The hormone vasopressin, which can be released during sexual intercourse, has a 100% correlation with pair-bonding in prairie voles. The gene that codes for vasopressin release during sex completely determines whether or not voles will form permanent pair-bonds. There is also research demonstrating a link between vasopressin and pair-bonding in humans, though of course the link is much less strong. Oyxtocin is also believed to play a role in human pair-bonding, and it is likewise released during sex.