New research from the University of California, Berkeley suggests the hormone oxytocin helps speed up the early stages of friendship formation, sharpening the sense of preferring a familiar peer over a stranger. The work, published in Current Biology, adds nuance to oxytocin’s popular image by focusing on how quickly and selectively social bonds take shape.
Oxytocin is released during a range of social and bodily experiences, including touch, sex, childbirth and breastfeeding, and it acts in the brain as a neuromodulator. While it is often linked with closeness and trust, scientists have also associated oxytocin signaling with social defensiveness, including stronger in-group and out-group behavior.
The team studied prairie voles, a species widely used to examine social bonding because individuals form stable, selective relationships. Instead of focusing only on mating pairs, the researchers emphasized peer bonds that resemble human friendships, such as choosing to huddle and groom with one familiar partner rather than spending time with strangers.
What changed without oxytocin receptors
Using prairie voles engineered to lack oxytocin receptors, the researchers found the animals were slower to form a peer preference. In tests where typical voles show a strong preference after about 24 hours, the receptor-deficient animals often needed up to a week to reliably choose a familiar partner.
The difference was not simply that the animals became less social overall. The findings point to reduced selectivity, meaning the altered animals were less consistent about who they sought out and were quicker to lose track of established partners when placed into new group settings.
Friendship selectivity, not just sociability
In a mixed-group, multi-room setup designed to mimic a party-like environment, typical voles spent early time near known companions before gradually mingling. Voles without oxytocin receptors mixed more freely from the start, behaving as if prior peer connections carried less weight.
In another test measuring social motivation, female voles usually worked harder to access a familiar peer than a stranger. The receptor-deficient animals still showed motivation for a mate, but not for a friend, indicating that oxytocin signaling may matter more for the reward value of peer bonds than for mating bonds.
A new look with oxytocin nanosensors
To examine whether the brain compensated for missing receptors by releasing more oxytocin, researchers used an oxytocin nanosensor that fluoresces when it detects the molecule. Measurements indicated no excess oxytocin release and, instead, lower release from fewer sites in the nucleus accumbens, a region central to social reward.
The results help explain why friendships formed more slowly and were less stable in challenging social conditions. Researchers say the work could inform future studies of psychiatric conditions where social bonding is disrupted, while underscoring that oxytocin’s role is complex and context-dependent.
The study also fits into a growing body of vole research suggesting oxytocin is not strictly required for bonds to exist, but can strongly affect how efficiently they form. By separating friendship-like bonds from mating behavior, the authors argue that the biology of peer relationships deserves attention in its own right.
