Original Article The Westermarck effect revisited: a psychophysiological study of sibling incest aversion in young female adults ☆ Delphine De Smet ⁎ , 1 , Linda Van Speybroeck 1 , Jan Verplaetse Ghent University, Department of Legal Theory and Legal History Gent, Belgium a b s t r a c t a r t i c l e i n f o Article history: Initial receipt 7 December 2011 Final revision received 19 September 2013 Keywords: Incest avoidance Sibling detection Psychophysiology Aversion Westermarck effect Maternal perinatal association 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 forinvestigating incestaversion that involved measuring psychophysiological responses during an imagery task. As such, we analyzed data on 63 heterosexual female students who viewed pictures of sexualand non-sexualactivities 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 maternalperinatalassociation 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 wh 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. © 2014 Elsevier Inc. All rights reserved. 1. Introduction In order to explain the widespread dissemination of incest avoid- ing behavior, the Finnish anthropologist Edward Westermarck (1891, 1908) hypothesized thathumans spontaneously develop a sexual aversion toward individuals they have lived with during childhood. He believed that natural selection favors such a mechanism as it prevents inbreeding and establishes childhood coresidence as a reli- able indicator of biological relatedness. While it initially attracted few attention,this mechanism,dubbed the Westermarck effect, gained increasing empiricalsupport through field studies. By examining exceptionalcases of unrelated children growing up together, these studies showed that such children are not typically attracted to one another sexually in later life (cf.the study of co-raised children in Israeli kibbutzim by Shepher, 1971), and that—when pressed to marry one another—they demonstrate lower fertility rates, higher divorce rates or more frequent extra-marital relationships (cf.the study of Taiwanese minor marriages by Wolf, 1995 and the study of Lebanese cousin marriages by McCabe, 1983). However, their underlying methods have been subject to the criticism that they cannot exclude alternative explanations for the attested findings. Shor & Simchai (2009) for instance recently casted doubt on the notion that early-life cosocialization really led to sexualaversion in the case of Israeli kibbutzim. They suggested that sexual aversion should not be analyzed through the presence or absence of marriages, but instead by probing the feelings ofthe people who grew up in these peer groups. Based on in-depth interviews with such individuals,they concluded that sociological factors—such as the importance of social cohesion and group unity—provide sufficientexplanation for the absence of sexual relationships.Questionnaire-based research of Lieberman & Lobel (2012) however confirmed that in male subjects the duration of coresidence with opposite-sex peers in Israeli kibbutzim predicts higher self-reported sexual aversion to them.In female subjects, the opposite pattern was found (i.e. increasing sexual attraction with longer coresidence), which according to the authors may be due to a ceiling effect in females' responses.Recentfield work among the North Sumatran Karo Batak suggested that the Evolution and Human Behavior 35 (2014) 34–42 ☆ Contributions and competing interests: The authors designed the study. D.D.S. and L.V.S.acquired the data.D.D.S.performed the statisticalanalysis.L.V.S.wrote the Inquisit script. D.D.S. and L.V.S. contributed equally to this paper. The authors alone are responsible for the content and writing of the paper. No conflict of interest is reported. ⁎ Corresponding author. Research Unit ‘The Moral Brain,’ UGent, Department of Legal Theory and Legal History, Universiteitstraat 4, B-9000 Gent, Belgium. E-mail address: delphine.desmet@ugent.be (D. De Smet). 1 Equally contributed, alphabetical order. 1090-5138/$ – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.evolhumbehav.2013.09.004 Contents lists available at ScienceDirect Evolution and Human Behavior j o u r n a l h o m e p a g e : w w w . e h b o n l i n e . o r g