus, an adaptive problem women had to face was losing their partner’s resources and commitment, which could be diverted to a rival woman, with whom the man gets emotionally involved, and their offspring. In this context, women are more likely than men to report greater upset in response to a partner’s emotional than sexual infidelity. Moreover, since fertilization occurs internally within women, men cannot be sure that the child a woman is bearing is genetically their own. The sexual infidelity of a man’s partner would threaten his certainty of paternity, and it would thus be more likely that men report greater upset than women in response to a partner’s sexual than emotional infidelity (Buss, 1989). From an alternative socialization perspective (for a relevant discussion, see DeSteno & Salovey, 1996; Fenigstein & Peltz, 2002), however, the above-mentioned differences are explained in relation to individuals’ gender socialization histories and, more precisely, social role training and acquisition. In other words, it would be the social experiences of men and women and their social role training and acquisition that would shape their responses to jealousy triggers. For instance, a man would be more likely to report greater upset in response to a partner’s sexual than emotional infidelity because he has been socially taught to behave in this manner, whereas the opposite would be valid for a woman.

It seems, then, that both evolutionary and social constructionist theorists can explain the demonstrated sex differences in response to sexual versus emotional infidelities. In response, evolutionary researchers have generated predictions that pit the two accounts against one another and investigated other family members’ responses to infidelity. For example, researchers collected data from participants who were parents (Fenigstein & Peltz, 2002; Shackelford et al., 2004) or had at least one sibling (Michalski, Shackelford, & Salmon, 2007), asking them to report how upset they would be over the sexual or emotional infidelity of their child’s or sibling’s partner, respectively. In the context of an inclusive fitness (or kin selection) theoretical framework (see Hamilton, 1964, as cited in Buss, 2011), natural selection is expected to shape one’s reactions toward their partner’s infidelity, as well as a kin’s partner’s infidelity, by virtue of shared genes and aiming to maximize reproductive fitness (see Fenigstein & Peltz, 2002; Michalski et al., 2007). In other words, the greater the reproductive costs for a child or individual with a sibling, the greater the cost for parents or siblings (see also Michalski et al., 2007). It is noted that this relationship might be moderated by the closeness of kin’s relationship (namely, it might be stronger for full siblings, rather than for cousins; see also Daly, Salmon, & Wilson, 1997). Given the above, from an evolutionary perspective, parents and individuals with a sibling would be expected to report greater upset in response to (a) the sexual rather than emotional infidelity of their son’s/brother’s partner and (b) the emotional rather than sexual infidelity of their daughter’s/sister’s partner, with the critical variable being the sex of their child or sibling rather than participants’ own sex. In contrast, a heuristic application of the socialization perspective would predict that (a) fathers would be more likely than mothers to report greater upset in response to the sexual than emotional infidelity of their child’s (son or daughter) partner and (b) mothers would be more likely than fathers to report greater upset in response to the emotional infidelity of a child’s (son or daughter) partner.

Three specific studies have primarily offered support for the evolutionary predictions. The results of Fenigstein and Peltz (2002) were largely consistent with the evolutionary hypothesis—that is, both mothers and fathers regarded sexual infidelity as more upsetting when committed by a son’s rather than a daughter’s partner, and emotional infidelity as more distressing when it involved a daughter’s rather than a son’s partner. These findings were replicated by Shackelford, Michalski, and Schmitt (2004) in a following study. In an attempt to examine whether previous findings apply only to the parent–child relationship or extend to other kin relationships, Michalski, Shackelford, and Salmon (2007) conducted a similar study, involving individuals with at least one sibling. In line with an evolutionary analysis of reproductive costs, the sex of the sibling (rather than the respondents’ gender) was once again the key variable predicting the responses of participants with siblings of a reproductive age.

The present study adopted a similar methodological strategy in order to examine whether the level of upset reported by a participant to the occurrence of a hypothetical sexual or emotional infidelity by a sibling’s partner is more likely to be a function of the sibling’s sex (in line with evolutionary predictions) or the participants’ gender (as a socialization account would predict). However, it should be noted that the limited previous studies (e.g., Michalski et al., 2007) do not make explicit reference to the sexual orientation of the participants’ siblings or children. If participants’ siblings/children are viewed as heterosexual, what still remains unexplored is whether participants’ responses are triggered by the sex of the sibling or, correspondingly, by the sex of the sibling’s partner. Specifically, greater levels of upset in response to the sexual rather than the emotional infidelity of a brother’s/son’s partner might be triggered either by the sex of the sibling/child (a man, facing the adaptive problem of paternity uncertainty) or by the sex of the sibling’s/child’s partner (the cheating woman, who might be fertilized by another man). Similarly, greater upset over the sexual rather than the emotional infidelity of a sister’s/daughter’s partner might be triggered by either the sex of the sibling/child (a woman, facing the risk of losing the man’s resources, commitment, and protection for her offspring) or of the sex of the sibling’s partner (the cheating man, who might divert resources and commitment to a rival woman).

