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

Because the human eye color represents a simple, predictable and reliable pattern of inheritance. The Norwegian scientists found that blue-eyed men are more attracted by blue-eyed women than by brown-eyed women, because the eye color can be like a paternity check.

In conformity with the genetic laws of inheritance, the brown eye gene is dominant, while the blue eye gene is recessive. That means that a person must have a double set of blue-eye gene in order to have blue eyes, while a brown eyed person can have either a double set of brown-eye gene or a brown eye gene with a blue-eye gene.

When both genes for the traits are identical, the individual is called homozygote, while when the individual bears two gene variants for a trait is called heterozygote. So, blue eyes can be only homozygote and this trait comes from the genetic background of both parents, while for brown eyes one gene is enough.

But how will be the children? When both parents have blue eyes, all their children will have blue eyes; there is no gene for brown eyes. If both parents are brown heterozygotes, a quarter of the children will have blue eyes and the rest brown eyes.

When one parent is brown eyed heterozygote and the partner has blue eyes, the children will be 50:50 blue and brown eyed.

But when just one partner is homozygote brown eyed, all the children will have brown eyes, no matter the genetic background of the other partner.

So, in a couple of blue eyed partners, a brown eyed child means cheating by the woman. If the wife is brown eyed, the blue eyed partner does not have a reliable clue in the color of the children's eyes.

The researchers found that blue eyed men are attracted by a feature less likely to cover their partners' sexual infidelity. 88 male and female students were put to designate facial attractiveness of models on a computer. They looked at pictures of young, unknown faces, in two eye variants: the natural eye color (blue/brown) and the opposite (brown/blue).

Women - regardless of their eye color - had no preference for male models linked to eye color, and similarly, brown-eyed men had no preference for the models according to their eye color. But blue-eyed men were more attracted to blue-eyed models than brown-eyed ones. As for the brown eyed men, the eye color of their children does not offer any clue and they show no preference. Women - because they are anyway biological parent - show no eye color preference.

In other experiment, a group of 443 young adults of both sexes and different eye colors reported the eye color of their partners. Blue-eyed men had the largest percent of partners of the same eye color. "It is remarkable that blue-eyed men showed such a clear preference for women with the same eye color, given that the present experiment did not request participants to choose prospective sexual mates, but only to provide their aesthetic or attractiveness responses…based on face close-up photographs." said Laeng.

"Blue-eyed men may have unconsciously learned to value a physical trait that can facilitate recognition of own kin."