A 3-year-old is handed six sets of colorful stickers.

'You can keep all of them,' he is told. 'Or you can give some to a child you don't know. He doesn't have any stickers. Do you want to keep all of your stickers? Or do you want to give some to a child you don't know?'

That was the basic script for a study that took place recently in an Israeli playroom which doubled as a social-science laboratory. A child-care-professional-turned-researcher asked 136 children, aged 3 and 4 years old, to step one at a time into the playroom to shed light unwittingly on a hot topic in behavioral science: Are children altruistic?

It seems they are, and part of the explanation may be genetic, according to the study, published last month in the online scientificjournal PLoS One. About two-thirds of the children chose to give one or more sets of stickers to an unknown recipient, described to them only as a child who had no stickers. There were no significant differences in generosity between boys and girls.

Among those who declined to share, many had something in common: a variation in a gene, known as AVPR1A, that regulates a hormone in the brain associated with social behaviors. Researchers found that this genetic variant was associated with a significantdecrease in willingness" target="_blank" title="n.情愿，乐意，自愿">willingness to share.

Until recently, only limitedresearch existed on altruism in children, and what it showed was younger children acting less generously" target="_blank" title="ad.慷慨地">generously. 'Younger children appear to weigh costs to the self more than do older children when deciding whether to assist others and are less attuned to the benefits,' says a professional guide called the Handbook of Child Psychology.

But young children all along have displayed greater levels of altruism than what most adults might expect. 'If parents think that generosity isn't possible at age 2, they won't try to encourage it,' says Nancy Eisenberg, an editor of the handbook and an Arizona State University psychology professor.

In another study published by PLoS One last week, scientists at the University of Washington said they found a sense of fairness and altruism in 15-month-old babies, tested in part on their willingness" target="_blank" title="n.情愿，乐意，自愿">willingness to share a favorite toy.

While genetics may play a part in people's willingness" target="_blank" title="n.情愿，乐意，自愿">willingness to share, environmental influences from home, school and the world at large seem to have a much greater role, social scientists say. That can be useful for parents since even children too young to talk seem able to absorb and imitate acts of empathy and generosity.

Positive reinforcement can help cultivategenerousbehaviors. Upon witnessing generosity in a child, a parent should resist any impulse to bestow a gift and instead offer praise of the child's character as well as his behavior. 'If you tell a kid he's a generous person, he's more likely to act generously" target="_blank" title="ad.慷慨地">generously again than if you buy him a toy,' Dr. Eisenberg says.

Generosity can be habit forming. Brain-imaging studies using MRIs have shown that being generous and being described as generous can engage the so-calledreward circuitry in the brain, prompting the release of dopamine-like neurotransmitters that are associated with positive feelings.

Capacity for what the Israeli study calls the warm glow that arises from giving of one's own property to others is believed to be in part a matter of genetics. The gene AVPR1A regulates a hormone associated with social behaviors, including affiliation and altruism. That gene functions differently from person to person, such that altruistic behavior may not induce that warm glow in everyone, the researchers suggest.

The researchers noted that generosity tends to increase as people mature and develop stronger notions of morality and fairness.

Of the 136 children invited to share some of their sets of stickers, two participants stood out for giving away their entire supply. Asked in a videotaped interview why he gave away all his stickers, one child responded: 'That's how you become happy,' according to Reut Avinun, a Hebrew University psychologist and lead author of the study.

Researchers say future investigations should follow subjects into adulthood to see if any association exists between generosity and success. The two children who gave away all their stickers, for instance, could represent future saints or future martyrs, says Christian Smith, director of the Science of Generosity Initiative at University of Notre Dame, which recently received a $5 million grant from the John Templeton Foundation to spur generosityresearch globally.