Promotor Prof. dr. Jan Verplaetse Department of Interdisciplinary Study of Law, Private Law and Business Law Decaan Prof. dr. Marc Boone Rector Prof. dr. Anne De Paepe De moraalpsychologie van incestaversie Faculteit Letteren & Wijsbegeerte Delphine De Smet The moral psychology of sibling incest aversion Evolutionary origins and legal future Proefschrift voorgelegd tot het behalen van de graad van Doctor in de moraalwetenschappen 2014 Acknowledgements The people who deserve my greatest acknowledgement are my parents, Daisy Vandebuerie and Philippe Bovyn. They had and continue to have a great and uttermost positive influence on who I am and on the things that I value in life. Through all life stages they have provided me with adequate support and guidance, always encouraging me to find my own way in life. I cannot put down in words how much I am thankful to them. Secondly, I am greatly thankful to my advisor, prof. dr. Jan Verplaetse, first of all for guiding me through the process of writing a master’s thesis, and after that, for inviting me to apply for the job of assistant at the former department of Legal Theory and Legal History at the Law Faculty of Ghent University. He gave me the opportunity to start up this research, and has been a terrific guide throughout the whole process. During the course of my PhD research I was very thankful to be cooperating with, and receive much-welcomed advice and mentorship from prof. dr. Johan Braeckman, dr. Dieter Jehs, prof. dr. Debra Lieberman, dr. Joshua Tybur, dr. Katinka Quintelier, dr. Michiel van Elk, dr. Lien Van Speybroeck and all the members of the Moral Brain Research Group and of the Human Evolution and Behavior Network (HEBeN). I wish to express special thanks to prof. dr. Debra Lieberman for inviting me for a research stay at the University of Miami, which took place from April to July 2013. This research stay however would also not have been possible without the travelling grant that was given to me by the FWO Flanders, for which I am thankful. I was happy to have colleagues around me to mutually share research related enjoyments, and obviously sometimes also frustrations. I would like to express thanks to all the colleagues of the former department of Legal Theory and Legal History at the Law faculty of Ghent University, and all the members of the Moral Brain Research Group. Over the course of the years some of these colleagues became friends. Special thanks to Stephen Billion, Eveline Seghers and Nancy Van Nuffel for the support and the happy moments we’ve shared. v During the course of my research I was lucky to enjoy the companionship of office buddies dr. Valérie Smet (2010-2012) and Charles Delmotte (2013-2014). Thanks to them I enjoyed an excellent working atmosphere. I was lucky to receive the support and friendship of several people outside of academia. For this I wish to express special thanks to Jan Buyse, Mieke Carrein, Sien Gernaert, Rudi Jehs, Regis Lefebre, Stefanie Van Heule, Martine Verbeke, and Stefanie Vandewalle. Lastly, I choose to thank three people that mean a great deal to me, each in their own way. I thank Dieter Jehs, for having been a great companion along the way, and for his continued support up until the final stages. I thank Tiago Soares Bortolini, for being a beloved friend and confidant despite all the kilometers between us. And I thank Davy Teirlynck for so much kindness and love. vi List of Tables Table 1 Overview of physiological output prior to data reduction: 6 SCR data (SCR a to f), 12 fEMG data (fEMG a to l) and 12 HR data (HR a to l) 47 Table 2 Paired samples t-tests: 1st vs. 2nd interval variables and mild vs. explicit sex variables 50 Table 3 Repeated measures ANOVAs on fEMG, HR and SCR variables (after first data reduction) 51 Table 4 Number and type of brothers 52 Table 5 Independent samples T-tests for differences in fEMG responses between females without brothers and females with at least one brother 54 Table 6 fEMG responses and coresidence duration, attitudes towards sexuality in the family, and disgust sensitivity 54 Table 7 EMG responses and brother relation characteristics (subjects with 1 brother) 55 Table 8 Paired samples t-tests: 1st vs. 2nd interval variables and mild vs. explicit sex variables (males, N=66) 66 Table 9 Paired samples t-tests: 1st vs. 2nd interval variables and mild vs. explicit sex variables (females, N=104) 67 Table 10 Frequencies of strong (SPI) and weak (WPI) indicators for shared parental investment for oldest opposite-sex sibling. 