Thus, taking a step further and extending Michalski and colleagues’ study (2007; see relevant suggestions in the Discussion section), we recruited individuals with a gay or lesbian sibling in an attempt to examine whether it is the sex of the sibling or that of the sibling’s partner that triggers sex differences in response to a sibling’s partner’s infidelity. To the best of our knowledge, no study so far has looked into this topic. It should be noted that several researchers assume that homosexuality does not significantly interfere with social or emotional adaptations. For example, Kenrick, Keefe, Bryan, Barr, and Brown (1995, p. 1167) noted that “… although homosexuality is obviously not adaptive in itself, it seems to involve biological mechanisms, carried along via sexual reproduction.” In such a context, even individuals with a different sexual orientation (see Bailey, Gaulin, Agyei, & Gladue, 1994; Symons, 1979, as cited in Harris, 2002) should exhibit the same sex differences in the evolved psychological design of mating preferences and jealousy and be sensitive to similar fitness threats (e.g., reproductive costs stemming from losing exclusive access to a partner; see Buunk, Angleitner, Oubaid, & Buss, 1996; Sheets & Wolfe, 2001).

It is fair to notice that findings of the—limited in number—existing studies are often inconsistent or unclear. For instance, Bailey, Gaulin, Agyei, and Gladue (1994) reported lower sexual jealousy for gays compared to heterosexual men and higher sexual jealousy for lesbians compared to heterosexual women (see also Dijkstra et al., 2001). Harris (2002) found that upset was greater for an emotional infidelity of a partner not only for gays but also for heterosexual men (in contrast to the robust pattern observed in several previous studies; see Buss, 2011). There are, however, findings that support the view of sociosexual adaptations as aspects of a single, biologically driven, psychological module (see Sheets & Wolfe, 2001, for a discussion). For example, in the same study, Harris (2002) reported that virtually all the lesbian participants regarded emotional infidelity as more upsetting than sexual infidelity (a finding that is consistent with the evolutionary perspective and with relevant evidence for heterosexual women). Moreover, in the above-mentioned study, Bailey and colleagues (1994) revealed significant sex differences in all seven mating preference scales used (including the sexual vs. emotional jealousy scale) but no differences between each homosexual group and the same-sex heterosexuals in most comparisons conducted. Specifically, researchers reported no difference between lesbians and heterosexual women in the distress reported over a partner’s emotional infidelity, in sociosexuality, and in interest for uncommitted sex. Moreover, gays and heterosexual men did not differ in the reported interest for uncommitted sex, their interest in visual stimuli, the unimportance of partner’s status, or the importance of partner’s physical attractiveness. The fact that the difference between each homosexual group and the same-sex heterosexuals was less than 15% of the heterosexual sex differences led Bailey and colleagues (1994) to suggest group similarities (rather than simply a failure to reject the null hypotheses) and to conclude that sex seems to have a greater impact on mating psychology compared to sexual orientation.

Future studies are expected to shed more light on this area. It is noted that the rationale of the present study was developed in the context of the aforementioned suggestions for deeply rooted sex-linked evolutionary dynamics that operate regardless of sexual orientation. Extending Michalski and colleagues’ (2007) work, we have strategically recruited individuals with siblings as a means to disentangle the evolutionary and socialization hypotheses. Furthermore, we have focused on individuals with gay or lesbian siblings, aiming at an empirical examination of what triggers participants’ responses (the sex of the sibling or of the sibling’s partner). Viewing jealousy reactions as aspects of a sex-driven psychological module, and in line with an inclusive fitness (kin selection) account, we expected participants to perceive threats for their siblings’ fitness as potentially costly for themselves, by virtue of shared genes, and based on their siblings’ sex, rather than sexual orientation. In other words, we expected to replicate Michalski and colleagues’ (2007) findings, showing that both male and female participants will report more upset over the sexual versus emotional infidelity of their male sibling’s partner and more upset over the emotional versus sexual infidelity of their female sibling’s partner, despite the fact that our participants’ siblings are gays and lesbians and, hence, the sibling’s partner’s sex is the opposite of what it was in Michalski and colleagues’ study.

