For more than three decades, scientists have worked to understand the link between eye gaze and cognition in babies. They have found that children begin following where people—typically moms—look during their first year of life, and the development of this function can signify how other skills, such as language and empathy, will progress.

A new study from Penn, Harvard, and Yale shows that a species of monkey called rhesus macaques exhibits developmental patterns around this skill similar to the way gaze following changes with age in humans.

The research is one of the first such analyses of the entire lifespan of these non-human primates in a natural habitat. Michael Platt, a Penn Integrates Knowledge professor with appointments in the Perelman School of Medicine, the School of Arts & Sciences, and the Wharton School, and his colleagues published their findings in Proceedings of the Royal Society B. Funding was provided by the National Institute of Mental Health.

“Social attention and gaze following are some of the best predictors of whether a child will later be diagnosed with a social disorder like autism,” Platt says. “The developmental unfolding of these behaviors is something that’s pretty basic to complex social primates like people and rhesus macaques. It’s a common shared response to living in complex social environments.”

Platt has studied these monkeys for more than 20 years in the lab, and for eight years on Cayo Santiago, an island off the coast of Puerto Rico. For his most recent work, the team observed nearly 500 monkeys on Cayo, ranging from 2 weeks to 28 years old. When a pair of researchers would encounter a lone monkey, one experimenter—called the actor—would sit five feet from the animal while the other would record the interaction.

Once the actor got the monkey’s attention by clapping or speaking loudly, she turned her head upward and said the word “now” to start a 10-second window. Another experimenter filmed the monkey’s face to catch whether the animal also looked up, and for how long and how frequently.

“Where possible, we would repeat this on the same animal,” Platt explains, “as a way of assessing whether the monkeys learned that there was nothing really important that the experimenter was looking at.”

In a follow-up study of 80 macaques, the actor alternated between looking up and down to determine whether this would change how often the monkeys followed the researcher’s lead. (It didn’t.)  

Rhesus macaques are very similar to humans when it comes to gaze, the researchers conclude. This behavior starts around 5 months and levels off around the equivalent to human teenage years. There are also gender differences—something previous research has suggested is true in humans, as well.

Next, the researchers plan to analyze data they collected on older monkeys, particularly those less likely to follow gaze.

“We know people who maintain stronger ties—who are more socially engaged—tend to be happier, live longer. They tend to be healthier,” Platt says. “Here we see this in a monkey, which suggests it might be part of the overall life history pattern built into all social primates. We still don’t understand why.”