Abstract
Although sibling sexual abuse (SSA) is one of the most common forms of sexual abuse, it has been particularly neglected in previous research. Hence, characteristics of this form of abuse and its longer term implications are not well understood. The aims of the current review were to precisely characterize the phenomenon of SSA and to condense the implications known to date of SSA on survivors. We included 15 studies with a total sample size of 14,680 individuals. Our results indicate that SSA has some unequivocal features such as an early onset, an extended duration and frequency, and a particularly high intensity (i.e., involvement of coercion, force, superiority, and manipulation). Our findings also revealed that SSA is linked to later depression, anxiety, impaired self-esteem, and sexual functioning. The findings of the current review suggest that (1) SSA is common, (2) SSA has various negative effects on survivors’ mental health, and that (3) SSA and its implications have been and to date are marginalized in research and practice. Results are discussed with a special focus on clinical implications.
Keywords
Sexual abuse holds an important position among all cases of child abuse not only because of its high prevalence (Stoltenborgh et al., 2015) but also due to its massive impact on victims’ current and future life (Nelson et al., 2002; Tyler, 2002). Previous research in this field mainly focused on intrafamilial sexual abuse, more specifically on parental sexual abuse (Cankaya et al., 2012; Faller, 1989), whereas sexual abuse committed by a sibling—albeit thought to be the most common form of sexual abuse (Krienert & Walsh, 2011; Laredo, 1982; O’Brien, 1991)—has gained marginal attention. Therefore, the phenomenology of sibling sexual abuse (SSA) and its longer term impacts on survivors’ health and functioning are not well understood. The purpose of this article is to systematically review the existing empirical literature in this field.
SSA has been defined as “sexual behavior between siblings that is not age appropriate, not transitory and not motivated by developmentally appropriate curiosity” (Morrill, 2014). Although SSA may not always involve physical force, it often consists of coercion and/or manipulation (Caffaro & Conn-Caffaro, 1998; Mc Veigh, 2003; O’Brien, 1991). SSA can have a myriad of negative effects on survivors’ mental health (Carlson et al., 2006; Gioro, 1991; Laredo, 1982; Mc Veigh, 2003; Stroebel et al., 2013). Accordingly, the prevalence in clinical populations was found to be very high, estimated as 60%. While, considering the general population, researchers estimate that 15% of the female population and 10% of the male population had any kind of sexual contact with their siblings (Greenwald & Leitenberg, 1989), the prevalence of actual SSA in the general population is unknown.
One possible explanation for the lack of clarity in the estimations of this phenomenon may be due to underreporting and minimization that often occur in survivors of child abuse in general and also in victims of SSA (Caffaro & Conn-Caffaro, 1998; Mc Veigh, 2003). However, this neither explains why SSA was found to be one of the most underreported forms of child abuse (amongst survivors, see Caffaro & Conn-Caffaro, 1998) nor why SSA is also being massively underreported and minimized in reports of parents and professionals in health and social care (Khan & Rogers, 2015; Meyers, 2014; Tener et al., 2018). Yates (2020) lays out various factors that might explain these findings. First, there is no clear definition differentiating SSA from the sibling rivalries, jealousies, and quarrels that are considered to be an inevitable part of growing up (Caffaro & Conn-Caffaro, 1998; Khan & Rogers, 2015; Perkins et al., 2017). There is also no sufficiently comprehensive definition distinguishing mutually initiated sibling sexual behavior, i.e., as part of experimentation and exploration in the scope of healthy sexual development, from SSA (Caffaro, 2014; Rowntree, 2007). Second, SSA might often be overlooked, unrecognized, or disbelieved by parents but also by professionals due to the abhorrence at the thought of sibling incest (Ballantine, 2012; Tidefors et al., 2010). Third, due to common beliefs that SSA is relatively harmless as compared to other forms of (sexual) abuse, SSA might be increasingly minimized and downplayed (Adler & Schutz, 1995; Sanders, 2004).
These beliefs about SSA as a less severe form of sexual abuse are reflected by the way it is addressed in the literature and the words researchers used when addressing it, such as “How to handle sexual play between our children” (Kluger, 2020) or “Sibling Sexual Experiences: Normal Exploratory Behavior, Nonabusive Incest, and Abusive Incest” (Cicirelli, 1995). All these factors contribute to a marginalization of SSA and with that to a massive gap in our knowledge about this phenomenon and its implications.
By summarizing previous research in the field of SSA, the aim of this review is to (1) precisely characterize the phenomenon of SSA and (2) condense the known implications of SSA on survivors’ health and functioning.
