What Really Causes Jealousy?

Ah, the green-eyed monster. It rears its head in interpersonal relationships of all ilk, from romance, to sibling rivalry, from keeping up with the Jones, to flyin’ elbows in the workplace. To better understand this most basic of human emotions, researchers at the University of California turned to some of our fuzzy brethren in the animal kingdom: male titi monkeys. In a study published in Frontiers in Ecology and Evolution, scientists created the first monogamous primate model for the neurobiology of jealousy.

The titi monkeys were used because, much like humans, they form a strong pair bond as adults—becoming attached to one another, guarding their mate and becoming upset if separated.

For the study, the researchers focused on male titi monkeys, who will physically hold their partner back from interacting with another male. The researchers purposefully made them feel jealous by putting them in view of their lady monkey, while she was in the company of another male monkey. (Cue the drama!) Brain scans were taken, and hormone levels were measured, while the male monkeys were feeling jealous.

The researchers found that the parts of the brain that had heightened activity was the cingulate cortex and lateral septum. The lateral septum is associated in formation of pair bonds in primates, while the cingulate cortex is associated with feelings of social rejection. When it came to the hormones, there were also changes. Jealous males showed elevated levels of testosterone, associated with aggression and competition, as well as cortisol, a marker of social stress.

The researchers also used a rodent-based model, which previous researchers have used to study social monogamy. Together with that, this titi monkey data suggests that the strong bond between pairs is formed in the brain parts associated with social memory and reward, while holding on to the bold is associated with avoiding the negative emotions of loss.

“Monogamy probably evolved multiple times so it is not surprising that its neurobiology differs between different species,” wrote Karen Bales, Ph.D., the study lead and unit leader of the Brain, Mind and Behavior Unit at the California National Primate Research Center. “However, it seems as though there has been convergent evolution when it comes to the neurochemistry of pair bonding and jealousy.”

Whether monkey, rodent or human, jealousy is a strong and powerful emotion, deeply felt. Luckily we have better tools to deal with it, perhaps, than the average lab animal.