69 Table 11 Females: Bivariate correlations between fEMG when imagining sex with a brother and (1) coresidence duration with oldest brother, (2) genetic relatedness with oldest brother, (3) cues for shared parental investment with oldest brother and (4) openness towards sexuality in the family 72 Table 12 Males: Bivariate correlations between EMG measurements and openness towards sexuality in the family and disgust sensitivity 76 Table 13 Males: Bivariate correlations of EMG measurements and (1) coresidence duration with oldest sister, and (2) cues for shared parental investment with oldest sister 77 Table 14 Repeated Measures ANOVA for differences in fEMG responses between specific word primes within each priming condition (family/not family) 94 Table 15 Repeated Measures ANOVA for differences in subjective rating responses between specific word primes within each priming condition (kin/not kin) 95 Table 16 Comparative overview of IAT effect sizes in a selection of recent studies that use Greenwald et al.’s (2003) scoring algorithm 145 vii Table 17 Illustration of correlations between IAT and explicit counterparts in a selection of recent research involving sexuality measures 148 Table 18 Pearson’s Correlation Coefficients of Moral Attitudes regarding Consensual Sibling Incest and Self-Reported Emotional Intensity 158 Table 19 Is a cognitive reappraisal of harm-related arguments mainly responsible for changes in moral attitudes? 161 Table 20 Summary of research questions, methods and results of the studies described in chapters 7 and 8 166 viii List of Figures Figure 1 Lieberman et al.’s (2007) model of the computational architecture of human sibling detection 14 Figure 2 Tooby et al.’s (2008) model of the computational architecture of the sexual and altruistic motivational systems 15 Figure 3 Figure displaying electrode placement for facial electromyography (figure derived from Fridlund & Cacioppo, 1986) 32 Figure 4 Image showing electrode placement for measurement of facial electromyography in the levator labii region 32 Figure 5 Image showing showing electrode placement for measurement of skin conductance responses 35 Figure 6 Procedural sequence of one trial. Instructions displayed were either ‘Imagine that you perform the following act with your partner’ or ‘Imagine that you perform the following act with your brother’ 45 Figure 7 error bars (mean + 95% C.I.) of EMG, SCR and HR variables. 49 Figure 8 Error bars (mean and 95% mean C.I.) of EMG responses in females (N=104) 67 Figure 9 Error bars (mean and 95% mean C.I.) of EMG responses in males (N=66) 68 Figure 10 Error bars (mean and 95% mean C.I.) of HR responses in males (N=66) 68 Figure 11 Error bars (mean and 95% mean C.I.) of SCR responses in males (N=66) 69 Figure 12 Scatterplot of fEMG 1 responses set out against coresidence duration with oldest brother from subject’s birth until the age of 18 (female subjects) 73 Figure 13 Scatterplot of fEMG 1 responses set out against strong predictors for shared parental investment with oldest brother from subject’s birth until the age of 6 (female subjects) 73 Figure 14 Scatterplot of fEMG 2 responses set out against strong predictors for shared parental investment with oldest brother between subject’s ages of 7 and 12 (female subjects) 74 Figure 15 Overview of experimental designs of fEMG studies: implicit and explicit priming of ‘incest’ and experienced aversion. For each experimental design, the time lapse, procedures and stimuli are given for one trial. 88 Figure 16 Immediately after display, each image was rated on this visual analogue scale. 91 Figure 17 Mean fEMG responses towards neutral, pathogen, heterosexual sex and homosexual sex images in each experimental task. ‘K’ refers to images ix preceded by a word referring to kinship, ‘NK’ refers to images preceded by a word not referring to kinship. 95 Figure 18 Mean self-report measures towards neutral, pathogen, heterosexual sex and homosexual sex images in each experimental task. ‘K’ refers to images preceded by a word referring to kinship, ‘NK’ refers to images preceded by a word not referring to kinship. 96 Figure 19 Mean fEMG responses towards neutral, pathogen, heterosexual sex and homosexual sex images, displayed separately for each priming condition (i.e., kinship/not kinship) 97 Figure 20 Mean self-report measures towards neutral, pathogen, heterosexual sex and homosexual sex images, displayed separately for each priming condition (i.e., kinship/not kinship) 98 Figure 21 Changes to the Law of Incest in Scotland, 1567–1986 (Leeming, 1996, p. 321). 111 Figure 22 Schematic overview of empirical findings on moral decision making about consensual sex between adult siblings, and predictors of inter- individual variability 130 Figure 23 Comparison of the procedural structure of a classic Implicit Association Test (IAT) and a Single-Category Implicit Association test (SC-IAT) 138 Figure 24 Mean values and standard deviations of moral attitudes towards consensual sex between adult siblings, displayed for each experimental phase. 156 Figure 25 Mean values and standard deviations of moral attitudes towards marriage between adult siblings, displayed for each experimental phase.