Infant primates when separated from their mothers show a pattern of protest, followed by resignation and depression, including the neurochemical transformations that typically occur in depression which are strikingly similar to the pattern

found in human beings. Early separation from the mother predisposes primates at older ages to a condition that appears to be analogous to human anaclitic depression.

Suomi conducted a series of remarkable experiments in which he bred Bonnet monkeys to be highly reactive to separation, and a contrasting group to be low reactives. He conceptualized these differences as reflecting stable underlying neurophysiological reactivity. These temperamental styles have been demonstrated to be highly stable over long periods of development in those animals that are at the extremes of the continuum (the upper and lower 20%). High reactives are sensitive even to those species specific behaviors of the mother, such as leaving to mate, and they remained reactive to separation over their life course.