This book investigates whether human female estrus has been evolutionarily lost or if it remains present but concealed throughout the ovulatory cycle. Randy Thornhill and Steven W. Gangestad, both established researchers in behavioral evolution, challenge conventional academic wisdom by proposing that women possess two distinct sexualities—estrus and extended sexuality—each serving different evolutionary functions. By synthesizing comparative biology and behavioral data, the authors construct a theoretical framework that situates human female sexuality within the broader context of vertebrate phylogeny.

Experts in evolutionary psychology and biology recognize this work as a significant contribution to the debate surrounding human reproductive strategies. Readers frequently note the academic density of the prose and the rigorous, data-driven approach the authors employ to support their controversial hypothesis.