Scent tells a wolf quite a tale: where an animal went, how fast it was traveling, and how long ago. For a wolf, the scent of an animal’s urine tells the sex and whether the animal is in good health. Poor health is a promising sign: the prospect for outrunning the prey is improved.
Despite having a comparatively poor sense of smell, olfaction remains a powerful sense in humans, often subliminal in its tug to guiding behavior. Pheromones, the prevalent communication form between insects, and a major facet in the lives of many animals, plays a significant role in human relations as well.
Much of the allure between the sexes is chemistry. Literally. Both men and women are affected by the scent of a potential mate. There is a built-in biological propensity to prefer a mate with a different, and hence complementary, immune system. This is detected by the scent of major histocompatibility complex (MHC). All cells in the body have these protein-processed marker molecules. Widening the gene pool in this way potentially confers enhanced protection to offspring.
This is a strong subconscious attraction, and so practically undeniable. Both women and salmon succumb to this lure.
A woman’s preference for men varies during her menstrual cycle. A woman’s sense of smell is keenest near ovulation.
During ovulation, a coupling with a masculine man has a stronger allure. Masculinity signals includes facial symmetry (an indicator of health), muscle tone, a more masculine voice, and dominant behaviors.
A woman’s behavioral signals subconsciously change during ovulation: a higher-pitched voice, desire to appear more attractive, an urge to flirt. Less masculine men become less attractive, though the cause may not be consciously recognized.
Hormonal birth control disrupts this natural biorhythm and alters MHC. The desires of women on the pill during ovulation are unchanged. Their preference for partners with different immunities disappears.
Birth control pills also affect a woman’s libido and mood, and may affect long-term relationships. A woman who partners with a less masculine man while on the pill may feel dissatisfaction coming off the pill. The urge to stray may have a say with a less manly partner. This does not happen to a woman with a masculine sexual partner.
Men do not exhibit shifting interest in an ovulating woman taking birth control pills, as the cues are not detectable.