In one of the classes I teach at The Australian National University — “Evolution and Human Behaviour” (BIAN 3124) — I teach the students about the diverse evolutionary approaches that have been use to study human behaviour.
In my section on evolutionary psychology, I use a little experiment with the students to drive home some of the key concepts. I present them with a handful of questions about infidelity and we, invariably, find that the males in the class report much more concern about sexual infidelity than the females. The opposite is found for emotional infidelity. We, thus, replicate the finding of a sex difference touted by evolutionary psychology as one of their great successes (Buss 2018; Sagarin et al. 2012).
You see, the fitness payoff to males for investing in offspring is always tempered by the probability that they are being “tricked” into investing in an offspring that is not their own. Women don’t have to worry about this. One way for men to win this game is to adopt behaviours that serve to maximize the probability they have sired the offspring for which they are caring.
Jealousy is viewed as a strategy that can provide this result. Since this has been a recurring theme throughout human evolutionary history, evolutionary psychologists argue that you should see more intense jealousy evoked by sexual infidelity in men. In women, emotional jealousy is predicted to be more common. These patterns should hold up regardless of cultural background. It should be a universal.
On the face of it, the hypothesis has fared well when tested in a variety of settings, including cross-cultural ones.
As a human behavioural ecologist, I’m relatively uncomfortable with the concept of universality — and refer you to my paper “Why do the Karo Batak Prefer Women with Big Feet?” for more discussion (Kushnick 2013). People adapt to their environments via culture and biology, and since humans live in such a diverse range of socioecological settings I would expect human behaviour to be more complex than that. For most things, I expect humans to adopt flexible, conditional strategies.
According to pioneers of the field, Winterhalder and Smith (2000), said:
[Human behavioural ecology] usually frames the study of adaptive design in terms of decision rules, or conditional strategies: “In the context of X, do α; in context Y, switch to β.”
Why shouldn’t this logic apply to sexual jealousy as well? The benefits of acting jealously to stave off the chance of becoming a cuckold should vary with a number of socioenvironmental factors, not the least of which is the degree to which a man will provide investment (time, resources, etc) to his putative offspring. This is exactly the logic which led my colleague Brooke Scelza (an anthropologist at UCLA, who — like me — did her PhD under the supervision of the aforementioned Eric Alden Smith) to design a project whereby a number of us would conduct the experiment in different small-scale societies. Brooke takes you behind the scenes here.
I conducted the experiment in rural North Sumatra, Indonesia, amongst a group of people referred to as the Karo that I have been working with for over 10 years. The Karo are one of the so-called “Batak” groups who share some similarities, like a stated preferences for marriages with their matrilateral cross cousins — an aspect of their society that I have studied (Kushnick et al 2016).
The results of this research, conducted with over 1000 subjects in 11 societies, have recently been published in Nature Human Behaviour (Scelza et al 2019). The article sits, unfortunately, behind a paywall, but the publisher has made a readable, but not downloadable version of the PDF available for free.
We found that jealous response, indeed, varied with the degree to which males were involved in parental care in those societies, as well as with the strictness of sociosexual norms, which are correlated with the male’s familial role in those societies.
We consider this a big win for human behavioural ecology, but a small step in the understanding of human nature and behaviour.
The research has received some press coverage:
And Brooke was chuffed to see this comment from David Buss on Twitter:
This is such a cool paper. A great demonstration of how evolutionary hypotheses can predict cultural variation as well as universal sex differences…in this case in the powerful emotion of jealousy. https://t.co/fS3eMlhpqC
— David Buss (@ProfDavidBuss) July 23, 2019
Buss DM (2018). Sexual and emotional infidelity: evolved gender differences in jealousy prove robust and replicable. Perspectives in Psychological Science, 13, 155-160.
Kushnick, G. (2013) Why do the Karo Batak prefer women with big feet? Flexible mate preferences and the notion that one size fits all. Human Nature, 24, 268-79. PDF Link
Kushnick G, Fessler DMT, Zuska F (2016) Disgust, gender, and social change: Testing alternative explanations for the decline of cousin marriage in Karo society. Human Nature, 27, 533-555. PDF Link Blog Data
Sagarin BJ, et al (2012). Sex differences in jealousy: a meta-analytic examination. Evolution and Human Behaviour, 33, 595-614.
Scelza BA, Prall S, Blumenfeld T, Crittenden A, Gurven M, Kline M, Koster J, Kushnick G, Mattison SM, Pillsworth E, Shenk M, Starkweather K, Stieglitz J, Sum, C-Y, Yamaguchi K, McElreath R (2019) Patterns of paternal investment explain cross-cultural variance in jealous response. Nature Human Behaviour. PDF Link