Whether you’re a social butterfly or a lone wolf, the brain circuits that define social behaviors begin forming early in life and mature over a lifetime. But how the social brain develops has remained unclear, and new research explores oxytocin – often referred to as the “love hormone” – for answers.

Oxytocin earns its loving nickname because the brain releases the hormone during moments of social bonding, such as those between a parent and child or romantic partners. But beyond this role, oxytocin has long been thought to play a more direct role in social circuit development, and a recent study published in the Journal of Neuroscience put this idea to the test with zebrafish.

Zebrafish are social creatures with evolutionarily similar brain circuitry to humans. Scientists can genetically alter them before observing their behavior across an entire lifespan, making them ideal for studying social behavior. So to understand the role of oxytocin-producing neurons in social brain development, researchers selectively removed those neurons from their brain circuits early in life and examined the consequences to social behavior once the zebrafish reached adulthood.

The researchers evaluated the zebrafish behavior by first separating a fish from a larger group with a transparent barrier, then observing how the lone fish reacts to its isolation. Like a person with FOMO ("fear of missing out") from a party next door, socially healthy zebrafish stay close to the transparent barrier – seemingly longing to join the group on the other side. However, zebrafish with a disrupted social circuit explore their own tank with no preference to socialize.

Researchers found that zebrafish with their oxytocin neurons removed early in life showed less preference to socialize as adults. However, eliminating these cells in adulthood did not affect social behavior, suggesting that oxytocin shapes the social circuit early in life during a critical developmental window. They also found that removing oxytocin neurons early impaired other social brain components, including those required for attention, decision making, and reward.

Together, this suggests that the famous "love hormone" may define our long-term social preferences early in life. But unlike a Pixar movie, fish are not humans, and there is still more to learn about social brain development.