You can't see them, feel them, or smell them, but they may have a huge influence on whom you find attractive. No, they aren't Cupid's invisible henchmen, although their effect might be just as magical. They're pheromones, chemical messengers emitted by one creature to evoke a response in another of the same species.
Pheromones are blends of chemicals, including acetates and aldehydes, and different combinations are thought to send different signals. For decades, scientists have studied them in bugs. A female moth, for example, emits pheromones from a gland on her abdomen. A male moth is aroused by the signal, flies up to 30 miles to find the female, and mates with her. Since 1959, we've synthesized insect pheromones to control pest populations - for example, to confuse males so they can't find the females.
We also know a lot about pheromones in animals. Hamsters need them to recognize other hamsters from their social groups, as well as to choose a mate. Pheromones help determine the pecking order among male elephants, and they enable mother rats to show their offspring what's edible and what's poisonous.
Human pheromones, on the other hand, are the subject of much debate. Their role in human attraction is controversial, in part because the nasal ducts that animals use to sense pheromones appear to be inactive in people (some researchers claim that's not true - we just haven't found the nerve that makes it possible).
Most researchers agree human pheromones exist. This agreement is due in part to a 1971 study that showed that the menstrual cycles of women who live or work together tend to synchronize over time. Researchers believe that this may be the result of picking up on each other's pheromones.
Other studies have compared men's and women's responses to hormone-like chemicals, such as androstadienone (derived from testosterone, the male sex hormone) and estratetraenol (related to estrogen, the female sex hormone). A 2001 study from Sweden used magnetic resonance imaging (MRI) to look at the hypothalamus in the brain, which influences the release of sex hormones in the body. They found that only women's hypothalamuses respond to androstadienone, and only men's respond to estratetraenol. Exactly how and why are not known. (That hasn't stopped entrepreneurial types from selling perfumes that promise to help customers attract bedmates like catnip does felines.)
Research has turned up some other fascinating discoveries. For example, a study published in 2005 showed that in a group of gay men, the hypothalamus responded to the odor of a testosterone derivative in the same way it did in a group of female subjects. It's not clear if this is a case of sexual orientation influencing biology, or the other way around.
While scientists sniff away at the legitimacy of human pheromones and their role in sexuality, what should lovelorn singles do? Appearance and personality probably have a greater impact on your chances of attracting and keeping a partner than chemical signals ever will. Even the researchers who study the power of pheromones say that their effects are mediated by psychology and social interaction, so it's probably wise to skip the overhyped perfumes and overhaul your table manners instead.
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