Vol. 10, No. 1, 2004 Page 1


What makes some children far more aggressive than others? A new study suggests that genetic makeup may be far more important than upbringing.

Rhesus macaques are a "female-bonded" species, in which related females from multiple generations live together. Noting that the aggressive behaviors of young female monkeys are very similar to those of their mothers in these social groups, Dario Maestripieri designed an experiment to determine how much of this similarity stemmed from genetic influences, and how much was a result of exposure to maternal aggression after birth. He selected 10 female infants which he switched immediately after birth to unrelated mothers, who then raised them. There was no physical or visual contact between the infants and their original parents or social groups.

"I was surprised by what we found," Maestripieri says. Observing the young monkeys for three years, he detected no significant associations between the aggressiveness of the babies and the aggression levels of their adoptive mothers. On the contrary, the offspring closely resembled their biological mothers in their rates of both aggression and social contact—particularly during the last two years of the investigation. For instance, monkeys who often used threats and slaps had biological mothers who also exhibited such tendencies.

"It is noteworthy that behavioral similarities between offspring and their biological mothers occurred despite differences in age and reproductive conditions," Maestripieri says. "These similarities may have been even more marked if the offspring had been observed as adults, and if their mothers had been observed in a period without dependent infants."

Interestingly, the offspring were not similar to their biological mothers in measurements of grooming and submissive behavior. Says Maestripieri, "This raises the possibility that these behaviors are more flexible and dependent on individual or social learning than contact and aggression."

Commenting on the study, primate expert Joan Silk says, "This study adds to a growing body of evidence that temperament and behavioral predispositions vary among individuals and that temperamental differences are stable over the life course…. These findings have important implications for understanding how evolution shapes behavior and temperament in primates and humans."


"Similarities in affiliation and aggression between cross- fostered rhesus macaque females and their biological mothers," Dario Maestripieri, Developmental Psychobiology, Vol. 43, No. 4, November 2003, 321-327. Address: Dario Maestripieri, Animal Behavior Research Group, University of Chicago, 5730 S. Woodlawn Avenue, Chicago, IL 60637, dario@uchicago.edu.

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"Social behavior among monkeys may be more nature than nurture," news release, University of Chicago Medical Center, December 3, 2003.

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