William I. Thomas opens Sex and Society with a bold physiological claim: that the fundamental difference between plant and animal life lies in the plant's focus on storage versus the animal's expenditure of energy. This contrast sets the stage for his central argument—that the metabolic and motor differences between men and women have profoundly shaped social forms. The book is a collection of revised essays, each self-contained yet united by the thesis that women's reproductive functions and men's greater restlessness and strength drove the development of primitive industry, morality, exogamy, and even modesty. Thomas draws heavily on ethnographic reports from Australia, citing Spencer and Gillen, Curr, and Fison, to ground his claims in observed practices rather than pure speculation.

Metabolism as Social Engine

Thomas begins not with culture but with biology. He posits that women's bodies are adapted for storing energy—necessary for pregnancy and lactation—while men's bodies are built for expenditure, leading to greater physical strength and a restless, exploratory drive. This metabolic dichotomy, he argues, underlies the sexual division of labor in primitive societies. Women's stationary condition tied them to domestic tasks and child-rearing, while men's mobility enabled hunting, warfare, and trade. Thomas supports this with data on differences in lung capacity, muscle mass, and even susceptibility to disease, though he acknowledges that social factors also play a role. The argument is deterministic but carefully hedged: he notes that individual variation exists and that civilization may alter these patterns.

Elopement, Capture, and the Politics of Marriage

In the chapter on exogamy, Thomas examines marriage practices among Australian tribes, focusing on elopement and capture. He cites detailed accounts: a young man might arrange with a girl to run off, sending a secret message like “Will you find me some food?” as a proposal. Parents were kept ignorant, and if caught, the girl's female relatives beat her with yamsticks. Thomas uses these examples to challenge theories by Lubbock and Spencer that marriage by capture was widespread. Instead, he argues that capture was rare and often tied to avenging parties. The real driver of exogamy, he suggests, was the monopoly of women by older men, forcing younger men to seek wives outside their group. This section shows Thomas at his most ethnographic, weaving primary sources into a critique of earlier sociological models.

Modesty and the Adventitious Character of Woman

Two linked essays explore the psychology of modesty and the social construction of femininity. Thomas argues that modesty is not innate but arises from the need to protect women's reproductive value, leading to clothing and seclusion. He traces how modesty varies across cultures, from the use of aprons to full veiling. In “The Adventitious Character of Woman,” he contends that women have been defined by their relationship to men—as daughters, wives, mothers—rather than as independent agents. This “adventitious” status, he claims, is a product of social control, not biology. The argument is nuanced: Thomas does not deny biological differences but insists that many traits attributed to women are culturally imposed. He draws on examples from Native American and African societies to show how women's roles shift with economic and political conditions.

The Mind of Woman and the Lower Races

Perhaps the most controversial essay, this chapter compares the mental capacities of women and so-called “lower races.” Thomas argues that both groups have been shaped by similar evolutionary pressures—namely, a lack of varied stimulation leading to conservatism and emotionality. He cites brain size studies and observations of sensory acuity, but he is careful to note that environment, not heredity, is the primary cause. Women's minds, he claims, are not inherently inferior but have been stunted by limited education and social roles. Similarly, he argues that “primitive” peoples are not intellectually deficient but have developed different cognitive skills suited to their environments. The essay reflects the racial and gender assumptions of its time while also pushing against biological determinism, making it a complex artifact of early twentieth-century social science.

Readers should approach Sex and Society as a document of its era—a pioneering but uneven attempt to synthesize biology, ethnography, and sociology. Thomas's reliance on Australian fieldwork gives his arguments a concrete foundation, but his evolutionary framework and occasional racial generalizations require critical scrutiny. The book rewards those interested in the history of gender theory and the early days of American sociology, offering a clear view of how scholars once tried to bridge the gap between body and society.