Oops, the scientist dropped his clothespin. Not to worry – a wobbly toddler raced to help, eagerly handing it back. The simple experiment shows the capacity for altruism emerges as early as 18 months of age.

Toddlers’ endearing desire to help actually signals fairly sophisticated brain development, and is a trait of interest to anthropologists trying to tease out the evolutionary roots of altruism and cooperation.

Psychology researcher Felix Warneken performed a series of ordinary tasks in front of toddlers, such as hanging towels with clothespins or stacking books. Sometimes he “struggled” with the tasks; sometimes he deliberately messed up.

Over and over, whether Warneken dropped clothespins or knocked over his books, each of 24 toddlers offered help within seconds – but only if he appeared to need it. Video shows how one overall-clad baby glanced between Warneken’s face and the dropped clothespin before quickly crawling over, grabbing the object, pushing up to his feet and eagerly handing back the pin.

Warneken never asked for the help and didn’t even say “thank you,” so as not to taint the research by training youngsters to expect praise if they helped. After all, altruism means helping with no expectation of anything in return.

And – this is key – the toddlers didn’t bother to offer help when he deliberately pulled a book off the stack or threw a pin to the floor, Warneken, of Germany’s Max Planck Institute of Evolutionary Anthropology, reports Thursday in the journal Science.

To be altruistic, babies must have the cognitive ability to understand other people’s goals plus possess what Warneken calls “pro-social motivation,” a desire to be part of their community.

“When those two things come together – they obviously do so at 18 months of age and maybe earlier – they are able to help,” Warneken explained.