Method
Literature Search Strategy
Searches were conducted using PubMED, PsychInfo, PsychNet, and Published International Literature of Traumatic Stress. Search terms were the following: “sexual interaction between siblings,” “sibling incest,” “sex between siblings,” “child sexual abuse among sibling,” “brother sister incest,” “sibling sexual abuse,” “problematic sexual behavior between siblings,” “sexual curiosity between siblings,” “sexual activity between siblings,” “sibling sexual experimentation,” “sibling sexual exploitation,” “sexual behaviors between siblings,” “mutual sexual acts siblings,” “sexual exploitation between siblings,” “sexual dynamics siblings” (Figure 1). Search items were linked via the operator “OR”. The literature search was explorative and not constrained or led by any theoretical framework. After consulting a university librarian, search term engine applications such as “term finder” (PsychNet) or “MeSH terms” (PubMed) were not used in order to ensure consistency across all databases. Additional papers were found through other sources, which included checking reference lists of relevant articles. Figure 2 maps the literature review process in a flowchart (Moher et al., 2009). Our search resulted in a total of 460 articles and duplicates were removed using EndNote (Thomson Reuters, United States). The 309 remaining papers were then manually checked. These articles were assessed using the following eligibility criteria: (a) They were published in English in a peer-reviewed journal, dissertations, or books; (b) they reported empirical data; (c) their focus was childhood SSA; and (d) they included adults who had a childhood SSA history or children who were currently exposed to SSA. Papers were excluded if (a) they focused on other types of abuse without SSA and (b) we could not access them. After applying the eligibility criteria, 15 studies remained and were included in the review.
A summary of search terms used in web-based search. Note. All search items were linked via the operator “OR.”
Flowchart of study selection process.
Design of Project
Given the broad and diverse body of literature, a scoping review was chosen to allow the authors to provide a descriptive overview, synthesizing findings from a range of study designs and methodologies (Pham et al., 2014).
Data Extraction
Following the literature search, authors screened articles and extracted data. To ensure no articles were wrongfully excluded and all information extracted was accurate, the senior researcher worked closely with the research assistant to monitor consistency and reliability. Finally, all included papers and extracted information were reviewed by an additional researcher not involved in the first process. Prior to the screening, the senior author and the research assistant screened 10 papers to assess reliability and consistency. Results indicated 100% level of agreement regarding eligibility of studies.
Results
Descriptive Information
A total of 15 papers (16 samples) investigated SSA. The majority of papers involved samples from North America (66.7%), followed by Canada (26.7%) and Europe (6.7%). Since 2011, this area has gained growing research interest, with 46.7% of papers published between 2011 and 2018. The total number of participants across papers was 14,680. The total sample size of the studies ranged from 17 to 974 (Tables 1 and 2). Most studies recruited community-based samples with a history of SSA experiences (Bevc, 1998; Carlson, 2011; Carlson et al., 2006; Collin-Vèzina et al., 2014; Crowder, 2002; Cryr et al., 2002; Gioro, 1991; Griffee et al., 2016; Morrill, 2014; Stroebel et al., 2013), three studies reported on clinical samples (D. H. Brown, 1997; Laviola, 1992; Rudd & Herzberger, 1999), and two reported on forensic samples recruited via official reporting systems or forensic medical services (Falcão et al., 2014; Krienert & Walsh, 2011).
Studies on Adults: Characteristics of the Included Research Papers.
Note. SSA = sibling sexual abuse; N/A = not available; y = years.
Studies on Children: Characteristics of the Included Research Papers.
Note. SSA = sibling sexual abuse; N/A = not available; y = years.
The majority of the studies reported on adult samples (Bevc, 1998; D. H. Brown, 1997; Carlson, 2011; Carlson et al., 2006;Gioro, 1991; Griffee et al., 2016; Laviola, 1992; Morrill, 2014; Rudd & Herzberger, 1999; Stroebel et al., 2013) and five studies reported on children (Collin-Vèzina et al., 2014; Crowder, 2002; Cry et al., 2002; Falcão et al., 2014; Krienert & Walsh, 2011).
