Characteristics and risk factors for sibling incest

Abstract

Sibling sexual behaviour, despite historical and cross-cultural incest taboos and biologically driven incest avoidance, poses a persistent problem. We tested factors theorized to be associated with sibling incest in a cross-sectional online survey of 1,863 respondents with siblings mainly from North America and Germany. We found that 13% of participants reported engaging in sexual contact with a sibling, typically starting at the age of 10, and that step-siblings and half-siblings were more likely to engage in sibling incest than full siblings. Curiosity and games were the primary motivators; being coerced was more prevalent among female and younger participants. The study underscores both individual (e.g., impulsivity, concurrent childhood sexual behaviour problems) and family level factors (e.g., presence of step-sibling, positive attitudes toward nudity, sexual abuse by parent) influencing liability to engage in sexual behaviours with a sibling. Findings were robust across English- and German-speaking participants, suggesting our results are generalizable. Professionals addressing problematic child sexual behaviour should assess for concurrent sibling incest, and evaluate positive family attitudes towards nudity, sexual abuse by parents, and reduced disgust to sibling incest as potential risk factors for sibling incest. The findings stress the need for comprehensive sexual education in blended households, where age gaps and diminished genetic relatedness contribute to sibling sexual behaviour.

Fig 1. Prevalence (%) of sibling incest divided by sex and sibling type.

Fig 2. Mediating effect of sexual interest in a sibling.

Asterisks denote significant effects, p < .001. N = 1,807. Values represent standardized coefficients. By itself, constant proximity was not significantly related to whether sibling incest had occurred. Instead, constant proximity was related to a decreased chance that sibling incest had occurred, through its relationship with sexual interest in a sibling.

Fig 3. Comparison of correlates (adjusted odds ratio) across sibling incest type.

n_Coercive = 1,190; n_Non-coercive = 1,213. Only correlates that had a significant AOR value for either group (i.e., participant coercive or participant non-coercive) were included in the figure. ^aSignificant for both outcomes at p < .05. ^bSignificant for participant-instigated coercive at p < .05. ^cSignificant for participant-instigated non-coercive at p < .05.