The Westermarck effect revisited: a psychophysiological study of sibling incest aversion in young female adults

Research on human inbreeding avoidance has uncovered at least two central cues to the detection of siblings: witnessing an infant being taken care of by one's mother (i.e. maternal perinatal association) and growing up in close proximity to a child (the so-called Westermarck effect). Both cues have been supported by fieldwork in populations under specific cultural conditions, and by survey research mainly in student populations. Both types of research have relied often on self-reporting. Unfortunately, this method is frequently colored by ceiling effects and notions of social desirability. In order to circumvent this problem, we explored a complementary method for investigating incest aversion that involved measuring psychophysiological responses during an imagery task. As such, we analyzed data on 63 heterosexual female students who viewed pictures of sexual and non-sexual activities while imagining performing these activities with either their partner or their brother. In female subjects with only (one or more) older brothers—a proxy for lacking maternal perinatal association with an opposite-sex sibling—the duration of coresidence with brother(s) predicted activity in the mm. levator labii superioris and alaeque nasi, facial muscles that are highly active when a subject expresses facial disgust. The strength of these responses was also predicted by the frequency of having bathed and shared a bedroom with a brother in early childhood; two activities that may serve as additional cues for relatedness as it can be expected that they are typically performed with genetically related children. As a result, the psychophysiological approach not only complements the use of self-reports in recent research on incest aversion, but also has the potential to fine-tune well-established cues for sibling detection, or to uncover additional ones.