Scientists identify a brain region that differs between males and females and may influence social behavior

Scientists have uncovered a previously unknown cluster of brain cells that may help explain differences in social behavior between males and females. The small neural circuit appears to function like an on–off switch, showing a striking pattern of activity that differs sharply by sex, an unusually clear contrast compared with most known brain sex differences, which tend to be more subtle and overlapping. Researchers also found that the circuit’s activity shifts with social and reproductive status, suggesting the brain may use this mechanism to adapt behavior across key life stages. 

A new study led by Dr. Tamar Licht and Dr. Dan Rokni from the Institute of medical research Israel-Canada (IMRIC)  at the Hebrew University of Jerusalem, published in the Proceedings of the National Academy of Sciences (PNAS), has identified a small, previously unknown cluster of brain cells that behaves like a biological switch, showing a clear difference between females and males and changing activity in response to social and reproductive status. The discovery offers a rare example of a brain sex difference that appears unusually distinct and may provide insight into how the brain adapts social behavior across major life transitions. 

The newly identified cluster is located in the medial amygdala, a region of the brain long known to play a central role in social behavior, emotional processing, and reproduction. What makes this cluster especially striking is its binary pattern of activity. In females, the cluster is consistently active under baseline conditions. In males, this brain region is typically inactive and becomes active only after major changes in social or reproductive state. Before sexual maturation, the region shows similar levels of activity in both males and females; however, after maturation, its activity disappears in males only. The region becomes active again when the male engages in sexual contact, suggesting that social and reproductive context plays a key role in regulating this brain area.

Most known sex differences in the brain involve gradual changes across multiple regions and typically show significant overlap between males and females. In contrast, this newly identified neural cluster appears to follow a much sharper “on/off” rule. The researchers also found that the cluster’s activity does not appear to depend on circulating sex hormones alone.

The study further highlights how sensitive the brain can be to life-stage changes. In males, the cluster’s activity shifted following reproductive experience, suggesting that the brain can “flip” neural states in ways that may support behavioral adaptation. The team also found that the cluster can be influenced by prolactin, a hormone known to shape social and parental behavior, and is known to be secreted following sexual contact, pointing to a possible mechanism that connects hormonal signals with long-term changes in brain activity and behavior. 

“Most sex differences in the brain are subtle and distributed,” said Dr. Licht. “What surprised us here was the clarity of the signal. This is a discrete group of neurons that behaves almost like a biological switch, reflecting sex and social state in a very robust way.” 

The findings may offer a new framework for understanding how the brain might organize sex-related differences in social behavior and how experiences such as reproductive transitions or shifts in social environment can rewire neural circuits. By identifying a clearly defined group of cells that integrates sex, experience, and hormonal responsiveness, the study opens new possibilities for investigating broader questions about parenting, bonding, emotional regulation, and why certain neuropsychiatric conditions may affect males and females differently.