When children witness "good" or "bad" behaviour, their brains show an immediate emotional response.
These neurodevelopmental findings are the first to link implicit moral evaluations to actual moral behaviour.
"Moral evaluation in preschool children, similar to adults, is complex and constructed from both emotion and cognition," said Jean Decety, professor of psychology and psychiatry at University of Chicago.
"However, we found that only differences in neural markers of the latter predict actual generosity," Decety noted.
As children grow up, they tend to show an increase in generosity.
To find out where such generosity comes from, the team monitored the electrical brain activity of kids, aged 3 to 5.
Children were given ten stickers and told that the "rewards were theirs to keep."
The children were told that the next child to come in would not be given any stickers and they were asked if they wanted to share any of theirs to this anonymous other child.
In case the kids were generous, they could place the stickers they were willing to give into a box.
The children on an average shared a little under two of their ten stickers.
The neural evidence indicated that more thoughtful reappraisal of helping and harming scenes predicted whether a child would share his or her stickers.
"The study may offer useful insight for parents this holiday season looking for their children to join in the spirit of giving," Decety suggested.
"These findings provide an interesting idea that by encouraging children to reflect upon the moral behaviour of others, we may be able to foster generosity," Decety concluded.
The study appeared in the journal Current Biology.
