Differences between biological and sociolegal incest offenders: A meta-analysis
Introduction
The global prevalence of child sexual abuse has been estimated as 18% for girls and 8% for boys (Stoltenborgh et al., 2013, Stoltenborgh et al., 2011; see also Prevoo, Stoltenborgh, Alink, Bakermans-Kranenburg, & van IJzendoorn, 2016), with approximately one third of these cases being perpetrated by family members (Ogrodnik, 2010). There can be serious mental and physical health costs associated with child sexual abuse (e.g., Fergusson et al., 2013, Ratican, 1992, Roberts et al., 2004) and incest victims have been found to experience greater negative consequences than victims of child abuse committed by non-relatives (Stroebel et al., 2012). Given the prevalence of child sexual abuse, as well as the mental and physical health costs associated with this behavior, understanding the causes of child sexual abuse is vital for prevention and may have implications for the treatment of the perpetrators.
Two broad factors that explain the onset and persistence of sexual offending against children are atypical sexuality and antisocial tendencies. Atypical sexuality includes paraphilias such as pedophilia and hebephilia (sexual interest in prepubescent and pubescent children, respectively) as well as excessive sexual preoccupation (sometimes described as hypersexuality or sexual compulsivity). Antisocial tendencies comprise antisocial traits such as impulsivity, risk-taking, callousness, and offense-supportive attitudes and beliefs. These two factors have been highlighted in theoretical explanations of the onset of child sexual abuse perpetration (e.g., Quinsey, 1986, Seto, 2008, Seto, 2013). Indicators of atypical sexuality are highlighted as motivations for sexual offending against children, while antisocial tendencies are highlighted as facilitators (Pullman et al., 2016, Seto, 2008). Indicators of atypical sexuality and antisocial tendencies are two of the strongest predictors of sexual and nonsexual recidivism in sexual offenders, indicating these variables play important roles in the persistence of these behaviors (Hanson and Morton-Bourgon, 2004, Hanson and Morton-Bourgon, 2005).
Incest avoidance mechanisms and incest taboos prohibit incest (Thornhill, 1991, Wolf, 2014). Therefore, we would expect incest offenders to be more problematic than extrafamilial offenders on indicators of atypical sexuality and/or antisocial tendencies, in order to overcome these incest prohibitions. A recent meta-analysis, however, found that incest offenders against children were less problematic on multiple indicators of atypical sexuality (pedophilia, hebephilia, sexual self-regulation) and antisocial tendencies (criminal history, general self-regulation problems, psychopathy) compared to extrafamilial offenders against children (Seto, Babchishin, Pullman, & McPhail, 2015). This suggests that atypical sexuality and antisocial tendencies alone are not adequate to explain the crimes committed by incest offenders.
Only 3% of the samples included in the Seto et al. (2015) meta-analysis had exclusively biological incest offenders, defined as those who commit a sexual offense against a biologically related child (e.g., son/daughter, niece/nephew and grandchild). In 29% of the samples, incest groups comprised a mixture of biological and sociolegal incest offenders, where sociolegal incest was defined as those who commit a sexual offense against a socially or legally related child (e.g., stepchild, adopted child or child of a common-law partner). In 68% of the samples, the composition of the incest offender group was unknown. It is therefore unclear to what extent the results reported by Seto et al. generalize across different types of incest offending.
Among sex offenders against children, it is common practice to differentiate offenders based on their relationship to their victims, that is, between incest and extrafamilial offenders. However, another distinction that has been less widely recognized is that between biological and sociolegal incest offenders (see Seto, 2008). There are important reasons to suspect there would be differences between biological and sociolegal incest offenders. From a biological perspective, sexual behavior involving a sociolegally related child is not incest because it is not accompanied by the potential costs of inbreeding depression and reduced offspring fitness. Inbreeding depression refers to the reduced survival and reproduction of offspring born from closely related individuals, as a result of combining harmful recessive alleles (see Charlesworth & Willis, 2009, for a review). Any offspring born from these incestuous unions would, on average, have greater morbidity or mortality and therefore would reduce the fitness of both parents.
Moreover, the genetically related child may experience psychological or physical injuries that reduce her later reproduction (e.g., avoidance of sexual relationships or getting a sexually transmitted infection that causes infertility). These consequences further impact the fitness of the perpetrating relative, because inclusive fitness theory states that traits are selected for their impact not only on direct reproduction but on the reproduction of close genetic relatives (Hamilton, 1964). Thus, evolutionary theory expects selection against reproductive behavior between close genetic relatives. These same costs—inbreeding depression and reduced inclusive fitness as a result of harm to the child—do not apply to sociolegal relatives. Instead, sociolegal relatives are inhibited by cultural taboos against incest and the resulting legal and social sanctions.
