Monkeys get jealous too, and a new study has identified the responsible part of the brain, which is likely to be the same in humans. It is hoped that understanding the neuroscience of jealousy will assist people striving to control it.
There's a social assumption that jealousy is normal in romantic relationships but pathological elsewhere, like in friendships. Yet the spectrum of how jealous people get is exceptionally broad.
Research trying to explain this variation has generally used prairie voles, which are easy to work with, but not exactly close relatives of humans. Professor Karen Bales of the University of California, Davis has done plenty of vole research herself, but wanted to investigate brains more similar to our own. She turned to coppery titi monkeys (Callicebus cupreus), which form long-term pair bonds, become distressed when separated from their partners, and guard their mates.
"Male titi monkeys show jealousy much like humans and will even physically hold their partner back from interacting with a stranger male," Bales said in a statement.
In Frontiers in Ecology and Evolution Bales reports that when male titis were caged so they could see either their own mate or an unknown female with an unfamiliar male, sharp differences emerged.
Males that watched their mates interact with other males had higher levels of the stress hormone cortisol afterward, as well as higher testosterone. The more they observed, the higher their cortisol levels became.
When the brains of these jealous monkeys were studied using PET and MRI scans, they had higher uptakes of a marker for sugar consumption in three parts of the brain, compared to the monkeys who had just watched two strangers.
Of the parts of the brain studied, the cingulate cortex, known to be associated with social pain in humans, was judged the most significant. "Increased activity in the cingulate cortex fits with the view of jealousy as social rejection," Bales said.
Bales found both similarities and differences between primates and rodents. "Monogamy probably evolved multiple times so it is not surprising that its neurobiology differs between different species," she said. "However it seems as though there has been convergent evolution when it comes to the neurochemistry of pair bonding and jealousy."
Bales told IFLScience; “The project could be extended to humans,” but it is challenging to induce jealousy in people who know they are part of a research project. “We're probably a long way from a treatment for excessive jealousy,” she added, “but this could certainly be a first step.” Her team are working to identify receptors for the hormones oxytocin and vasopressin which, in combination with this work, could provide clues on how to control jealous emotions.