Measures of SSA
Four studies used self-developed SSA measures. Bevc (1998) developed a 15-item questionnaire asking participants to indicate whether a specific sexual behavior occurred with a sibling (yes/no, e.g., “a sibling touching your private areas of the body or sex organs”). Carlson et al. (2006) developed a semistructured interview as well as the Severity of Sibling Incest Scale while the first was then also used in Carlson (2011). Laviola (1992) also developed a structured interview to assess characteristics of SSA. Rudd and Herzberger (1999) developed a questionnaire assessing both qualitative (e.g., “Do you remember how the abuser initiated the sexual abuse?”) and quantitative characteristics (e.g., “In your everyday life, how often do you think about the sexual abuse?”) of SSA. Two studies utilized existing measures of sexual abuse to characterize the SSA and its severity. More specifically, D. H. Brown (1997) used the Sexual Exposure Questionnaire (Rowan et al., 1990) asking for both characteristics and severity of the abuse and Cyr et al. (2002) used the Sexual Abuse Rating Scale assessing characteristics of sexual abuse (Friedrich, 1992). Two studies adjusted existing measures (Gioro, 1991; Morrill, 2014). Gioro (1991) modified the Finkelhor Sexual Abuse History Questionnaire to include a section on sexual abuse by brothers and, furthermore, only the “Sexually Punitive Scale,” the “Sexual Self-esteem Scale,” and the “Parental Support Scale” of the original Finkelhor Sexual Abuse History Questionnaire were included (Finkelhor, 1979). Morrill (2014) adjusted the Conflicts Tactics Scale (Straus, 1979) to exclusively address SSA, with 36 items addressing prevalence and severity of both physical and psychological sexual violations such as vaginal intercourse, touching, fondling, oral sex, and forced exposure to pornography. Two studies used information from official reports, that is, from the Canadian Incidence Study on Child Abuse and Neglect, as in Collin-Vézina et al. (2014), and, as in Crowder (2003), from investigations of the Ontario child welfare agencies and from questionnaires completed by agency workers.
Samples’ Characteristics
The demographic characteristics of the samples are summarized in Table 3. As can be seen, on average, 81.1% of subjects were female (D. H. Brown, 1997;Carlson, 2011; Carlson et al., 2006; Collin-Vézina et al., 2014; Crowder, 2003; Cyr et al., 2002; Falcão et al., 2014; Gioro, 1991; Griffee et al., 2016; Krienert & Walsh, 2011; Laviola, 1992; Morrill, 2014; Rudd & Herzberger, 1999; Stroebel et al., 2013). On average, 84.7% of the subjects were White (D. H. Brown, 1997; Carlson, 2011; Carlson et al., 2006;Crowder, 2003; Krienert & Walsh, 2011; Laviola, 1992; Morrill, 2014), 16.8% African-American/Black (Carlson et al., 2006; Krienert & Walsh, 2011; Morrill, 2014), 1.7% Asian (Morrill, 2014), and 2.1% Latina/Latino (Carlson et al., 2006; Morrill, 2014).
Demographic Characteristics of All Included Samples.
Among the studies that included adults, the average age was 31.9 years (SD = 5.3; Bevc, 1998; D. H. Brown, 1997;Carlson, 2011; Carlson et al., 2006; Gioro, 1991; Griffee et al., 2016; Laviola, 1992; Morrill, 2014; Rudd & Herzberger, 1999; Stroebel et al., 2013). The socioeconomic status of participants was reported by five studies and was mostly lower middle class (D. H. Brown, 1997; Carlson, 2011; Carlson et al., 2006; Gioro, 1991; Rudd & Herzberger, 1999). Six studies reported on the education of SSA survivors revealing that the major part of each sample received some kind of post–high school education as being either college students or college graduates (Carlson, 2011; Carlson et al., 2006; Falcão et al., 2014; Griffee et al., 2016; Laviola, 1992; Morrill, 2014). Most of the adult SSA survivors were employed part- or full-time, as six studies reported (Bevc, 1998; D. H. Brown, 1997;Carlson, 2011; Carlson et al., 2006; Laviola, 1992; Rudd & Herzberger, 1999). On average, around one third of SSA survivors were married (D. H. Brown, 1997;Carlson, 2011; Carlson et al., 2006; Laviola, 1992; Rudd & Herzberger, 1999), one third were single (D. H. Brown, 1997; Gioro, 1991; Laviola, 1992; Rudd & Herzberger, 1999), and one third reported to be separated or divorced (J. Brown et al., 2005; Carlson, 2011; Carlson et al., 2006; Laviola, 1992; Rudd & Herzberger, 1999).
Among the studies that included children, the average age was 9.1 years (SD = 1.57) (Crowder, 2003; Cyr et al., 2002; Falcão et al., 2014; Krienert & Walsh, 2011). Households in which SSA occurred included three or more children, whereas the natural parents often did not live together and/or marital problems were prevalent (Crowder, 2003; Falcão et al., 2014; Gioro, 1991). Furthermore, parental histories of childhood maltreatment (Crowder, 2003), parental psychological distress and mental health issues (Collin-Vézina et al., 2014; Cyr et al., 2002), as well as alcohol and/or drug abuse were common in the families in which SSA occurred (Collin-Vézina et al., 2014; Cyr et al., 2002). Three studies assessed the socioeconomic status of SSA survivors’ families. These three cohorts of SSA survivors grew up in low-income families (Crowder, 2003; Cyr et al., 2002; Gioro, 1991).
