No study has led to the isolation of true human sex pheromones, though various researchers have investigated the possibility of their existence. Sex pheromones are chemical signals, pheromones, released by an organism to attract an individual of the opposite sex, encourage them to mate with them, or perform some other function closely related with sexual reproduction. While humans are highly dependent upon visual cues, when in proximity, smells also play a role in sociosexual behaviors. An inherent difficulty in studying human pheromones is the need for cleanliness and odorlessness in human participants. Experiments have focused on three classes of putative human pheromones: axillary steroids, vaginal aliphatic acids, and stimulators of the vomeronasal organ. Axillary steroids are produced by the testes, ovaries, apocrine glands and adrenal glands. These chemicals are not biologically active until puberty when sex steroids influence their activity. The activity change during puberty suggest that humans communicate through odors. Several axillary steroids have been described as possible human pheromones: androstadienol, androstadienone, androstenone, androstenol, and androsterone. Androstenol is the putative female pheromone. In a 1978 study by Kirk-Smith, people wearing surgical masks treated with androstenol or untreated were shown pictures of people, animals and buildings and asked to rate the pictures on attractiveness. Individuals with their masks treated with androstenol rated their photographs as being "warmer" and "more friendly". The best-known case study involves the synchronization of menstrual cycles among women based on unconscious odor cues, the McClintock effect, named after the primary investigator, Martha McClintock, of the University of Chicago. A group of women were exposed to a whiff of perspiration from other women. Depending on the time in the month the sweat was collected, there was an association with the recipient woman's menstrual cycle to speed up or slow down. The 1971 study proposed two types of pheromone involved: "One, produced prior to ovulation, shortens the ovarian cycle; and the second, produced just at ovulation, lengthens the cycle". However, recent studies and reviews of the methodology have called the validity of her results into question. A 2013 meta-review of existing studies showed that the syncing of ovarian cycles likely did not exist. Androstenone is postulated to be secreted only by men as an attractant for women and is also thought to affect their mood positively. It seems to have different effects on women, depending on where a female is in her menstrual cycle, with the highest sensitivity to it during ovulation. In 1983, study participants exposed to androstenone were shown to undergo changes in skin conductance. Androstenone has been found to be perceived as more pleasant to women at a woman’s time of ovulation. It is hypothesized that this may be a way for a male to detect an ovulating female who would be more willing to be involved in sexual interaction.
