Testing for broad cognitive ability, the researchers identified high, middle, and low performing monkeys, determined by a general intelligence score. General intelligence, or “g,” is a hallmark of human cognition, often described as similar to IQ. The effect of “g” in primates may offer insight into the evolution of human general intelligence.

The study, published in the journal PLoS One, is the first to examine differences of broad cognitive ability in primates within a single species. Previous studies of general intelligence in primates primarily concerned variation between species.

The research was led by Konika Banerjee, a research assistant in the Department of Psychology at Harvard University. Banerjee’s co-authors are Marc Hauser, professor of psychology, and James J. Lee all of Harvard, along with Christopher Chabris of Union College, Fritz Tsao of Hillsdale College, and Valen Johnson of the University of Texas Medical School at Houston.

“We found that there was substantial individual variation in performance on these tasks,” says Banerjee. “A significant proportion of that variation can actually be accounted for by something that looks very similar to the general intelligence, or ‘g’ factor, in humans. It appears to be the case that tamarins have something very similar to our general intelligence.”