Men appear to get greater satisfaction than women when witnessing retribution, research suggests.
Scans were used to monitor brain activity
Scientists monitored brain activity in people while they watched someone they either liked or disliked apparently suffering pain.
While women showed signs of empathy with people they both liked and disliked, men appeared to enjoy pain being inflicted on their foes.
The study, by University College London, is published in Nature.
A series of tests was undertaken involving 32 male and female volunteers.
They were joined by four actors whose true identity was kept secret from the rest of the group.
In the first part of the experiment, the volunteers played a monetary investment game.
During this session, the actors established themselves as either likeable by playing fair, or unlikeable by holding on to money unfairly.
Next, the actors apparently received mild electric shocks while the volunteers were monitored for their responses.
When the "fair" players were shocked, both female and male volunteers showed increased activity in the pain-related centres of the brain - the fronto-insular and anterior cingulate cortices.
When the "unfair" actor received a shock, the women taking part in the experiment showed a similar empathy with them.
In contrast, the male volunteers showed no increased activity in the empathy-related pain areas.
They did, however, show a surge of activity in the reward centre of the brain - the nucleus accumbens.
The researchers noted that the volunteers tended to avoid people who had cheated them, and their antipathy was confirmed by questionnaire responses.
'Sense of justice'
Lead researcher Dr Tania Singer said: "Men expressed more desire for revenge and seemed to feel satisfaction when unfair people were given what they perceived as deserved physical punishment.
"This type of behaviour has probably been crucial in the evolution of society as the majority of people in a group are motivated to punish those who cheat on the rest.
"This altruistic behaviour means that people tend to protect each other against being exploited by society's free-loaders, and evolution has probably seeded this sense of justice and moral duty into our brains."
Dr Singer said it was possible that men reacted more sharply because the punishment being meted out was physical, rather than psychological or financial."
However, she said: "This investigation would seem to indicate there is a predominant role for men in maintaining justice and issuing punishment."
Dr Colin Wilson, a neuropsychologist at Belfast's Green Park Healthcare Trust, said the findings were interesting - but much more work would be needed before one could conclude that there were profound differences between the sexes.
He said: "It might be that women tend to have more reflective, thoughtful responses, and are less likely to make quick, punitive judgements.
"But what would be really interesting would be if the same findings were found if the punishment was a social insult or a put down, rather than physical."