The ovulatory shift hypothesis holds that women experience evolutionarily adaptive changes in subconscious thoughts and behaviors related to mating during different parts of the ovulatory cycle. It suggests that what women want, in terms of men, changes throughout the menstrual cycle. Two meta-analyses published in 2014 reached opposing conclusions on whether the existing evidence was robust enough to support the prediction that women's mate preferences change across the cycle. A newer 2018 review does not show women changing the type of men they desire at different times in their fertility cycle.
Overview
The theory proposes that women's behavior may change during the most fertile time in their ovulatory cycle. At high fertility, the theory holds that women may become more physically active and avoid male relatives.
The hypothesis separately proposes that hormonal changes across the cycle cause women, when they are most likely to get pregnant, to be more attracted to traits in potential short-term male sexual partners that indicate high genetic quality, leading to greater reproductive success. It has been proposed that genetic traits like compatible major histocompatibility complex gene profiles are considered more attractive. Newer studies do not support female changes in desired reproductive partners when more fertile.
Estrus in humans
Most female mammals experience reproductive fertility cycles. They typically consist of a long period of low fertility, and a brief period of high fertility just prior to and including ovulation. In humans, this is called the ovulatory cycle, or menstrual cycle. The period of high fertility is also called the fertile window, and is the only time during the cycle when sex can result in conception.
Females of most mammalian species display hormonally-induced physical and behavioral signals of their fertility during the fertile window, such as sexual swellings and increased motivation to mate. Some species will not—or cannot—engage in sex at all outside of this window. This phase of sexual receptivity and proceptivity, estrus, is often referred to as being "in heat".
Human females, however, engage in sex throughout their ovulatory cycles, and even beyond their reproductive years. Additionally, they do not show obvious physical signals of high fertility. This has led many researchers to conclude that humans lost their estrus through evolution. It has been hypothesized that this could be due to the adaptive benefits of concealed ovulation and extended sexuality.
However, research has shown that human females may in fact experience subtle but distinct physiological, behavioral, and cognitive changes during the high-fertility phase of their ovulatory cycle, and that both men and other women can detect signals that indicate high-fertility in a woman, which may indicate that humans have retained an estrus-like state.
Evolution of ovulatory cycle shifts
Estrus evolved to facilitate reproduction and maximize reproductive success, or the success of passing on one's genes by producing offspring that are most likely to survive and reproduce themselves. The ovulatory shift hypothesis proposes that motivation and desire to mate should increase during the fertile window, and that females should seek and attract the best possible mate at their highest fertility. An ideal mate could have many qualities: resources to care for offspring, the physical ability and social status to protect a mate and offspring, a compatible personality for a long-term pair bond, etc. Evolutionary theory and sexual selection theory suggest that an organism’s top priority should be to maximize survival and reproductive success. Thus, the ovulatory shift hypothesis proposes that women possess a dual sexuality, where during the fertile window, a woman should prioritize attracting and choosing a mate with the best genetic quality, or “good genes”, since this is the only time she can become pregnant and pass on heritable genetic qualities to her offspring. However, at low-fertility, a woman should prioritize a mate with "good parenting" traits, such as willingness and ability to invest in parenting, resources to devote to offspring, and compatibility for a long-term partnership. These differing traits are sometimes referred to as the "sexy cad" vs. the "good dad".
It has also been hypothesized that high-fertility preferences should be strongest when evaluating a short-term sexual partner, but low-fertility preferences should be strongest when evaluating a long-term relationship partner. A woman can gain the benefits of good genes through only a single sexual encounter, and good dad traits are only relevant for a long-term pair bond.