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In XX century was discover the phenomenon wherein the menstrual cycles of women living in close quarters tend to synchronize. This phenomenon has been observed in women living together. It has been found in room-mates, close friends, lesbian couples and most strongly between mothers and daughters. It has also been noted in mice, hamsters and rats. It as suggested that ovulation (process of releasing egg from ovary) is socially regulated and this lead to what is called menstrual synchronicity (McClintock effect).
Synchronized periods known as the McClintock effect, also known as menstrual synchrony or the dormitory effect, is a theory that proposes that the menstrual cycles of women who live together (such as in prisons, convents, bordellos, or dormitories) tend to become synchronized over time.
It is thought to be analogous to the Whitten effect, which is the synchronization of the estrous cycle and has been noted in small animals such as mice and guinea pigs. In contrast to the Whitten effect, which is driven by male pheromones, the McClintock effect is postulated to have only female pheromonal involvement.
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The phenomenon about women synchronized periods was called as the McClintock effect for the author of the first article to discuss the phenomenon, Menstrual Synchrony and Suppression, by Martha McClintock, which appeared in Nature in 1971. It seems she was at a conference where the phenomenon of synchronized ovulation in mice was broached. Ms. McClintock commented that the same thing happened in human females as well, as evidenced in dormitory life. While at first wary of the assertion, the scientists challenged her, as an undergraduate, to address the issue scientifically. McClintock did just that when she pursued the topic for her senior thesis at Wellesley and then published it when she was working on her graduate degree at Harvard.
This was the first paper to suggest any operation of pheromones in humans. While it did not definitively prove that pheromones were acting causally, it did offer tantalizing evidence. Further research by McClintock and others (Russell et al., 1980; Stern and McClintock, 1988; etc) did prove that it was pheromones causing the synchrony. Other studies have also found influences of male pheromones on women's menstrual cycles as well as evidence of female pheromones effecting men's hair growth.
McClintock effect (synchronous menstrual cycles between girls)
Mechanism: pheromones (kind of chemical like hormone) are released by specialized skin glands concentrated under the arm, these are airborne chemical which are not detected as odor by the nose but they are sensed in the nose by vomeronasal organ (VNO, so called Jacobson's organ). When a lady is menstruating she releases this substance into the air, this will stimulate the VNO in close lades and signals reach the hypothalamus leading to hormonal changes and induce menstruation and some behavioral changes.
Two pheromones have been discovered which are involved in such synchronicity and released in different phases of the cycle (period).
Jacobson's Organ and the Sixth Sense
(By Anne Marie Helmenstine, Ph.D., About.com)
Traditionally humans have been thought to come equipped with five senses: sight, hearing, taste, touch, and smell. Animals possess several extra senses, including altered vision and hearing, echolocation, electric and/or magnetic field detection, and supplementary chemical detection senses.
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In addition to taste and smell, most vertebrates use Jacobson's organ (also termed the vomeronasal organ and vomeronasal pit) to detect trace quantities of chemicals.
While snakes and other reptiles flick substances into Jacobson's organ with their tongues, several mammals (e.g., cats) exhibit the Flehmen reaction. When 'Flehmening', an animal appears to sneer as it curls its upper lip to better expose the twin vomeronasal organs for chemical sensing. In mammals, Jacobson's organ is used not simply to identify minute quantities of chemicals, but also for subtle communication between other members of the same species, through the emission and reception of chemical signals called pheromones.
In the 1800s, Danish physician L. Jacobson detected structures in a patient's nose that became termed 'Jacobson's organ' (although the organ was actually first reported in humans by F. Ruysch in 1703).
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Since its discovery, comparisons of human and animal embryos led scientists to conclude that Jacobson's organ in humans corresponded to the pits in snakes and vomeronasal organs in other mammals, but the organ was thought to be vestigial (no longer functional) in humans. While humans don't display the Flehmen reaction, recent studies have demonstrated that Jacobson's organ functions as in other mammals to detect pheromones and to sample low concentrations of certain non-human chemicals in air. There are indications that Jacobson's organ may be stimulated in pregnant women, perhaps partially accounting for an improved sense of smell during pregnancy and possibly implicated in morning sickness.
Since extra-sensory perception or ESP is awareness of the world beyond the senses, it would be inappropriate to term this Sixth Sense 'extrasensory'. After all, the vomeronasal organ connects to the amygdala of the brain and relays information about the surroundings in essentially the same manner as any other sense. Like ESP, however, the sixth sense remains somewhat elusive and hard to describe.
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