This paper is openly available in a view-only version at: https://rdcu.be/bLiqw
In 2010 I began working in Namibia with a community of Himba pastoralists, intending to study parent-child dynamics. But in my first round of interviews, two important themes emerged: first that women (and men) frequently had multiple, concurrent partners, and second that there was a whole suite of social norms that appeared to allow concurrency to function in a way that preserved the respect and integrity of formal marriages. Over and over I was told that one could love both their husband and another man, and that in fact, many people would be uninterested in having a spouse who could not attract other partners. Did this mean that Himba did not experience jealousy? Furthermore, evolutionary psychologists predicted that because men and women face different adaptive problems, they will respond differently to threats of infidelity, with men more upset than women by sexual versus emotional infidelity. Would Himba, with their high frequency of concurrency and low levels of paternal investment, respond similarly?
To answer these questions, I set out a few years ago to explicitly study jealous response among Himba. In line with the logic of evolutionary psychology, Himba conformed to the expected sex difference. When faced with a forced choice between two types of infidelity, men, significantly more than women, chose sexual infidelity to be more upsetting than emotional infidelity. However, Himba responses looked different from those seen in previous studies. The majority of both men and women thought the sexual infidelity was worse. In addition, in opportunistic responses to the task, Himba often stated that either type of infidelity was “fine.” These results sparked the present study, which aimed to go further than the Himba study could, by conducting a more wide-ranging test of the predicted universal sex difference, and at the same time aiming to provide greater understanding of cross-cultural variation in jealous response. I sought out collaborators who worked in populations spanning various modes of production and social systems and who had been working at their fieldsites for many years, so that we could leverage their expertise to interpret our results.
At the core of our paper, Patterns of paternal investment predict cross-cultural variation in jealous response, we wrestle with issues of universality and variation, and we found that, as we waded deeper into the proverbial muck of cross-cultural work, we did not find either explanation entirely satisfying on its own. On one hand, we replicated previous work and found strong evidence across eleven diverse populations that there was a clear sex difference in how men and women responded to threats of sexual versus emotional infidelity. We didn’t always find it (Tsimane for example all found sexual infidelity more upsetting, regardless of sex), but the pattern was consistent enough that we view it as robust support for the original hypothesis.
But more interesting (we think) than the consistency of this result, was the variation we found underlying people’s reactions to scenarios of infidelity. Some of this variation could be attributed to individual differences, like age or marital status, and some is likely to be due to cultural differences that we did not measure. But we were able to use evolutionary theory to make predictions about how jealous response would vary, finding that the severity of jealous response in a particular population was reflective of local conditions. In particular, we predicted, and found, that in populations where the costs of infidelity were lower, either because men were not expected to invest much in their children, or because the opportunities for desertion were more favorable (e.g. a female-biased adult sex ratio), jealous response would be less severe.
This study is part of a large body of work, sparked by a call to expand behavioral science studies beyond the typically used W.E.I.R.D. populations, that examines a psychological phenomenon from a cross-cultural perspective. Where our study differs from most others, is that in addition to using evolutionary theory to derive the initial test (in this case for a universal sex difference in jealous response), we also used it to make predictions about the variation we expected to find. An integrative approach to evolutionary social science has the potential to go beyond the false dichotomy of explaining a phenomenon as either universal or variable, and we are pleased to include this paper among those that think about the plasticity of psychological mechanisms.