We tested the following research questions and hypotheses:

Research Question 1: Is upset reported by participants in response to the hypothetical infidelity (sexual and emotional) of a sibling’s partner affected by the sibling’s sex (evolutionary perspective) or the participant’s gender (socialization perspective)? In line with the evolutionary account, we expect the sex of the sibling to differentiate the reported upset in response to the sexual (vs. emotional) infidelity of a sibling’s partner (Hypothesis 1).

Method

Results

The mean scores, SDs, and score ranges for the measures of upset in response to a sibling’s partner’s sexual and emotional infidelity, per participants’ own sex and their gay/lesbian siblings’ sex are provided in Table 1.

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Table 1.

Descriptive Statistics (Range, Mean, and Standard Deviation) for Each Infidelity Subscale (Sexual and Emotional), Per Participants’ Own Sex and Their Siblings’ Sex.

Participant’s Sex	Sibling’s Sex	Infidelity Subscale	Range	M	SD
Male	Male	Sexual	18–34	29.06	4.51
		Emotional	11–35	22.25	7.37
	Female	Sexual	10–35	25.63	7.95
		Emotional	18–35	30.00	4.97
Female	Male	Sexual	20–34	29.36	4.11
		Emotional	13–32	25.64	5.68
	Female	Sexual	16–34	24.19	5.72
		Emotional	21–34	29.88	3.61

Note. The sibling’s sex is also the hypothetical partner’s sex. Scores ranged from 0 to 35.

In order to examine the first research question of the study and, specifically, whether participants’ upset levels toward the hypothetical infidelity (sexual and emotional) of their sibling’s partner depend on their own sex and/or that of their gay/lesbian sibling, a mixed factorial analysis of variance was conducted. Participants’ sex and siblings’ sex were entered as the between-subject factors and the upset in response to the sexual and emotional infidelity of a sibling’s partner was entered as the within-subject factor.

The main effects along with the three-way interaction and the two-way interaction between participant’s sex and upset levels did not reach statistical significance (all ps > .05). There was, however, a significant two-way interaction between the sibling’s sex and upset in response to the sexual and emotional infidelity of a sibling’s partner, F(1, 58) = 46.36, p < .001, η² = .44 (see Figure 1). In line with the first hypothesis, the sex of the sibling was the key factor differentiating participants’ responses toward the hypothetical infidelity (sexual/emotional) of their sibling’s partner.

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Figure 1.

Comparison of the upset reported by participants in response to the sexual and the emotional infidelity of a male or female sibling’s partner.

To further examine the significant two-way interaction, we conducted two paired-sample t-tests: one within the group of participants with a gay sibling and another within the group of participants with a lesbian sibling, comparing participants’ upset levels in response to a sibling’s partner’s sexual and emotional infidelity. Participants with a gay brother reported greater upset in response to their sibling’s partner’s sexual infidelity (M = 29.20, SD = 4.25) than emotional infidelity (M = 23.83, SD = 6.75), t(29) = 5.30, p < .001, d = .86. Moreover, participants with a lesbian sister reported being more upset in response to their sibling’s partner’s emotional infidelity (M = 29.94, SD = 4.27) than sexual infidelity (M = 24.91, SD = 6.85), t(31) = 4.50, p < .001, d = .79. The results are consistent with Hypothesis 2a, which stated that the sex of the sibling (rather than the sex of the sibling’s partner) would trigger participants’ responses toward the hypothetical infidelity of a sibling’s partner.

Discussion

The current study contributes to the existing literature on sex-specific triggers of jealousy in several ways. It has attempted to extend previous studies by testing competing socialization and evolutionary hypotheses about how human jealousy is designed to respond. Specifically, we examined sex differences in infidelity-related distress, as reported by individuals who are genetically a step removed from the sexual behaviors of “significant others” (in this case, of their siblings’ partners’) but are still potentially influenced by the reproductive consequences of that behavior. In line with our first hypothesis, the distress experienced by a participant to the occurrence of sexual or emotional infidelity by a sibling’s partner seemed to be more a function of the sibling’s sex than of the participant’s gender. Our findings are thus consistent with the suggestion that sex differences reflect sex-differentiated evolved psychological design rather than gender-differentiated socialization practices, social role training, and acquisition.

