Women are programmed to have affairs as a way of testing for better partners, according to research.

Humans, and women in particular, have evolved to pursue affairs in case they decide to leave their partners.

This "mate switching hypothesis" suggests that humans have developed instincts to constantly test their own relationships and check for better long-term options.

This applies particularly to childless women whose choice of partner can have a huge impact on their subsequent ability to raise children, the researchers found.

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The US study goes against the widespread assumption that humans are meant to be monogamous and that break-ups are signs of failure.

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"Lifelong monogamy does not characterise the primary mating pattern of humans," said David Buss, author of the research paper.

Buss, a professor of evolutionary psychology at the University of Texas, added: "Breaking up with one partner and remating with another may more accurately characterise the common, perhaps the primary, mating strategy of humans."

The researchers cited our ancient ancestors who rarely used to live beyond the age of 30 - so experimenting to find the best partners would have been integral to survival, the Sunday Times reported.

They said people chose partners who had the best chances of long-term survival, but also had someone else as a "back-up" in case their current partner died.

The paper cites Jennifer Love Hewitt, whose character in the television series The Client List said: "Husbands are like pancakes: there's no shame in throwing the first one out."
 
Professor Buss suggests that women keep track of their partner's "mate value", comparing it with that of other single men.

They also assess "relationship load" - the costs imposed by partners who behave badly or fail to provide.

He said: "Affairs serve as a form of mate insurance, keeping a back-up mate should a switch become warranted in the future."

He even suggested that women in positive relationships benefited from affairs, saying: "A regular mate may cheat, defect, die, or decline in mate value. Ancestral women lacking a back-up mate would have suffered a lapse in protection, and resources."