Blue eyed men prefer blue eyed women as their partners because this can provide the reassurance that the woman's babies are theirs too.
When surveyed, blue-eyed men find pictures of women with the same eye colour significantly more attractive than those with brown eyes, whereas neither brown-eyed men nor brown-eyed women show any preference for eye colour, Bruno Laeng of the University of Tromsø, Norway, and his team have discovered.
The effect is seen in real relationships, too. Blue-eyed men are more likely to be romantically involved with a woman of the same eye colour than they are with brown-eyed women, or brown-eyed men are with a partner of any eye colour.
Blue eyes are a recessive trait, Laeng explains, so two blue-eyed parents should produce a blue-eyed child, while a child with any other eye colour must have been fathered by another man.
Blue-eyed men seeking a partner unconsciously know this, Laeng claims, and select women of similar eye colour to ensure they can more easily spot if they have been cuckolded.