Humans and monkeys show similar skill in estimating uncertain outcomes.

When information is incomplete but a choice must be made, individuals sometimes can rely on past experiences to help them assess uncertain outcomes in terms of the probabilities of payoffs. Monkeys (Cebus apella) and humans (Homo sapiens) were presented with a test in which they first made quantity judgments between two clear options. Then they made choices in which only one option was visible, and they had to estimate the quantity for the other option. Both species were guided by the past outcomes, as they shifted from selecting the known option to selecting the unknown option at the point at which the known option went from being greater than the average rate of return to being less than the average rate of return. This comparability across species suggests that tallying ongoing average rates of return from repeated choices occurs spontaneously and likely serves an adaptive purpose when dealing with uncertainty in the environment.