Fatherhood significantly reduces men's testosterone levels, a study has shown.
Caring for a baby reduces male 'competitive' behaviour
US researchers compared levels of the male sex hormone among single men and married men with and without children amongst Chinese students.
Those who were fathers had the lowest levels of all, the Proceedings of the Royal Society study found.
A UK expert said the fall was nature's way of ensuring men behaved in a "civilised" and non-aggressive way around newborn offspring.
The researchers, from Charles Drew University of Medicine and Science in Los Angeles, Harvard University and the University of Nevada asked 126 men aged 21 to 38 to fill out a questionnaire.
The men also provided two saliva samples to researchers, in the morning and afternoon.
The 66 unmarried men had slightly higher levels of testosterone than the 30 married men who did not have children.
But the 30 fathers had significantly lower testosterone levels than men who did not have children, whether married or not.
The researchers, led by Dr Peter Gray of Charles Drew University, said elevated testosterone levels were associated with mating efforts - "male to male competition, mate seeking", while lower levels were linked with relationships - and particularly with fatherhood.
They added: "In other words, the lower levels of testosterone in fathers may reflect both their withdrawal from the competitive arena and their involvement in paternal care."
Dr Nick Neave, a psychologist at Northumbria University, told the BBC News website the study findings fit in with a recognised pattern, seen in people and animals, related to the bringing up of young.
"There are obviously social elements to parenting. But we are a biological species, and are not that far removed from animals - although we like to think we are."
He said the hormone reduction was aimed at making males stay around to help care for the child.
Dr Neave added: "Nature doesn't want testosterone levels to be high at a time when there is a baby.
"It's a very frustrating time for men, very draining, so feelings of anger could be taken out on the offspring.
"It's nature's way of making males civilised - at least for a short time."
Dr Neave added: "It would be interesting to study men who did not exhibit this paternal behaviour as fathers; those who batter children or behave antisocially.
"It may be their behaviour is due to social reasons, but it might be they have relatively high levels of testosterone, or they are more sensitive to circulating levels of the hormone in the blood."
However, he said testosterone levels also varied with age - with young men having higher levels, so that could also make a difference.