Blue-eyed men prefer blue-eyed women, apparently because eye color can help reveal whether their partner has been faithful, researchers said on Monday.

"Before you request a paternity test, spend a few minutes looking at your child's eye color," Bruno Laeng and colleagues at the University of Tromso in Norway said in the study.

Under the laws of genetics, two parents with blue eyes will always have blue-eyed children, it said. So a blue-eyed man can know his blue-eyed wife or partner has cheated on him if their child has brown eyes.

"Blue-eyed men may have unconsciously learned to value a physical trait that can facilitate recognition of own kin," the scientists said in the journal Behavioral Ecology and Sociobiology.

The scientists asked 88 students to rate the attractiveness of models based on pictures manipulated so that half of them had blue eyes and the other half had brown eyes. The blue-eyed men in the group showed a preference for blue-eyed women.

But brown-eyed men, who cannot find any clues about paternity from a child's eye color, had no preferences by eye color. Women showed no preference for brown- or blue-eyed men, regardless their own eye color.

A quarter of children born to two brown-eyed parents who have both brown and blue-eye genes among their ancestors will have blue eyes. The rest will be brown.

In a second study, 443 young adults of both sexes were asked about the eye color of their partners -- blue-eyed men were also the group with the highest proportion of partners with the same eye color.