Women normally (with exceptions) prefer the smell of men who are immunologically different from them. The desire for mates who have differences in sequences in Major Histocompatibility Complex (MHC) genes is probably the result of selective pressures to give offspring more genetic variations against diseases to increase the odds that at least some of a woman's offspring would survive. But Craig Roberts of the University of Liverpool found that women prefer the faces of men who have more similar MHC genes.

Previous research on smell suggests that humans prefer odours from potential partners who are genetically dis-similar. But new research in which women rated the facial attractiveness of men suggests the exact opposite. So sight and smell appear to be giving contradictory messages about which partners to choose.

The curious thing about this effect is that MHC variations are tied to facial shape variations. Are the genes for each located on the same chromosome? Do MHC variations cause facial shape differences?

“It’s a subtle effect,” says Craig Roberts of the University of Liverpool, who led the team which made the discovery. “We’re not saying it’s something that rules who we find attractive.”

Roberts and colleagues recruited 92 women and 75 men, and recorded differences in their MHC by analysing DNA from blood samples. Then each woman was asked to rate photographs of six men, three with similar and three with different MHC.

The results showed that, visually, the women preferred men with similar MHC. The preference applied both to long and short relationships, but was strongest for potential long term relationships.

Amerinds (a.k.a. indigenous peoples of the Americas) have fewer MHC variations than Europeans and Europeans have fewer than Africans (and this fact majorly contributed to the massive die-offs of Amerinds when the first European explorers showed up carrying many diseases that were new to the Americas). It would be worth investigating whether the same patterns of attraction to the opposite sex with similar or different MHC genes are found in Africans, Europeans, and Amerinds.

Since the Amerinds have fewer MHC variations and less selective pressure was present for MHC variations did they also experience less selective pressure for preference to mate with people who are immunologically different? Do the Amerinds have perhaps even indifferences to smell differences caused by MHC differences? Is the opposite the case among Africans? More generally, do particular MHC types which are found in some races or sub-racial groups produce smells that are either extremely desirable or extremely revolting in some other racial group?