Because of their greater vulnerability to mortality, juveniles might be expected to be more conservative than adults in response to novel and potentially threatening situations. Results from research on vervet monkeys (Cercopithecus aethiops sabaeus) in the field and in captivity suggest that the opposite is true. Juveniles in the field were more likely to approach a human observer, and in captivity they had shorter latencies to enter a new area, approach a novel food container, and come within a meter of a strange adult male when compared to younger and older animals. In all four test conditions, risk-taking was an inverted U-shaped function of age. Latency to approach in unfamiliar and potentially dangerous situations declined from birth to two years of age, then increased with age to adulthood.