Over half of all human cultures in the ethnographic record practice polygamy, the simultaneous marriage of more than one woman to a single man. Both men and women have individual incentives to enter into polygamous marriages, and if they do so by their own consent, it’s not obvious that anyone is harmed. Why should polygamy be illegal and should the law be changed? Two cases are pending in the US Supreme Court and a case considered by the Supreme Court of British Columbia in 2012 upheld the law.
The British Columbia decision relied heavily on evolutionary considerations, based on information provided by two experts, Joseph Henrich and Todd Shackleford. ETVOL is pleased to provide the relevant portion of the judge’s decision to show how knowledge about human sexuality and cultural evolution figured in the judge’s decision. We’re also pleased to provide a link to a 2012 academic article titled “The Puzzle of Monogamous Marriage” written by Henrich, Robert Boyd, and Peter Richerson, based in part on the report that Henrich wrote for the court.
The laws of a nation are a set of formally adopted norms enforced by punishment–a topic central to cultural evolution. The rationale for any given law is based on a consideration of its consequences for individuals and society as a whole. Lawyers must consult experts to obtain the best available information relevant to a case, and the fact that two evolutionists were consulted for this case provides an example of how evolutionary approaches to human behavior and culture are becoming mainstream, at least in Canadian society (it will be interesting to follow the US Supreme Court decisions). ETVOL invites readers to comment on the details of the case and whether knowledge derived from evolutionary science was used appropriately or inappropriately.
Here is a portion of the judge’s report:
1. Evolutionary Psychology
 Two eminent academics in the field of evolutionary psychology gave evidence in this proceeding.
 Evolutionary psychology, as described by one of these experts, is the study of the features of the mind with specific reference to our ancestral past as a way of trying to understanding how and why we behave in the present.
 As his principal witness in the reference, the AGBC called Dr. Joseph Henrich, an Associate Professor in the Psychology and Economics Departments at the University of British Columbia. He also holds a prestigious Tier 1 Canada Research Chair in Culture, Cognition and Evolution. Dr. Henrich was qualified as an expert in psychology, particularly evolutionary psychology, in economics and in anthropology, as well as in the interdisciplinary field of culture, cognition and co-evolution.
 Dr. Henrich, over the course of four months, conducted an extensive review of the academic literature on polygyny in the sciences and social sciences. The resulting report is entitled “Polygyny in Cross-Cultural Perspective: Theory and Implications.”
 The Amicus, in turn, called Dr. Todd Shackelford, Chair of the Department of Psychology at Oakland University (Rochester, Michigan). Dr. Shackelford enjoys an international reputation in his field and was qualified as an expert in evolutionary psychology and in conflict between men and women in monogamous relationships.
a) Dr. Henrich
 In some respects, Dr. Henrich’s report is similar to the literature reviews conducted by several of the other experts. However, while those reviews are largely limited to cataloguing the consequences of polygyny found in the literature, Dr. Henrich develops a theoretical model that explains why our evolved psychology favours polygyny and how the consequential effects of polygyny are the necessary by-products of a polygynous social structure. He then uses a wide range of evidence to test his theory.
 What this evidence ultimately leads Dr. Henrich to conclude (at 2) is that:
· A non-trivial increase in the incidence of polygyny, which is quite plausible if polygyny were legalized given what we know about both male and female mating preferences, would result in increased crime and antisocial behaviour by the pool of unmarried males it would create.
· Greater degrees of polygyny drive down the age of first marriage for (all) females on average, and increase the age gap between husbands and wives. This generally leads to females marrying before age 18, or being “promised” in marriage prior to age 18.
· Greater degrees of polygyny are associated with increased inequality between the sexes, and the relationship may be causal as men seek more control over women when women become scarce.
· Polygynous men invest less in their offspring both because they have more offspring and because they continue to invest in seeking additional wives. This implies that, on average, children in a more polygynous society will receive less parental investment.
· Greater degrees of polygynous marriage may reduce national wealth (GDP) per capita both because of the manner in which male efforts are shifted to obtaining more wives and because of the increase in female fertility.