Women in one study ranked a partner's smell above his looks
Asked what mattered for attraction, women in one study put body odour ahead of appearance. A leading explanation lies in immune genes: people often prefer the scent of partners whose genes differ from their own. Yet the effect seems to flip for women on hormonal contraception, who in one study favoured men more like themselves.
The genes in question form the major histocompatibility complex, known in humans as HLA, which helps the immune system recognise threats. Because these genes are expressed codominantly, a child who inherits two different sets can recognise a wider range of invaders. More than thirty olfactory receptor genes sit in the HLA class I region, hinting at a mechanism by which the nose might detect immune-linked odours. Women not using hormonal contraceptives tended to prefer the scent of men whose MHC differed from theirs and of men carrying two different HLA versions. Men, by contrast, showed no such preference for heterozygous women, though they favoured the scent of women with rare variants.
Three explanations compete. One says mixing immune genes strengthens offspring; another, the inbreeding avoidance hypothesis, says the preference steers people away from relatives, since scent can signal kinship; a third holds that varied immune genes help resist fast-evolving parasites. Couples with repeated failed conceptions may share considerably more genes than other couples.
Detection starts in the olfactory epithelium, a yellow-brown patch of about 6.5 square centimetres high in the nasal cavity, which relays signals through the olfactory bulbs to the brain. Fresh male sweat contains androstenol, which women tend to find appealing, but once exposed to air it yields androstenone, which many women find highly unpleasant. Around ovulation, women in studies preferred the smell of men with higher testosterone.
Whether men can smell a woman's fertility is less clear. One study found men rated T-shirts worn in the follicular phase as more pleasant, but a slightly larger study found no significant difference, and a third recorded changes in men's saliva testosterone. Oral contraceptives may also shift preferences, so a woman who starts or stops the pill might find her partner's appeal changing.
Source: Body odour and sexual attraction