Studying individuals with siblings of a homosexual orientation has also enabled us to examine suggestions made for deeply rooted evolutionary dynamics that operate irrespectively of sexual orientation. In the relevant study of Michalski and colleagues’ (2007), both male and female participants reported more upset over the sexual than emotional infidelity of their (presumably heterosexual) male sibling’s partner and more upset over the emotional than sexual infidelity of their (presumably heterosexual) female sibling’s partner. It is noted that the pattern of findings is the same in our study, despite the fact that our participants’ siblings were gays and lesbians and, hence, the sibling’s partner’s sex was the opposite of what it was in Michalski and colleagues’ study.

Studying individuals with siblings of a homosexual orientation also allowed us to extend Michalski and colleagues’ (2007) work, in determining what triggers participants’ responses (the sex of the sibling or of the sibling’s partner). Specifically, in line with Hypothesis 2a, participants with a gay brother were found to report more upset over sexual (vs. emotional) aspects of a sibling’s partner’s hypothetical infidelity, and participants with a lesbian sister reported becoming more upset over emotional (vs. sexual) aspects of a sibling’s partner’s infidelity. Thus, we conclude that the responses toward the hypothetical infidelity of a sibling’s partner are driven by the sex of the sibling rather than that of the sibling’s partner.

The latter finding is not consistent with the double-shot hypothesis (see DeSteno & Salovey, 1996; Harris & Christenfeld, 1996), suggesting that certain types of infidelity might be regarded as more upsetting by some individuals than others, simply because they represent a double—and thus, more troubling—type of infidelity. For instance, in women mostly, sexual infidelity is assumed to strongly implicitly suggest emotional infidelity too, whereas in men mostly, emotional infidelity is expected to imply associated sexual infidelity. In such a context, it would thus be the gender of one’s partner that drives the degree of upset in response to each infidelity type. Specifically, applied to the present study, the sexual (rather than emotional) infidelity of a sibling’s partner would be expected to cause more distress to participants with a lesbian sibling (whose partner is a female), since it would also imply emotional infidelity on the partner’s behalf. On the other hand, the emotional (rather than sexual) infidelity of a sibling’s partner would be more distressing for participants with a gay sibling (whose partner is a male), since it would also imply sexual infidelity on the partner’s behalf (i.e., a double shot of infidelity in both cases). However, as mentioned above, we found the exact opposite, in line with the evolutionary perspective we adopted. Future research studies are expected to shed more light on this issue by examining how upset gays and lesbians (rather than individuals with gay and lesbian siblings) would report being in response to the sexual or emotional infidelity of their partner.

A sample of participants with gay/lesbian siblings is not easy to find, especially in Greece, where homosexuality is not yet an open issue. It is for this reason that snowball sampling was used to track down this hidden population. Future studies might include participants in an online survey with full anonymity preserved, in an attempt to utilize a larger sample of participants, thus allowing for other variables to be considered (e.g., effects of participants’ or siblings’ age). Besides larger sample sizes, future studies could also rely on forced-choice questions in an attempt to investigate whether the type of measures used affect the observed patterns (see also Becker et al., 2004; Confer & Cloud, 2011).

Researchers could also attempt to meet the ethical challenges of examining participants’ responses toward their siblings’ partners’ actual infidelity. According to a recently published meta-analysis (Sagarin et al., 2012), although sex differences in jealousy are documented in relation to a partner’s hypothetical infidelity, there is limited evidence with actual infidelity in focus. This line of research might provide valuable insights concerning the nature and origin of jealousy (see Gorman & Blow, 2008), and in the long run, this could inform the development of successful interventions (e.g., in couples therapy), specialized for the needs of each individual.

Finally, it is noted that the present study was conducted in Greece, a collectivistic-traditional Mediterranean culture, where a limited number of studies have addressed evolutionary psychology topics in general. The Greek context differs significantly from the postindustrialist, individualistic North American culture, where most evidence on sex-specific triggers of jealousy stems from. Our findings overall converge with previous research using heterosexual siblings (and children), suggesting the operation of similar, deeply rooted evolutionary dynamics that operate regardless of sexual orientation or culture (see Buunk et al., 1996). Examining whether such findings on jealousy, as well as on other social or emotional adaptations (e.g., mating preferences), extend to other cultural settings could add to the weight of the evidence, suggesting that evolutionary principles underlay our understanding of human thought, emotion, and behavior.