Infants as young as eight months old like to see bad behaviour punished and don't like those who commit anti-social acts, according to new research that suggests that humans carry out complex social evaluations at a surprisingly early age.

A group of Canadian and U.S. scientists tested a variety of scenarios on 100 babies using hand puppets that looked like animals. The babies watched these puppets act either negatively or positively toward other characters. They then saw puppets either giving toys (rewarding) or taking toys (punishing) from the "good" and "bad" puppets.

A UBC-led study shows that babies as young as eight months old want to see bad puppets punished for anti-social behaviour. (UBC)

When the infants were asked to pick out their favourite characters, they preferred the puppets that had punished the "bad" puppets more than those that treated others well.

A similar finding was observed among 64 older infants, who were 21 months old. These infants were asked to either give a treat to a puppet or take one away. The puppets in this case had previously either helped another puppet or had harmed the other puppet.

The toddlers ended up rewarding the "good" puppets with treats and punished the "bad" puppets by taking treats away from them.

"This study helps to answer questions that have puzzled evolutionary psychologists for decades," said lead author Kiley Hamlin, from the Department of Psychology at the University of British Columbia.

"Namely, how have we survived as intensely social creatures if our sociability makes us vulnerable to being cheated and exploited? These findings suggest that, from as early as eight months, we are watching for people who might put us in danger and prefer to see anti-social behavior regulated."

Hamlin, who worked with colleagues at Yale University in Connecticut and Temple University in Philadelphia, says such responses likely have many learned components. But because the fact that they are present so early in life suggests infants may also have an inborn ability to like those who give "bad" people what they deserve.

The study is published in the latest issue of the Proceedings of the National Academy of Sciences.