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 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.

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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.

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 new mothers should not worry about their partners straying after the birth, or feel anxious if they do not want 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. He studied more than 400 men in the Philippines.

"If you think about fathers in other mammalian species, they don't really help taking care of the children," he said.

"So it seems that natural selection has stepped up men's hormone system to respond to the needs of their offspring."