Women have higher sex drives in their thirties and early forties than in their younger years because they fear their "biological clock" is ticking, researchers claim.
Despite the fact their bodies may be in decline, women are more likely to have sexual fantasies and affairs as they approach 40, a study showed.
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 (aged 19 to 26), those whose fertility was declining (aged 27 to 45) and those who were approaching or had reached the 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.
Professor David Buss said: "Results indicate women with declining fertility have greater sexual motivations and increased sexual behaviours than do women with relatively high fertility.
"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."
Dr Pam Spurr, an expert in sex and behaviour, told the Daily Mail: "I often find it is women in their late thirties who seem to be having a lot of fun. There might be a subconscious level to this but also a conscious one – women are more informed about fertility levels falling after a certain age."
The research, by academics from the University of Texas, was published in the journal Personality And Individual Differences.
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