What drives women to cheat?
Scientists can now show females are constantly on the lookout for a better partner.
Women are said to get the 'fittest genes' from their affair partner and look after the baby with their more reliable long-term partner.
New research suggests women have been programmed by evolution to pursue affairs in case they decide to leave their partners.
The “mate-switching hypothesis” suggests that humans - especially women - have evolved to constantly test their own relationships and check for better long-term options.
The scientists believe it applies particularly to childless women whose choice of partner can have a huge impact on their subsequent ability to raise children.
The theory that affairs are women's natural back-up plan challenges the accepted notion that humans are intended to be monogamous.
It suggests humans have evolved to constantly be on the lookout for better long-term partners that their current ones.
David Buss, Cari Goetz and their team told the Sunday Times: 'Lifelong monogamy does not characterise the primary mating pattern of humans.
'Breaking up with one partner and re-mating with another - mate switching - may more accurately characterise the common, perhaps the primary, mating strategy of humans.'
Mr Buss is professor of evolutionary psychology at the University of Texas, and Mr Goetz is assistant professor of psychology at California State University, San Bernardino.
For our ancestors, disease, poor diet and poor medical care meant few lived past 30 - meaning experimenting to find the most suitable partner may have been key to survival.
Women would pick partners with the highest chance of survival, but have someone in reserve in case that person died.
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