Chapter 6 - Incest


  Incest avoidance and incest taboos are related but distinct phenomena (…).
Westermarck’s (1891/1921) proposed propinquity mechanism provides an elegant
explanation for incest avoidance and has good empirical support from nonhuman
animal research and quasi—experiments in humans.

Incest taboos are ubiquitous, and appear in both historical and cross-cultural
accounts of laws and customs regarding permissible sexual relationships. Overall,
however, the etiological understanding of incest lags the knowledge being
accumulated regarding the origins of sexual offending against children in general.

The gap remains large between

  • (a) theoretical research in evolutionary psychology and anthropology on incest
    avoidance and taboos and
  • (b) the more applied research carried out by researchers concerning incest offenders
    and their offenses.

This is very unfortunate, because many novel hypotheses could be tested to help
improve understanding of not only why incest occurs but why some children may
be more vulnerable than others – these hypotheses include those that consider
inclusive fitness, parental investment, and paternity uncertainty. Such research
could have important implications for prevention programs, particularly in the
identification of at-risk families and children.

From this chapter, I hope I have convinced you that the following factors can make
major contributions to researchers’ understanding of incest:

  • (a) degree of genetic relatedness, with the likelihood and nature of incest offending
    expected to vary by relatedness, distinguishing further from matrilineal versus
    patrilineal relatives;
  • (b) mother-infant association, as another important cue of relatedness in addition to
    the early proximity identified by Westermarck (1891 / l92l );
  • (c) perpetrator and victim gender, with perpetrators being much more likely to be
    male and victims being more likely to be female;
  • (d) type of sexual behavior, with larger effects obtained for incidents involving
    penile-vaginal penetration than non-productive sexual activities, and with
    differences in the likelihood of penile-vaginal penetration based on genetic
    relatedness and victim age;
  • (e) family dysfunction, with incest more likely in families characterized by instability and by parental conflict;
  • (f) the attractiveness of the male perpetrator, based on physical and other
    characteristics, such as status and wealth, which would influence his access to
    unrelated sexual partners and likelihood of opportunistic offending against a
    younger relative; and
  • (g) paternity uncertainty, influenced by the characteristics and behavior of the father,
    mother, and incest victim and connected to what is known about sexual jealousy and
    intimate partner violence.

Recent evidence supports some of these factors and a full accounting may eventually
find that an important proportion of incest offending can be accounted for by the
motivation-facilitation model described in Chapter 4 (this volume; the model in
which atypical sexual interests and antisociality can overcome incest avoidance and
taboo); by failures of incest avoidance as a result of low or no early propinquity or
maternal-infant association; paternity uncertainty; and by opportunistic offending.

However, much more work is needed to determine how much incest offending can
be explained by these different perspectives, what other explanations may be
important, and what researchers and clinicians can do to prevent incest.