When we feel a profound sense of trust in another person, our brain is performing a complex chemical calculation. At the heart of this calculation is oxytocin, a hormone synonymous with social bonding. However, the story of human connection and its darker counterpart, social aggression, is a tale of two hormones: oxytocin and vasopressin.

The foundation of this understanding comes from the work of Dr. C. Sue Carter, Director of the Kinsey Institute and a pioneer in prairie vole research. Prairie voles are rare among mammals for their lifelong pair-bonding. Dr. Carter discovered that upon nesting and mating, these voles experience a surge in oxytocin.

The vole blueprint: Bonding and defense

However, a second chemical actor soon enters the stage: vasopressin. While oxytocin facilitates the bond, vasopressin drives the protective response. In male voles, elevated vasopressin levels lead to mate guarding, a state of heightened aggression toward any vole outside the pair bond. Research published in Animal Behaviour in October 2015 confirmed this link, showing that when vasopressin activity is inhibited, previously aggressive males revert to calm, social behavior. To understand how these hormones shape adult personality, we must look at human adolescence.

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