Attractiveness is certainly a complex matter. Previous research has suggested that people (and rats!) prefer the smell of potential partners who are genetically dissimilar (judged on the basis of large molecules -- MHC -- which help our immune systems distinguish native from alien cells). But a new study has found women regard men with similar MHCs as more facially attractive. So sight and smell appear to be giving contradictory messages about which partners to choose. Of course, women are contrary creatures; just because we regard men as good-looking doesn't mean we'd choose them as mates!