From the New Scientist:
Facial attractiveness and smell give us contradictory messages about how to select mates, new research has revealed.
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.
…Although the results appear to contradict those applying to smell, Roberts and his colleagues offer an explanation to resolve the paradox, based on the notion that kith and kin – despite having similar MHC – can offer cultural and social advantages in child rearing.
The team suggests that “filtering” for mates takes place at two levels – the first based on facial likeness to select someone not too distantly related, and the second based on smell, essentially to avoid in-breeding.