Women have higher sex drives in their thirties and early forties than in their younger years because they fear their "biological clock" is ticking.
A study showed that despite the fact their bodies may be in decline, women are more likely to have sexual fantasies and affairs as they approach 40, reports telegraph.co.uk.
Experts said that as women approach the milestone, they may sense that their "window of opportunity" to have children is closing and their fertility is declining.
Their instinctual reaction is an increased appetite for sex, the researchers said, an explanation which could explain the rise of the "cougar" - a woman who seeks out flings with younger partners.
The survey of nearly 900 women divided respondents into three groups: those at their most fertile age (19 to 26 years), those whose fertility was declining (27 to 45 years) and those who were approaching or had reached menopause.
Those in the middle group were significantly more sexually driven, engaging in sex and having sexual fantasies more often than their older and younger counterparts and being more likely to have flings.
"Results indicate women with declining fertility have greater sexual motivations and increased sexual behaviours than do women with relatively high fertility," David Buss, a researcher, said.
"These findings lend further support to the influence of the biological clock on women's mating psychology to facilitate conception before the window of opportunity closes," he added.