Recent studies suggest that mother-infant bonding and addiction share some of the same brain circuitry, an area of the brain called the opioid system. The opioid system is involved in pleasure, pain, and addiction. Drugs like morphine work to suppress physical pain by acting on this system.

However, the observation that morphine can decrease social behaviors as well prompted scientists to investigate the connection between the opioid system and social relationships. They compared normal baby mice to baby mice lacking a crucial opioid receptor.

What they found was that while normal baby mice who have been separated from their mothers make distressed noises, baby mice that lack the crucial opioid receptor exhibit significantly less distress when mom's away.

Similarly, given the choice between the nest built by their mother and the nest of another mother, normal baby mice always chose the familiar nest while only a third of the receptor-deficient mice chose mom's nest.

The receptor-deficient mice don't lack in the smelling department. Nor do they cry any less when exposed to dangers such as the presence of a threatening adult male mouse or cold temperatures. It seems that the opioid system is indeed connected to social bonding.