Characteristics of the Abuse
The average age onset of SSA was 8.4 years (Bevc, 1998; D. H. Brown, 1997; Carlson et al., 2006; Crowder, 2003; Cyr et al., 2002; Griffee et al., 2016). In most of the cases, victims were younger than the perpetrator. In very few cases, victim and perpetrator were equal in age while no study reported the perpetrator being younger than the victim (D. H. Brown, 1997; Carlson et al., 2006; Collin-Vézina et al., 2014; Crowder, 2003; Cyr et al., 2002; Falcão et al., 2014; Krienert & Walsh, 2011; Laviola, 1992; Stroebel et al., 2013). On average, the victims were around 4.5 years younger than the perpetrator (D. H. Brown, 1997; Carlson et al., 2006; Cyr et al., 2002). For over 75% of SSA victims, the SSA lasted for more than 1 year (J. Brown et al., 2005; Carlson, 2011; Collin-Vézina et al., 2014; Gioro, 1991; Laviola, 1992; Rudd & Herzberger, 1999). For one third of victims, the SSA happened between two and 10 times in total (J. Brown et al., 2005; Griffee et al., 2016; Laviola, 1992; Stroebel et al., 2013). For around one third of victims, the SSA happened regularly, for example, once or twice a week (Bevc, 1998; J. Brown et al., 2005; Gioro, 1991; Laviola, 1992).
Clinical Outcomes
Evidence showed that the most common outcome of SSA was depression. Three studies reported that adulthood depressive symptom severity, assessed by the Brief Symptom Inventory (BSI; Derogatis, 1993) and the Center for Epidemiologic Studies Depression Scale (Radloff, 1977), was higher in SSA victims compared to individuals without experiences of SSA (Carlson, 2011; Gioro, 1991; Stroebel et al., 2013). In children, there appears to be a similar tendency. Collin-Vézina and colleagues (2014) reported higher percentage of depression in SSA victims compared to children without sexual abuse experiences. Cyr and colleagues (2002) supported this finding by reporting higher symptom severity of depression in children with a history of SSA than in children that have been sexually abused by their father or stepfather. Accordingly, on average, around three quarters of SSA survivors were currently receiving some kind of counseling (Carlson, 2011; Carlson et al., 2006; Gioro, 1991).
Two studies reported that adulthood anxiety measured by the BSI was more severe in victims of SSA compared to individuals without experiences of sexual abuse whereas studies reported very similar effect sizes on this matter (Carlson, 2011; Gioro, 1991). Similar findings have been reported by studies on children. One study suggests that anxiety may be more severe in victims of SSA than in individuals that have been abused by their father or stepfather (Collin-Vézina et al., 2014). Collin-Vézina and colleagues (2014) also reported a higher percentage of anxiety in children with experiences of SSA compared to children without sexual abuse experiences.
Three studies reported on self-esteem in SSA survivors consistently suggesting that SSA may have large negative effects on adulthood self-esteem (Carlson, 2011; Gioro, 1991; Morril, 2014). While Carlson (2011) reported lower self-esteem in SSA survivors than in individuals with no sexual abuse history, Gioro (1991) reported negative effects of SSA on adult sexual self-esteem. Additionally, findings by Morril (2014) suggest a negative relationship between SSA exposure level and self-esteem.
Three studies reported on sexual functioning in adults with a SSA history. They suggest poorer sexual functioning in survivors of SSA as compared to the control groups without history of any kind of sexual abuse (Gioro, 1991; Stroebel et al., 2013). In line with these findings, Carlson (2011) reported that over two thirds of adults with SSA history display sexual promiscuity, while less than half of the adults without SSA displayed sexual promiscuity.
Discussion
The aim of this review was to identify and synthesize the current evidence base on SSA and its implications. The findings of the current review suggest many salient characteristics of SSA including its early onset, its extensive duration, and the fact that, most commonly, victimized children were younger than the sibling committing the abuse. This suggests that superiority is a central circumstance in allowing for the older sibling to dominate and manipulate the younger. However, an aspect that might unequivocally distinguish SSA from parent–child incest, where power relations are also very relevant, might be its high intensity. The studies included in the current review suggest an extensive amount of coercion, force, and manipulation in cases of SSA (D. H. Brown, 1997; Carlson et al., 2006; Cyr et al., 2002; Krienert & Walsh, 2011). Furthermore, actual penetration might be more common in SSA cases as compared to cases of parent–child incest (Cyr et al., 2002).
