One other interesting detail from In Your Face, psychologist David Perrett’s 2010 exploration of human attraction. Perrett’s Perception Lab recruited 300 men and 400 women, all of whom had heterosexual partners and had been raised by two parents. They learned that romantic partners tend to look alike — the participants and their partners tended to have similar hair color and similar eye color.

This might be explained by a self-similar preference or narcissism, but on looking deeper into the data Perrett’s team found that the single best predictor of one’s partner’s eye color was the eye color of one’s parent of the opposite sex. If a woman’s mother had blue eyes and her father had brown eyes, she would most likely be partnered with a brown-eyed man. If a man’s mother had blue eyes and his father had brown eyes, his partner most likely had blue eyes. Similarly, the hair color of a man’s mother was the single best predictor of his partner’s hair color. “These results indicate that individuals choose partners who resemble their opposite-sex parent both in eye and hair color.”

(Anthony C. Little et al., “Investigating an Imprinting-Like Phenomenon in Humans: Partners and Opposite-Sex Parents Have Similar Hair and Eye Colour,” Evolution and Human Behavior 24:1 [2003], 43-51.)