In each of six experiments, two people stood in front of a platform, each with a grape attached to the platform. The experiments created competitive conditions, the monkeys were competing with the humans for the grapes.
The 115 monkeys tested only took the grape when they could tell that a person was not looking at them. The experiments varied from one person looking at the monkey and the other with their back turned, to one person having their eyes covered with cardboard and the other with only their mouth covered with cardboard.
"What we are interested in is whether monkeys can reason about the mental states of others," said Jonathan Flombaum, a graduate student in Yale's Department of Psychology and the first author of the study. "Seeing is the most elementary piece of this. The simplest mental state is perception, what you see."
The experiments were conducted with semi-free-ranging rhesus monkeys on the island of Cayo Santiago in Puerto Rico.
Flombaum said the research could help shed light on the causes of autism. "There are many studies showing that autistic children don't look at the eyes of others in the ways that other individuals do in the course of social interactions, and also that autistic children don't use eye gaze information to make social and mental judgments about others," he said.
The senior author was Laurie Santos, an assistant professor at Yale. The research was supported by a National Science Foundation graduate research fellowship and the Yale University Moore Fund grant.
Current Biology 15: 447-452 (March 8, 2005)