A "monogamy hormone" may cause men in committed relationships to keep away from women they find attractive, scientists claim.
The research focused on oxytocin, also known as the "cuddle hormone", that helps forge bonds between loving couples and parents and children.
Studies have also shown raised oxytocin levels make people more trusting.
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Scientists in Germany found squirting the hormone up men's noses had a distinct effect on those in committed relation- ships, and they kept a greater distance when encountering an attractive woman than men treated with an inactive "dummy" version of oxytocin.
However, the same was not true for single men.
Lead scientist Dr Rene Hurlemann, from the University of Bonn, said: "Previous animal research in prairie voles identified oxytocin as a major key for monogamous fidelity in animals.
"Here, we provide evidence it may have a similar role for humans."