These unique characteristics may contribute to the severe levels of psychopathology found in survivors of SSA. The most common outcome might be adulthood depression (Carlson, 2011; Gioro, 1991; Laredo, 1982; O’Brien, 1991; Stroebel et al., 2013). However, some of the included studies suggest that depressive tendencies can also occur earlier, that is, during childhood (Collin-Vézina et al., 2014; Cyr et al., 2002). Another correlate of SSA might be adult anxiety (Carlson, 2011; Gioro, 1991) and low self-esteem (Gioro, 1991; Morrill, 2014). This said, we noticed that there is an immense lack of evidence addressing longer term impacts of SSA on survivors. This gap in our knowledge is unfortunate especially because it inhibits researchers from taking the next step, that is, the development of specific interventions helping SSA survivors to process and to overcome the abuse. From a research perspective, we therefore strongly recommend the implementation of large and longitudinal studies on SSA to endorse the understanding of this phenomenon and to provide a basis for the development of interventions (for more details, please see Table 4). Official record data for such studies are available in many countries, as our review has shown. Future studies should also be utilized to develop and validate reliable measures to assess SSA.
Implications for Practice, Policy, and Research.
Note. SSA = sibling sexual abuse.
Limitations
The current review has a number of strengths as it sheds light on a severe but overlooked phenomenon that has devastating long-term implications and contributes to a better definition of SSA and the environmental conditions allowing for SSA to happen. Nevertheless, findings should be considered in the light of a few limitations. Due to the low number of studies involved in this review and the variation in the data across studies, only a qualitative synthesis of the studies was possible. With more studies involving empirical data, a meta-analysis could be a future target. Moreover, this review only includes articles published in English, limiting the generalizability of our findings. Although, as the analysis of the included samples has shown, SSA survivors differing in gender, age, socioeconomic status, race, ethnicity, and so on are reflected, it is possible that our inclusion criterion (i.e., studies published in English) systematically influenced the results of the current review. To cover this potential limitation, future studies in this field should ensure to choose recruitment pathways that enable participation for SSA survivors from diverse cultural and religious contexts. This would also provide with larger sample size, allowing a quantitative analysis and to statistically examine cultural and gender differences in the nature of the abuse and its later outcomes.
Together, the findings of the current review suggest that (1) SSA is common, (2) SSA has various negative effects on survivors’ mental health, and that (3) this phenomenon and its implications have been and to date are a neglected topic in research. There is an urgent need to better understand SSA and its implications in order to prevent and detect its occurrence and to help survivors to overcome experiences of SSA.
Implications for Practice, Policy, and Research
In addition to developing interventions after SSA has already happened, our review revealed valuable implications relevant for the prevention of SSA. More specifically, the first step toward preventing SSA should be to end the marginalization of this phenomenon in social care contexts. We strongly encourage professionals in the field of social care to take a more considerate and attentive perspective on SSA and to reinforce an open communication on this topic.
Besides marginalization that can clearly be considered a risk factor for SSA and its negative outcomes (Caffaro, 2014; Caffaro & Conn-Caffaro, 1998; Yates, 2020), we would like to direct special attention to other possible risk factors for SSA that we have identified based on the studies included in the current review (Table 4). First, it seems that SSA is more likely in households with three or more children, and households that provide an unstable environment due to marital problems, parental mental health issues, drug abuse, and/or parental experiences of childhood maltreatment. Second, studies suggest that SSA is more likely in low-income families (Crowder, 2003; Cyr et al., 2002; Gioro, 1991). Social care professionals should screen their cases for these risk factors and take their presence as a reason for further investigation with a special focus on SSA. In order to help professionals through this stage of development, specific trainings educating them about SSA and its risk factors should be implemented in the system.
An enhanced attentiveness for these risk factors by professionals is especially important as victims often minimize or downplay SSA experiences making it very challenging to detect SSA. Findings from focus group interviews of 19 female SSA survivors impressively demonstrate the phenomenon of SSA minimization by revealing that most victims regarded the SSA as mutually initiated, as the victim’s fault, or that it could not be abuse because it involved a brother (Rowntree, 2007). Undoubtedly, minimization of SSA by survivors is very consequential as it prevents them from seeking the appropriate support, potentially causing for the abuse to continue. Furthermore, marginalization may also result in persisting feelings of shame, helplessness, and guilt contributing to the development of depression and to an enhanced risk for other mental disorders (Yates, 2020). In clinical samples, SSA should therefore be assessed as a matter of principle (as a facet of childhood maltreatment) and its role in patients’ individual symptom pattern should be examined (Table 4).
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
