Most couples will testify that their sex life plummets on the birth of a new baby, with new mothers often worrying that they are no longer seen as attractive in the eyes of their partner.
But a new study suggests that parenthood not only affects the biology of mothers but also of fathers.
During the first year new fathers experience a drop of testosterone of around one third, with those who help out with childcare for three or more hours a day seeing a further drop of 20 per cent.
The new fathers who took part in the research also reported having less sex.
Researchers believe that ‘the sensitising effect’ is driven by the psychological and cultural impulse to protect a newborn and would have the same impact on adoptive fathers.
Men with less testosterone are likely to be less aggressive and more caring. Previous research has also shown that men with high testosterone levels feel less sympathy or need to respond to the cries of a baby.
It means that new mothers should not worry about their partners straying after the birth, or feel anxious if they do not want to have sex. They are biologically programmed to concentrate on looking after their children at the expense of their sex drive.
Dr Lee Gettler, of Notre Dame, carried out the largest study of its kind looking at how the biology of new fathers changes after the birth of their children.
“It’s not just mothers who go through pregnancy and birth and it’s not just mothers who biologically respond to parenthood. Fathers can biologically respond to the needs of children too,” he said.
“We found that men who became new fathers had a decline of testosterone of between 33-34 per cent. Men who were most involved in the day to day hands on childcare had the lowest testosterone levels.
“If you think about fathers in other mammalian species they don’t really help taking care of the children.
“So it seems that natural selection has stepped up men’s hormone system to respond to the needs of their offspring.
“Our species has evolved paternal instincts which are somewhat unique to our species compared with our closest relatives.”