Women with steady partners may still be tempted to sleep around - but mainly on certain days of the month, say researchers.

A BBC documentary to be broadcast on Wednesday explains how human sexual instincts are so strong that some women's preferences may alter significantly while they are ovulating.

While her partner might be a better bet to bring up children and support her, another man might carry genes which mean healthier, stronger children.

Serial cuckold

Morgan Wise, a train driver from Big Spring in Texas, found this out when his youngest son was found to have cystic fibrosis, a devastating lung disorder caused by a single faulty gene.

Both mother and father must carry the gene to produce a cystic fibrosis child, and Morgan duly went for a gene test to confirm he was a carrier.

The test proved negative - effectively proving that he was not the child's father.

He told the BBC: "The doctor said: 'You are not a carrier of cystic fibrosis.' I couldn't believe it."

There was worse to come. Subsequent DNA tests revealed that not one of Morgan's three sons was fathered by him.

One in 10

However, researchers suggest that this is by no means an isolated event.

One study suggested that one in 10 children are being raised by men who are unaware that they are not the father.

A study at the University of Stirling seems to pinpoint the instinct which might tempt some women to stray around the time of the month they are fertile.

Two groups of female volunteers were picked.

One was tested during their ovulation, the other at another point in her cycle.

Each was shown a computer image of a male face which they could adjust electronically to make appear more or less masculine, using features such as the thickness of the neck and the squareness of the jaw.

While the group not ovulating tended to prefer their men with slightly more feminine feature, at the point of ovulating, the women strongly preferred their men masculine.

This, say scientists, is down to instinct - while more feminine features might signify a man with less testosterone who is more likely to prove a steady partner, the stronger features they preferred at ovulation might indicate a better set of genes, producing a stronger or healthier child.

Human Instinct, presented by Professor Robert Winston, will be broadcast on BBC One on Wednesday at 2100GMT.