Smell may be the most important, albeit the most subtle, factor in physical attraction, because the nose can suss out complex mechanisms like sexual compatibility, although the conscious mind is unaware of it. The human body has more than 100 immune system genes known as the MHC, or major histocompatibility complex; these genes help our immune system to identify unwanted pathogens. The MHC determines, for example, whether an organ donor is compatible with a patient in need of a transplant.
MHC also determines histocompatibility, influencing human mating choices. In MHC experiments, women are generally more discriminate and fussy about smells and odors, possibly because they are more invested in the results of reproduction, bearing children, and caring for them.
In one famous study, researchers had male subjects wear new T-shirts for two consecutive nights. Female study participants later sniffed each shirt and deemed which ones had the most attractive scent. They preferred the shirts worn by men more immunologically dissimilar to themselves. This makes biological sense, as pairing dissimilar immune systems can mean healthier offspring. Some studies indicate that dissimilar MHC genes, which recognize foreign substances, resulted in lower rates of early miscarriages for couples.
