Predictability and coping with separation in infant squirrel monkeys.

Twelve infant squirrel monkeys were separated from their mothers once every 48 hr for a total of 20 separations. Half of the infants experienced separations for a fixed interval (FI) of 30 min. The remaining 6 infants experienced separations for a variable interval (VI) which averaged 30 min. During the separation periods, movement and vocalizations were recorded. Blood was sampled for later cortisol assay after the 1st, 5th, 10th, 15th, and final separation. The FI group showed increased levels of movement and vocalization over repeated separations when compared with the VI group. Cortisol showed significant increases over basal levels and remained elevated throughout the entire period in both groups. These findings suggest that predictability may be more stressful than unpredictability when organisms cannot control the predicted event.