Oxytocin: The “Love Hormone” Is Also the “Friendship Hormone”

Current Biology showed that prairie voles need oxytocin receptors (OXTR) to form selective friendships — a strong preference for a specific “friend” and less tolerance for strangers. Females with the Oxtr gene knocked out formed friendships more slowly, bonds lasted less well in “community” conditions, and the “reward” from communicating with a close relative was weaker. At the same time, knockout males and females showed reduced evoked oxytocin secretion in the nucleus accumbens, a node of the reward system. Conclusion: OXTR is not about “sociality in general,” but about selectivity — that same “us/them” on which human friendships are based.

Background of the study

• Why oxytocin at all? It is often called the “love hormone,” but more accurately, it is a neuropeptide that helps the brain mark socially significant contacts and “highlight” them as rewarding. It works through oxytocin receptors (OXTR) in reward system nodes (for example, in the nucleus accumbens).
• Why voles? Meadow voles are rare "social monogamists" among mammals: they form both long-term pairs and selective friendships with peers. So this is a convenient model for sorting out not "sociality in general" but selectivity - the ability to choose "our own" and keep a distance from strangers.
• What was already known. Many studies have linked the oxytocin system to attachment and reward. However, in 2023, a resonant article was published: voles with Oxtr turned off could still show signs of pair bonding. This raised the question: maybe OXTR is more important not for the pair as such, but for fine-tuning selectivity in different types of relationships?
• What gap is being closed. We had a poor understanding of the role of OXTR specifically in peer friendships: does it affect the speed of friendship formation, its strength in a “social mess” (group/dormitory) and the reward from communicating specifically with “your” companion, and not with any relative.
• How new technologies have helped. Optical oxytocin nanosensors (near infrared range) have appeared - they allow us to see in real time how oxytocin is released in key areas of the brain. This gives a chance to link behavior (choosing a friend) with neurochemistry right in the reward system.
• Why do people need this? Friendship is also about selectivity: who to let in closer, and where to keep the boundaries. Understanding the axis "oxytocin → OXTR → reward" in the model helps to put forward accurate hypotheses about the mechanisms of social selectivity in humans - from the norm to disorders where it is disrupted. This is not a recipe for "treating yourself with oxytocin", but a map of nodes that are worth studying.

What was done and why is it important?

People value not just social interactions, but selective relationships—friendships. Meadow voles are rare among mammals: they form long-term pairs and strong bonds with peers, making them a great species for “disassembling” the biology of attachment. The UC Berkeley team, together with colleagues from UCSF, bred voles with a CRISPR knockout of Oxtr and compared their behavior with “wild” animals: how quickly a preference for a specific cage partner emerges, how stable it is in a multi-chamber group environment, how much effort the rodent is willing to put in to gain access to “its own,” and how it reacts to strangers.

Key Results

• Slower to “grasp” friendship. Oxtr−/− females were significantly delayed in forming selective friendship attachments compared to controls.
• The connection is less strong. In a "dormitory" of several cells, the choice of "one of their own" slipped away from the knockouts faster - selectivity was lost.
• Reward from communication is lower. Oxtr−/− had defects in both general social reward and selective reward (towards a familiar companion vs. an unfamiliar one). That is, it is less pleasant and less meaningful for them to be “friends”.
• The biochemistry of friendship is weakened. Using optical oxytocin sensors, it was shown that evoked oxytocin release in the nucleus accumbens is reduced in males and females with knockout; there is no compensation from above.
• Less "guarding" reaction to strangers. A popular paper from UC Berkeley emphasizes that animals without OXTR took longer to form friendships and were less aggressive toward strangers — meaning they were worse at maintaining friendship "boundaries" (and this is also part of selectivity).

How does this relate to the "oxytocin controversy"

In 2023, a high-profile paper in Neuron showed that voles can form a pair bond without OXTR — at least in terms of “marital” attachment. The new paper clarifies the picture: OXTR is critical for friendship with peers — the receptors are not about “sociality in general,” but about fine-tuning selectivity (“friends — we love them, strangers — keep your distance”). So there is no contradiction, but there is a division of functions by types of relationships.

Tools: How did they measure the “chemistry of friendship”?

To see oxytocin in action, the team used near-infrared (CNT) nanosensors developed in Marquita Landry’s lab that fluoresce when exposed to oxytocin molecules, allowing real-time recording of peptide release in brain slices and mini-preps. This technology provided a rare opportunity to peer into neuropeptide dynamics directly in the target node (NAc) and link them to behavior.

What do people have to do with it?

The study is about rodents, but the mechanism is recognizable: friendship requires not only an attraction to people, but also a choice - whom to let closer, whom to keep at a distance. The data suggest that the axis oxytocin → OXTR receptor → reward system helps to "highlight" social favorites and "mute" strangers. For humans, this does not mean "treating ourselves with oxytocin", but there are points for careful hypotheses about the selectivity of relationships in social behavior disorders.

Limitations and what's next

• The model is rodent and the focus is peer friendship, not kinship or a romantic couple: transferring to people requires great caution.
• Knockout is a harsh intervention; in humans, variations are often more subtle (polymorphisms, expression, context).
• The next steps the authors and commentators suggest are to test male cohorts in the same tests, decompose the contribution of dopamine and other neuromodulators to “social reward,” and see how experience changes the sensitivity of the network (social learning).

Authors' comments

This is what the authors themselves emphasize:

• Not a "love hormone," but a selectivity mechanism. Oxytocin receptors (OXTR) turned out to be important not for sociability in general, but for choosing "one's own": without OXTR, voles are slower to form preferences and worse at maintaining friendly attachment in a social environment.
• How to Reconcile the Disputes about Pair Bonding. The work does not cancel out the data that some aspects of pair bonding are possible without OXTR. The authors clarify: peer friendship is a different format of relationships, and it is here that the contribution of OXTR is critical.
• Reward and motivation are key. Weak “selective reward” in OXTR knockouts suggests that the oxytocin system helps highlight socially significant contacts within the reward circuit (including the nucleus accumbens).
• Methodological advantages. In addition to behavioral tests, direct optical sensors of oxytocin were used, which links behavior with the real dynamics of the peptide in the target zone - not only correlations by bypass markers.
• Limitations. This is a rodent model and a complete knockout of the receptor; in humans, subtle variations in expression and context are more common. Extrapolation of findings to the clinic requires extreme caution.
• What's next? To understand which specific chains (oxytocin → dopamine, etc.) provide selectivity, how experience influences them, and whether it is possible to gently modulate these nodes without breaking social behavior as a whole.
• No “quick fixes.” The results are not a reason to “take oxytocin.” Rather, they are a node map for future research on social selectivity in disorders where friendships/partnerships are disrupted.

Conclusion

Oxytocin receptors turn out to be “selectivity switches”: without them, voles are drawn to communication, but lose the ability to choose and keep “their own.” And this is no longer about “love in general,” but about the architecture of friendship, where neurochemistry helps distinguish loved ones from others.

Research source: Black AM et al. Oxytocin receptors mediate social selectivity in prairie vole peer relationships. Current Biology, online ahead of print, August 4, 2025.