On an emotional level, “kindred spirits” sounds lovely. But when it comes to sexual desire, biology seems to prefer difference. Helen Fisher, for one, pointed me to the famous “sweaty T-shirt” experiment, conducted in 1995 by the Swiss researcher Claus Wedekind. He had women sniff the unwashed T-shirts of various men and asked them which scent they were most attracted to. Most women selected the T-shirts of men with genes markedly different from their own in a certain part of the immune system. Other studies confirmed these findings. Presumably this attraction to genetic variation is an evolutionary adaptation to prevent incest in our ancestral environments and improve the survival prospects of offspring.