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.”

Dr Gettler studied more than 400 men in the Philippines. He tested their level of testosterone when they were single aged 21 and then retested them at 26 when many had become new fathers.

Previously, small studies have suggested that testosterone may drop in fatherhood but this is the first large study that has quantified the impact as well as recording that levels of sex also fall.

The researchers say they are unclear why fatherhood would cause couples to have less sex having controlled for factors which could impact on intimacy, such as having a young child sleeping in the same room, the parents being more tired or having less time because of childcare.

They also do not believe that the fall in testosterone is linked to a fall in libido although it may be possible that women find their new partner less attractive because of the hormonal change, the researchers suggest.

It man also improve the stability of the relationship as previous studies have shown that men with higher testosterone are more likely to have marital problems and to be divorced.

“We found that newly married new fathers who experience greater declines in testosterone also reported less frequent sexual intercourse with their partners at follow-up,” said Dr Gettler.

“We don’t know what the mechanism is but we know new fathers are having less sex with their partners and there isn’t a strong relationship between testosterone and libido.”

Those we reported having sex with the partner less than once a week had the lowest testosterone levels.

During the first year, levels of prolactin, a hormone which helps new mothers to produce milk, are also raised.

The researchers also found that new fathers produced far more antibodies in their saliva which protects against cold and flu.

This could be linked to the fall in testosterone, which is known to suppress the immune system.

Dr Gettler said men should not be concerned that becoming a father will affect their masculinity.

He said: “There is a very strong cultural association between testosterone and masculinity but I think if you ask most men they would say that being a great father is being a great man.

“Evolution has shaped male biology and neurobiology to help him that role.”

Dr Gettler’s findings were presented at the American Association for the Advancement of Science (AAAS) annual meeting in Chicago.