The Ecology and Evolution of Human Reproductive Behavior

The complexity of human reproductive behavior has necessitated its examination through a variety of scientific disciplines, each focusing on specific elements of our biology, behavior, and society. However, this complexity also necessitates that we reintegrate the information learned from each discipline into a single framework, one rooted in the evolutionary principles that have shaped the development of all life on earth. In this dissertation, I use this framework to explore human reproductive behavior, with a particular focus on sexual coercion and fertility-mediated sexual behavior. In Chapter 1, I introduce the approach taken in this document, identify several key limitations, and outline the general structure. In Chapter 2, I conduct a comprehensive and interdisciplinary review that includes the fundamentals of sexual conflict and reproductive strategies; the evolution of human reproductive characteristics in response to socio-cognitive demands; the aspects of human sociality expected to influence reproductive behavior; the identified trends in human mating behavior; the proposed pressures behind concealed ovulation in primates; the essentials of the menstrual cycle; and the existing evidence for behavioral fertility in humans. In Chapter 3, I use a game-theory model to investigate the emergence of sexually coercive behavior across a variety of species, including humans, in which male coercion is a non-developmentally-determined reproductive strategy to identify several ecological and behavioral characteristics that predict the emergence of coercive behavior generally consistent with observed trends. In Chapter 4, I use face-trait research to investigate the degree to which women recognize and discriminate between images of men with personality traits associated with different male reproductive strategies as well as how these preferences might be mediated by her relationship and fertility status. In Chapter 5, explore the intersection of fertility, fertility belief, and sexuality, specifically testing the hypothesis that a woman’s sexual interest shifts in response to her fertility while taking into consideration her beliefs regarding her fertility. Finally, in Chapter 6, I review the primary take-home messages of this work and recommend that future research take these into consideration as they move forward. By taking an interdisciplinary approach rooted in evolutionary biology, this work reveals the need for an understanding of human reproductive behavior that incorporates a wider view of reproductive ecology. In doing so, we can gain a more accurate, comprehensive, and nuanced understanding of human reproductive behavior

Fertility, Belief, and Sexuality

The human ovulatory cycle lasts an average of 28 days and is highly variable both within and among women, making ovulation and the fertile window difficult to estimate in humans. Commonly used methods for estimating fertility assume length consistency during various cycle phases, an assumption that often miscategorizes women as fertile when not and vice versa. This experiment analyzes several of the common methods used to estimate fertility, including forwards and backwards-counting methods and actuarial methods, and compares them against hormonal testing to determine the most accurate method for fertility identification. Additionally, we used hormonal ovulation tests combined with daily journaling reports to determine the degree to which fertility affects sexual desire, likely sexual engagement, and general happiness, as well as a woman’s belief that she is fertile. We hypothesize that women will not be able to accurately predict their fertility and will show increased sexual interest and general happiness during the fertile window. We also hypothesize that estimation methods will have different results than those using confirmed fertility, but that the closest estimation will be backward counting methods estimating fertility as the six-day window ending at thirteen days from the start of menses