Genetic factors influence female infidelity and the number of sexual partners women have, British scientists said on Wednesday.

  They studied the responses of 1,600 pairs of identical and non-identical twins in a confidential survey to look at the impact of genes on behavior.

"We found that around 40 percent of the influence on the number of sexual partners and infidelity were due to genetic factors," Professor Tim Spector, director of the Twin Research Unit at St Thomas' Hospital in London, told a news conference.

But he added that environment and upbringing also play a part in explaining the variation in infidelity between women.

"The fact that psychosocial traits such as number of sexual partners and infidelity appear to behave as other common complex genetic traits in humans ... lends support to evolutionary psychologists' theories on the origin of human behavior," Spector said.

The scientists questioned the twins about their sexual behavior, total number of partners and their attitudes about infidelity. Twenty-two percent admitted that they had been unfaithful.

Genetic factors did not appear to influence the women's attitudes to infidelity. Many of the women in the study, even those who admitted being unfaithful, said they thought it was wrong.

The average age of the women was 50. A quarter were divorced. Women who had been faithful had about four sexual partners, compared to eight in the infidelity group.

"More than 90 percent of the women admitted to having had thoughts of infidelity at some time," said Spector, who reported the findings in the journal Twin Research.

The scientists suspect that many genes could be associated with sexual behavior. They believe genes on chromosomes 3, 7 and 20 could be involved.

In a separate study in the journal, involving nearly 2,000 sets of female twins, Spector and his team said genes also played a role in common sleep disorders.

"These results suggest a substantial genetic contribution to the symptoms of both obstructive sleep apnea and restless legs syndrome and that could be good news for people who suffer from these conditions if the genes responsible can be identified," said Spector.

Sufferers of obstructive sleep apnea (OSA) can stop breathing for short periods while sleeping. It causes poor sleep during the night and excessive sleepiness during the day.

The Twin Research Unit, which was set up in 1992, studies the role genes play in the development of common diseases and behavioral traits in men and women.