Researchers have discovered that women - unlike many men - instinctively prefer a monogamous relationship, mainly because they need help bringing up any resultant children.
But if they do stray it's more likely to be when they enter their 30s as their biological clock starts ticking.
Much like many celebrities, such as Kate Beckinsale and Amanda Holden, they perceive the age to be the beginning of their last chance for them to secure a good man to father their children.
It also coincides with them reaching their sexual peak and when they are most likely to have the most opportunities to have an affair.
The conclusion made by Dr David Schmitt of Bradley University, Illinois, was made after he collected data on the sexual habits of women from 48 countries across the world.
He found that while men's "sociosexuality" or promiscuousness peaks in their late 20s, women are most likely to be unfaithful to their partners in their early 30s.
Dr Schmitt told New Scientist that women were more likely to have an affair when their fertility first begins to wane.
"That's exactly the point where the odds of conceiving start to drop at a bigger rate, and it's also the point where the odds of having a child with a genetic problem or birth defect start to go up," he said.
Of course plenty of women have babies much later, but Dr Schmitt suggests that women's increased sociosexuality at around this time reflects an evolved reproductive strategy that maximises the chances of their conceiving and bearing a healthy child.
The research appears to be backed up by high profile marriage break ups. Miss Beckinsale, the British actress left her Welsh husband Michael Sheen after an affair when she was 30.
Amanda Holden divorced Les Dennis in December 2003 when she was 32 after being unfaithful with Neil Morrissey.
The research by Dr Schmitt also showed that the levels of promiscuousness in society also varied depending on the relative abundance of men.
In east Asian countries such as China, Japan and South Korea, where the population is heavily male biased there is a relatively low level of interest in uncommitted, casual sex.
Meanwhile, urban areas of the US with low ratios of men to women, had a correspondingly high level of short-term relationships and divorce.
Anne Campbell, a psychologist at the University of Durham, said: "This means men can call the shots, and what men usually want is casual sex."