Oxytocin has a hidden influence on who we call friends

For years, oxytocin has been tagged as the “cuddle hormone” – a brain chemical linked with romance, trust, and human connection. It’s known to be released during sex, childbirth, breastfeeding, and moments of warmth between people.

You might have even heard advice about boosting oxytocin with hugs, music, or exercise. But the picture is more complicated.

Recent research suggests oxytocin isn’t just about love and affection – and in some cases, it’s not even necessary for romantic bonding. The latest twist? Scientists have found that it plays a key role in forming friendships.

Oxytocin’s role in forming bonds

Researchers at the University of California, Berkeley, working with collaborators at UC San Francisco, turned to an unlikely model for friendship: the prairie vole.

These small, furry creatures are famous for forming long-term, selective relationships, both with mates and with peers. That makes them a useful stand-in for exploring the biology of human social life.

“Prairie voles are special because they allow us to get at the neurobiology of friendship and how it’s similar to and different from other types of relationships,” said Annaliese Beery, a UC Berkeley associate professor of integrative biology and neuroscience and senior author of the study.

The team focused on what happens when voles are missing oxytocin receptors – the brain’s landing pads for the hormone. Without them, the animals could still form friendships, but the process took much longer.

“Oxytocin seems to be particularly important in the early formation phase of relationships and especially in the selectivity of those relationships: ‘I prefer you to this stranger,’ for example,” Beery said. “The animals that didn’t have intact oxytocin signaling took longer to form relationships.

Missing hormone slows connection

In normal circumstances, prairie voles show their friendships by huddling, grooming, and even sitting on each other. But for those without receptors for the hormone oxytocin, the bonding delay was obvious.

The receptor-lacking voles also seemed less motivated to be with their friends, and less likely to avoid or act aggressively toward strangers.

“In other words, oxytocin is playing a crucial role not so much in how social they are, but more in who they are social with, their selectivity,” Beery explained.

The change ran deeper than behavior. Using a nanosensor developed in the UC Berkeley lab of chemical and biomolecular engineering professor Markita Landry, the team found that missing receptors altered the way oxytocin was released in the brain.

“That helped us understand the feedback consequences of lacking this receptor, and how oxytocin signaling was altered in the brain,” Beery said.

Friendship before romance?

Beery has long wondered whether friendships – or “selective peer relationships” in scientific terms – might have evolved before monogamous mating in animals like voles.

Her lab’s field studies have shown that many vole species naturally form these peer bonds, even when they aren’t monogamous.

“While most rodents prefer to interact with unfamiliar individuals, it turns out that the majority of vole species we’ve tested form peer-partner preferences, which is what we call these selective friendships,” Beery said. “So there seems to be this widespread tendency to bond.”

“But only a couple of those species are also monogamous. Someday, I hope to be able to tell you, ‘Do selective peer relationships precede the development of monogamy? Is that why monogamy has evolved so many times in this genus?’ I think this familiarity preference is deeply rooted.”

Friendships formed in a day

To explore how signaling from the hormone oxytocin shapes friendship, the researchers ran three main experiments.

In the first, known as the bond speed test, normal voles typically formed a clear partner preference within 24 hours of co-housing. In contrast, oxytocin receptor-deficient voles showed no such preference until about a week later.

“Wild-type animals form this incredibly robust preference within one day of co-housing, but the null mutants have no sign of a relationship after 24 hours,” Beery said. “After a week, they mostly get there, and the lifetime partners look no different from each other.”

“Our conclusion from that experiment is that oxytocin isn’t required to have a relationship, but it’s really important in those early phases of a relationship to facilitate it happening quickly and efficiently.”

Social loyalty under pressure

The second experiment, the “party test,” placed the voles in a large enclosure with multiple rooms and other animals.

Normal voles tended to stick with their known partner before mingling, while the receptor-lacking voles didn’t show that same loyalty.

“They can all separate, they can all come together, or they can hang out in any combinations that they want,” Beery said. “The wild-type animals keep track of who they know. It’s like if I went to a party with a friend, I would stand near that friend for the first part of the party and then I might start to mingle.

“The voles that lack oxytocin receptors just mixed. It was as if they didn’t even have a partner in there with them.”

Testing the value of friends

The third experiment, called the friendship rewards test, measured how hard the voles would work to reach a friend or mate.

In a lever-pressing setup, normal females pressed more to access a friend or mate than to reach a stranger. The receptor-deficient voles still preferred their mates but showed no extra effort for friends.

“Female wild-type voles typically press more to get their partner than to get a stranger, in both peer and mate relationships,” Beery said. “The oxytocin receptor deficient mutants also press more to get to their mating partner, but not for peer relationships.”

“That makes sense at some level because we think mate relationships are more rewarding than peer relationships, or at least they depend more on reward-signaling pathways.”

Bonds and boundaries in friendships

One surprising twist: the receptor-lacking voles’ reduced selectivity made them friendlier to strangers.

“You can see contributions of oxytocin signaling to both sides of selectivity,” Beery said. “On the prosocial side, it’s involved in wanting to be with a known friend or peer, while on the antisocial side, it’s aiding in rejecting an unfamiliar animal.”

The researchers observed that oxytocin influenced both affiliation and aggression in prairie voles, mirroring human findings on its role in in-group/out-group dynamics.

Oxytocin’s friendship connection

While this research focuses on voles, it raises big questions for human relationships. If oxytocin shapes not just our desire to bond, but who we choose to bond with, it could offer clues for conditions like autism or schizophrenia.

In these conditions, social connections can be harder to build and maintain.

For now, the work shows that oxytocin might deserve a new nickname. Forget the “love hormone.” Maybe it’s the “friendship hormone” we should be talking about.

The full study was published in the journal Current Biology.

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