“It is not love that is blind, but jealousy,” according to the writer Lawrence Durrell. Still, this ugly and durable emotion has served a vital evolutionary purpose. Jealousy is defined as a fear and rage response that preserves romantic bonds between sexual partners. Its function, it is believed, is to curb infidelity between parents, which advances the survival of their children and their subsequent reproductive success.
Romantic jealously is widely understood to be different for men and women because each gender has a different level of investment in reproduction. For a man to provide for genetically distant children decreases his reproductive success — and because men are uncertain whether they really are the father of said children, they are most susceptible to sexual infidelity. By contrast, women can rest assured that they are the mother of their own children; however, they are more dependent on men for resources, making them more sensitive to emotional infidelity, since it could threaten the supply of resources for herself and her child. While many subscribe to this view, the research has been admittedly inconclusive. Some studies attribute sex differences in romantic jealousy to cultural forces, while others have observed no gender differences.
Now, a team led by Hasse Walum of the Karolinska Institute in Sweden has broken new ground. He and his collaborators also investigated if there were sex differences when it came to romantic jealousy, but they also wondered whether there's a genetic component. That question hadn't yet been tested, and because of a gold mine of a research sample comprised of 1,048 monozygotic twins, 1,129 same-sex dizygotic twins, and 1,020 opposite-sex dizygotic twins, they were able to pursue it. (Monozygotic twins share 100 percent of their genes, and dizygotic twins share on average 50 percent).
Participants were presented with two hypothetical infidelity scenarios:
“Sexual jealousy: ‘You suspect that while your boyfriend/ girlfriend was on vacation s/he had a one nightstand. You realize that even if s/he did have sex with this other person, they will probably never see each other again. How upset do you think you would feel if this happened?’”
“Emotional jealousy: ‘You suspect that while your boyfriend/girlfriend was on a trip s/he fell in love with someone else. You realize that even if s/he did develop these feelings, s/he will probably never see this other person again. How upset do you think you would feel if this happened?’”
They were then asked to answer these questions along a 10-point scale, ranging from 1 (not at all) to 10 (extremely).
Walum and his team crunched the numbers. What did they find? Consistent with prior research, women reported higher levels of jealousy on both measures, and both men and women scored higher on sexual jealousy than on emotional jealousy. However, men reported greater jealousy in response to sexual infidelity by comparison to emotional infidelity. These findings square with the theory that men and women differ when it comes to the types of jealousy, that is, sexual vs. emotional.
The results also revealed that genetics was a significant factor – accounting for 30 percent of the equation. Yet there were no differences between men and women when it came to jealousy on a genetic level.
The authors highlight that their study provides additional evidence that men and women probably process infidelity differently. From an evolutionary perspective, it could result from exposure to different 'selection pressures' over the course of human evolution. And their finding that genetic factors play a role in romantic jealousy is in keeping with previous research establishing a relationship between genes and other mating behaviors, including marital quality, monogamy, and the probability of divorce. While the investigators acknowledge the limitations of this study and future research directions, their findings lend more insight into a cruel component of human nature.
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