It is often thought of as one of the qualities which distinguishes humans from animals.
And a new study has shown that the ability to tell the difference between right and wrong is a skill which even babies can possess.
Infants who show a good understanding of what is fair and unfair are also more like to share their possessions with others.
Babies in the study were able to differentiate between the equal and unequal distribution of food, showing an early awareness of fairness, scientists said.
There was also seen to be a link between how sensitive the babies were to fair behaviour and whether they would be willing to share a favourite toy.
Jessica Sommerville, associate professor of psychology at the University of Washington, said: ‘Our findings show that these norms of fairness and altruism are more rapidly acquired than we thought.
‘These results also show a connection between fairness and altruism in infants, such that babies who were more sensitive to the fair distribution of food were also more likely to share their preferred toy.’
The research, published today in the journal PLoS ONE, involved showing two short videos to 15-month-old babies.
In the first, a bowl of crackers was distributed between two people – first with an equal allocation of crackers, and then with one person getting more crackers than the other. The second video showed a jug of milk being shared between two people in a similar way.
Scientists measured how long each of the babies looked at how the food had been distributed, as babies pay more attention when they are surprised.
They discovered that babies spent more time looking at the allocation of food if one person got more than the other.
Dr Sommerville said: ‘The infants expected an equal and fair distribution of food and they were surprised to see one person given more crackers or milk than the other.’
Previous studies had shown that two-year-old children can help others, which is considered a sign of altruism, and that those who are between six and seven years old show a sense of fairness.
Babies are likely to pick up on ideas of fairness and unfairness ‘by observing how people treat each other’, Dr Sommerville said.
In a second task, scientists recorded whether or not the babies were willing to share a toy with a stranger.
Those who had been more surprised by the unequal distribution of food were found to be more likely to share a favourite toy than the babies who seemed more surprised when there was an equal distribution of food.
Dr Sommerville said: ‘The results of the sharing experiment show that early in life there are individual differences in altruism.’
Those who were willing to share a favourite toy had been ‘really sensitive to the violation of fairness in the food task’, she added.