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Like our mammalian relatives, though perhaps less deliberately, humans often use olfactory signals as a form of communication. We are emitting olfactory clues 24 hours a day. A person's scent will change according to their personal environment and state of health.

Similar to odorants produced by glands in the skin, is a small subclass of volatile compounds called pheromones. First discovered in the late 1960's, pheromones were found to be strong chemical attractants released by female moths to attract a mate. As more is discovered about these common chemical signals, the definition of pheromone continues to change. Not all pheromones are involved in sexual behavior, and pheromones are most certainly not specific to insects.

The most basic definition for pheromone is any volatile substance that elicits a physiological or endocrine-based change in the receiving organism. Pheromones have been found in organisms ranging from soil nematodes to primates. Ordinarily, pheromones cannot be detected through the normal olfactory system. Instead, pheromones are thought to function through a specialized sensory organ, the vomeronasal organ (VNO).

The VNO is a small, indented patch of membrane that lies just over the Vomer bone in the nose. Receptors in the VNO membrane work in a similar way to regular olfactory receptors, but are highly specific. VNOs in insects, mice, and larger mammals only respond to a few molecules, most of these are specific to the opposite sex of their species. Receptors in the VNO do not send signals into the olfactory bulb of the brain. Instead, receptor cells extend axons into the hypothalamus and amygdala, two "primitive" parts of the brain involved in hormone control and sexual response. It was believed that humans no longer had VNOs, much less one that worked. In 1991, however, scientists found the human VNO and discovered that it was fully functional.

Just prior to this discovery, a biochemist by the name of Dr. David Berliner made an interesting observation in his lab. "While studying the chemistry of human skin, Berliner had discovered a mysterious skin extract that, when left open to the air, put his lab workers in uncharacteristically good moods." Following detailed chemical analysis on male and female skin extracts, Berliner found that two unscented steroid compounds could trigger an electrical response in the human VNO. The two steroid compounds were found to be sex-specific in production as well as detection. Female VNOs were activated only in the presence of the male-specific compound, androstadienone, and male VNO's were similarly specific to the female-specific estratetraenol.

Work done by Luis Monti-Bloch at the University of Utah also showed a human behavioral response to these pheromones. Men exposed to less than one millionth of a gram of a synthetic female pheromone became "laid back and relaxed. Their heart and breathing rates slowed, while the capillaries in the skin of their hands dilated, and electrical recordings of the brain found an increase in alpha-wave activity, classic signs of relaxation." There is no evidence, however, indicating pheromones make a person more or less appealing to the opposite sex. In other words, pheromones have little effect on who we find attractive and who finds us attractive. Humans have come to depend so much on other techniques of mate selection that pheromones are no longer necessary to identify a member of the opposite sex. The emotional relaxation that occurs in the presence of human pheromone, then, may simply be the result of associative learning. To sense a person's pheromones, you must be very close. Even though you don’t consciously smell the pheromones, your brain associates their presence with pleasurable physical intimacy.

So, what's the bottom line? Humans emit and sense pheromones that can change behavior, however subtly. The field of human pheromone research is young, and there is still much to be learned.