Gender differences in visual habit formation in 3-month-old rhesus monkeys.

The rate of learning concurrent visual discriminations with 24-hr intertrial intervals, a measure of habit formation, was assessed in infant monkeys of both sexes and compared with the rate of learning in adults. In two separate series of animals, 3-month-old male monkeys learned an initial set of discriminations (but not later sets) more slowly than 3-month-old females. This gender difference was absent in 6-month-old monkeys and in adults, both of which learned the initial set at the same rate. Determination of plasma gonadal hormone levels at the time of testing revealed a significant correlation (rs = 0.95) between testosterone levels and learning scores on the initial set in the 3-month-old male monkeys (i.e., the higher the level the poorer the score). No such correlation was found in the 3-month-old female monkeys or in the 6-month-olds of either sex. These results suggest that high testosterone levels found perinatally in male monkeys temporarily slows maturation of the neural system underlying visual habit formation.