Sex, Evolution, and Behavior

Daly and Wilson argue that sex is the organizing principle of behavioral science: all social organization, from mating systems to parental care to sex-role development, is interpretable as the outcome of evolved reproductive strategies. The book rejects the assumption that biology and culture are competing explanations, insisting instead that behavior requires multiple complementary levels of analysis—adaptive significance, physiology, evolutionary history, ontogeny, and stimulus control. As the authors state in the preface, an item of behavior "cannot be fully comprehended until we grasp its adaptive significance and its physiology and its evolutionary history." This integrative stance distinguished the book from both strictly biological sociobiology and purely cultural social science, positioning reproductive strategy as the conceptual bridge between disciplines.

The book systematized the concept of reproductive strategies as the unifying framework for behavioral science. Daly and Wilson argued that organisms are "products of evolution by natural selection, a process that favors only those characteristics that pay off in reproduction," and that sex therefore "holds center stage in the behavioral sciences." They organized the book around strategic rationales: the adaptive significance of sexual reproduction, sex differences in behavior, comparative mating and social systems, parental behavior, and developmental aspects of reproductive function. This framework treated diverse phenomena—from vertebrate mating systems to human sex-role development—as manifestations of the same underlying logic. The approach became foundational for evolutionary psychology's later development, though its sociobiological lineage also attracted criticism from those wary of biological explanations of human behavior.

The second edition was a wholly rewritten text rather than a minor revision, integrating significant theoretical developments since 1978. A Nature review noted that the authors incorporated Eric Charnov's sex allocation theory, which unified disparate research on how individuals partition resources into male and female function, and integrated comparative and experimental tests of adaptive hypotheses that had accumulated in the intervening five years. Two new chapters on humans, physiology, and development were added. The second edition was published by Willard Grant Press/Wadsworth (1983) while the first was published by Duxbury Press (1978). The Nature reviewer praised the second edition as "even better than the first" but criticized its avoidance of mathematical formalism, particularly regarding confidence of paternity and sexual selection.

Sex, Evolution, and Behavior extends E.O. Wilson's Sociobiology: The New Synthesis (1975) while adapting it for a behavioral science audience. The authors trace the book's origins to a 1975-76 seminar at UC Riverside where participants discussed Wilson's work "with excitement and occasional acrimony," concluding that sociobiological functionalism offered a "potentially integrative perspective" for behavioral sciences despite widespread skepticism. Where Wilson synthesized sociobiology across all social organisms, Daly and Wilson focused specifically on reproductive behavior and sex, arguing that the strategic logic underlying sex makes it the natural center of behavioral science. They explicitly addressed the resistance biological approaches to human behavior typically encounter, devoting a chapter to refuting criticisms and explaining what biology offers the social sciences.

Sex, Evolution, and Behavior was the foundational text that established Daly and Wilson's reputation and set the agenda for their subsequent work. In a 2013 interview, Daly identified it as "for sure" his greatest contribution to the field. The book developed the comparative and theoretical approach they later applied to human violence, culminating in Homicide (1988) and the Cinderella effect research on stepparental violence. The framework of reproductive strategies and selection thinking that they articulated in this textbook became the methodological toolkit for their later analyses of homicide, infanticide, and sexual jealousy. A Nature review noted that while Homicide eventually accumulated more citations, the textbook laid the intellectual groundwork for treating human behavior as continuous with animal behavior, a core commitment of their entire research program.

Contemporary reviews identified several limitations. J. Philippe Rushton, reviewing the second edition, noted that the book's focus on sex and reproduction meant other sociobiological topics—altruism and aggression, for example—received insufficient coverage for course instructors who wanted broader treatment. He also observed that behavioral scientists and students would have preferred more emphasis on humans, and that the book adopted a "mildly deprecating tone" toward social-personality-developmental psychology. A Nature reviewer criticized the absence of formal mathematical presentation, particularly regarding sexual selection and confidence of paternity, arguing that "flippant treatment of formal mathematical arguments" led to confusion about what assumptions supported the authors' conclusions. These critiques reflected tensions between the book's accessible, interdisciplinary ambitions and the demands of specialized audiences.