The 'Love Hormone' Promotes Aggression Too

Everything you feel–whether overwhelmed with love, brimming with aggression, hunger, stress, or a little frisky–can be traced back to hormones. Among these powerful little chemicals is one that you've probably heard of: oxytocin. Better known by its sexy nickname, the “love hormone” is famous for its role in caring and nurturing behavior.

But there's a plot twist in the story of oxytocin. The same hormone that makes us warm and fuzzy can also stoke the fires of aggression under the right circumstances. Vasopressin, a chemical cousin of oxytocin, works with the love hormone to produce a different type of care behavior: defensive aggression.

Beware My Care

This synergy of oxytocin and vasopressin creates a “tend-and-defend” mechanism known as the “mama bear effect.” A mother’s love for her cubs is so strong that it is a go-to metaphor to describe the fierce protectiveness human mothers feel for their offspring. It’s no coincidence that we see parallels between these fierce mammals and ourselves. Love and defensive aggression are mediated by the same chemicals in all mammals: oxytocin and vasopressin. (Offensive aggression, especially competing for status, resources, or mates, is primarily governed by testosterone.)

Oxytocin and Care

Oxytocin evolved primarily to ensure offspring survive and thrive. Across all mammals, oxytocin is involved in the basic physiological processes of childbirth and lactation. The psychology of love came later. Evolution co-opted the same hormone to send signals to make these processes possible and make them feel good. (Childbirth certainly does not feel good, but research has shown that oxytocin has analgesic effects during intense stress.) The warm fuzzy feeling of love experienced during cuddling originated as a mechanism to promote mother-infant bonding during breastfeeding.

Surviving and Thriving

In bears, the mother does all of the heavy lifting. The father plays no role except in mating and will sometimes even kill his own cubs if left to stick around. No wonder mama bears have evolved such ferocity. These isolated family units–a lone mother and her cubs–are certainly stable enough for survival. Bears have been around for millions of years. But in social species like humans, strong social bonds are necessary to ensure offspring not only survive but thrive. (“It takes a village…”)

In humans, the love hormone that originally evolved to promote mothering was co-opted for yet another purpose: social bonding. Skin-to-skin contact has the same bonding effect between fathers and their infants, between romantic partners, and, really, between any mammals. Research has shown that oxytocin levels rise when cuddling or making eye contact in humans and dogs.

There is clear adaptiveness in evolving loving bonds between romantic partners, family members, and social communities. But to evolve love and care for one’s ingroup, one must be on guard for potential threats. This is where vasopressin, defensive aggression, and outgroup biases come in.

Ingroups and Outgroups

Countless human and animal studies have shown oxytocin’s role in fostering care behavior, social synchrony, empathy, trust, and cooperation. Paradoxically, a few studies have found that oxytocin increases aggression and the desire to punish. Two key insights are necessary to explain these puzzling findings.

The first insight is that ingroup care and outgroup aggression are two sides of the same coin. As soon as you have people you care about (your “ingroup”), anything not part of that inner circle of care becomes a potential threat. And it is in exactly this context of outgroup threat that oxytocin has been shown to facilitate aggression. For example, in economic games, participants administered oxytocin show more cooperative behavior with teammates but more aggressive behavior against rivals. And while oxytocin facilitates ingroup trust, it also increases the desire to punish individuals who violate trust. This aggression against rivals and traitors is a form of ingroup care, consistent with oxytocin’s role as the love hormone.

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The second insight is that it’s most likely not oxytocin directly producing this defensive aggression. Vasopressin, oxytocin’s chemical cousin, is produced in the brain when oxytocin is administered. And animal research has shown that vasopressin is associated with increased anxiety and vigilance, territoriality, and defensive aggression. Low doses of oxytocin increase care behavior as expected, but high doses of oxytocin have effects more similar to vasopressin, increasing threat sensitivity. This is surely an adaptive counterbalancing mechanism–if the love hormone took over completely, we would have a harmless teddy bear, not a fierce mama bear.

The Crucial Balance

This balance is crucial for the survival and success of social species like humans. Too much ingroup care without enough outgroup aggression could leave a group vulnerable to external dangers. Conversely, excessive outgroup aggression without adequate ingroup care could result in a fractured and dysfunctional social group.

The next time you think about the love hormone oxytocin, remember that there are forms of care that, on the surface, do not seem so loving and that this defensive aggression is made possible due to vasopressin, the sister hormone that oxytocin could not live without.