Incest: What we know (and don’t know)

Lesleigh Pullman

Although this topic definitely does not make for good dinner conversation, attempting to understand the causes of sexual offending against children is an ongoing endeavour. But we’ve done pretty well so far. Much is known about the causes of child sexual abuse generally. Two key motivational factors that have been highlighted are atypical sexual interests and antisociality. However, much less is known about the causes of incest. Incestuous sexual offending is puzzling not only from a social and moral perspective (i.e., the incest taboo), but also from a biological perspective. Inbreeding depression (what happens when we have offspring with close relatives) increases the likelihood of birth defects and infant mortality. Presumably, we have evolved strong mechanisms for avoiding incest because sex with our relatives is bad for the survival of our genes!

To commit their crimes, incestuous offenders must “overcome” not only the moral stigma associated with incest (i.e., the incest taboo) but also the biological costs of incest. While socially and morally abhorrent, sexual offenders against unrelated children are not subject to these same inhibitory factors. Therefore, in order to overcome these additional factors, we would expect incestuous offenders to have more problems in the key areas we know contribute to sexual offending against children (see also Seto, Lalumière, & Kuban, 1999). However, research shows that this is actually not the case. A recent meta-analysis by Seto, Babchishin, Pullman and McPhail (2015) indicates that, compared to sexual offenders against unrelated children, incestuous sexual offenders against children have fewer atypical sexual interests (e.g., pedophilia, hebephilia, other paraphilias), as well as antisociality indicators (e.g., offense supportive attitudes, victim empathy, psychopathy).

This is not to say that an antisocial orientation and atypical sexual interests are not important to explain incest. Indeed, incestuous offenders are more problematic in these domains compared to non-offenders. Instead, the literature suggests that there must be additional factors to explain incest given that: 1) strong biological mechanisms and social mores should be inhibiting this form of sexual offending, and; 2) the factors known to explain sexual offending against children generally don’t seem to be “strong” enough in incestuous offenders to overwhelm these inhibitory controls.

Investigation of these “other factors” can be made at multiple levels. At the ultimate level of explanation, we must ask ourselves why the ‘incest-avoidance mechanism’ is not functioning adequately in incestuous sexual offenders. In order to answer this question, we need to know exactly how this incest-avoidance mechanism works. At the proximate level of explanation, we must investigate the psychological mechanisms that promote incest avoidance and whether these same mechanisms are defective in incest offenders.

Westermarck (1891/1921) suggests that close physical proximity between people (especially during childhood) facilitates sexual indifference because physical proximity/ co-residence with another person is an indicator of genetic relatedness. Therefore, family members who do not have close physical proximity with one another are theorized to be at an increased risk of incestuous behaviour because the incest avoidance mechanism has not been triggered. This theory is supported by multiple studies showing that genetically unrelated children who are raised together from birth are unlikely to develop romantic and sexual relationships (Shepher, 1971; Spiro, 1958; Talmon, 1964).

In further support, a review of arranged marriages in Taiwan found higher rates of infertility and divorce when children – who were raised together from a young age and were not genetically related – were forced to marry, compared to arranged marriages where couples did not co-reside during childhood. Additionally, siblings who were separated during the first few years of life were more likely to engage in vaginal intercourse (which could lead to reproduction) compared to siblings who were not separated.

These studies provide a start to understanding the inhibitory factors for incest. However, very little research has been conducted on father-daughter incest. One notable exception found that non-incestuous fathers were more likely to be present in the home during their daughter’s early childhood compared to incestuous fathers. Conversely, Williams and Finkelhor (1995) did not find a statistically significant difference in the time spent at home between incestuous and non-incestuous fathers. However, they did find that incestuous fathers spent less time in a caregiver role with their daughters (e.g., changing diapers, washing, playing) compared to non-incestuous fathers.

Incest Disgust

At the proximate level, however, no one thinks “I’m not going to have sex with this person because it is bad for the survival of my genes”. That is, the idea of incest does not typically involve conscious thought. So, how does close proximity facilitate the development of incest avoidance? Typically, the idea of sex with a close relative evokes a strong feeling of disgust. Disgust is an emotional and physical reaction that has been shown to promote the avoidance of objects, situations and behaviours that may be harmful to evolutionary fitness. Incestuous behaviour would fall under this category. Once the system that tells us we are genetically related to someone has been triggered (through close proximity/co-residence), a disgust response develops toward the idea of sex with that particular individual.

In regard to sibling incest, this is a promising line of enquiry. One large study of participants who had siblings measured disgust toward sibling incest using two different methods. The first was the level of disgust participants expressed towards the idea of sexually interacting with a specific sibling. The second used morality as a proxy for disgust and asked participants to rank how morally wrong they found third-party sibling incest to be among a list of other behaviours (considered to be an unobtrusive measure of disgust toward incest). The authors found that, in the absence of other cues of genetic relatedness, the longer siblings co-resided, the more disgust they felt toward sibling incest.  However, this hypothesis has not been explored in father-daughter incest.

At the ultimate level of explanation, we would expect to see a lack of co-residence and physical proximity between incestuous fathers and their daughters prior to when the incestuous behaviour began. At the proximate level, we would expect incestuous fathers to experience less disgust toward the idea of a sexual relationship with their daughters. This would suggest that disgust is the proximate mechanism that ensures we do not engage in incestuous behaviours and reduce our fitness. These are both complimentary hypotheses that may help facilitate a better understanding of incest.

This is a fascinating yet puzzling area of research with many unanswered questions. I hope to build on the few answers we do have by investigating the ultimate and proximate explanations for father-daughter incestuous offending. And here is some shameless self-promotion: keep an eye out for the results of my research at the ATSA conference in the next few years!


Lesleigh Pullman completed her undergraduate degree (BA Honours, Psychology) from Carleton University where she studied the effect of mood on attitudes toward sexual aggression. She is currently a fourth year student in the combined MA-PhD program in experimental psychology at the University of Ottawa, supervised by Dr. Michael Seto. She is also an “adopted” member of the Sex, Crime and Evolution Research Lab at the University of Ottawa. Her research activity has thus far been focused in the area of sexual offending, particularly with adolescent sexual offenders and recently incestuous sexual offenders. Her research interest, however, are broad and include many different areas of forensic psychology, evolutionary psychology and sexology. Her graduate school career has been funded by the Ontario Graduate Scholarship, as well as the Social Sciences and Humanities Research Council Doctoral Fellowship. 

Suggested citation:
Pullman, L. (2015, Feb 22). Incest: What we know (and don’t know). [Weblog post]. Retrieved from

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