The costs associated with inbreeding depression vary by the degree of genetic relationship. The cost of inbreeding depression would be lower for an uncle offending against his niece than for a father offending against his daughter, because fathers share more common genes with their daughters. Similarly, the inclusive fitness cost of psychological or physical harm to genetically related children is lower for uncles or grandfathers than for fathers. These extended relatives (biological relatives other than fathers) will be referred to as non-paternal biological relatives.
Because of inclusive fitness and related parental investment, Daly and Wilson (1998) suggested that, on average, a stepparent expends less investment in a stepchild than a biological parent would toward a biological child. Although not explicitly stated by the authors, the same is true for a non-paternal biological relative compared to biological fathers. In a systematic review conducted by Archer (2013), the author found that children living with at least one stepparent had 7.7 times greater odds of being physically abused than those living with two genetic parents (median odds ratio across all studies reviewed). Similar results have been found for the incidence of sexual abuse in genetic parent versus stepparent families (Sedlak et al., 2010). Though most stepparents are loving and caring toward their sociolegally related children, Daly and Wilson suggested that lower parental solicitude is related to less inhibition in expressing anger and physical abuse, which increases the risk of harm to stepchildren. Given these theoretical and empirical differences between biological and sociolegal parents, it is possible that sociolegal incest offenders will exhibit indicators (e.g., atypical sexuality and antisocial tendencies) that are closer to extrafamilial offenders than to biological incest offenders.
The purpose of the current meta-analysis was to examine the extent to which biological and sociolegal incest offenders differ on theoretically and clinically relevant factors. If there are meaningful differences between these two groups, then future research should examine these differences, which may eventually lead to more effective risk assessment, treatment, and case management with incest offenders.
Sociolegal incest offenders are expected to be more similar to extrafamilial offenders against children than to biological incest offenders because inbreeding depression and inclusive fitness explanations do not apply to the crimes committed by either sociolegal incest offenders or extrafamilial offenders (Seto, 2008). We predicted that sociolegal incest offenders would be more problematic in the domains of antisocial tendencies and atypical sexuality compared to biological incest offenders, just as extrafamilial offenders are more problematic in these domains than incest offenders in general (Seto et al., 2015). Variables related to childhood abuse/neglect history, interpersonal deficits, and psychopathology were also examined because of their clinical importance in sexual offender management and treatment.
Section snippets
Defining incest
For the purposes of this meta-analysis, biological incest was defined as physical sexual contact or attempted contact by an adult toward a child (i.e., an individual under the legal age of consent in the jurisdiction where the study was conducted) who was genetically related to the offender, up to and including first cousins. This included those who offended against their biological child, niece/nephew, grandchild, sibling, or first cousin. Biological incest was defined broadly for this
Results
In total, 27 non-overlapping samples (31 individual studies) contributed to this meta-analysis. These studies were published between 1984 and 2012 (Mdn = 1993). Across the 27 samples, the sample size for biological incest offenders ranged from 11 to 1862 (Mdn = 33, Total N = 4192) and the sample size for sociolegal incest offenders ranged from 10 to 896 (Mdn = 33, Total N = 2322). Only one study included female offenders (4%), with 12.5% of the sample in that study being female. Seven samples were from
Discussion
Sociolegal incest offenders were found to have more general and sexual self-regulation problems (Trim-and-Fill: d = 0.28 and d = 0.31, respectively), impulsivity problems (Trim-and-Fill: d = 0.34), as well as alcohol (d = 0.25) and drug (d = 0.52) problems compared to biological incest offenders. While not statistically significant, the comparison between biological and sociolegal incest offenders for psychopathy as measured by the Psychopathy Checklist – Revised (Hare, 2003) produced a similar effect
Funding sources
This work was supported in part by a Social Sciences and Humanities Research Council Doctoral Fellowship (L.P.), a Canadian Institutes of Health Research Banting Fellowship (K.B.), and a Social Sciences and Humanities Research Council Insight Grant (M.C.S. & K.B.; Ref No. 435-2015-0319). The funding bodies were not involved in the design or conduct of the study, collection, management, analysis or interpretation of the data, or preparation, review or approval of the manuscript.
Acknowledgments
We would like to thank L. Maaike Helmus for her helpful comments on an earlier version of this manuscript.
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References marked with an asterisk indicate studies included in the meta-analysis.