Sex and Reason is a 1992 book about human sexuality by the economist and federal judge Richard Posner, in which the author attempts to explain sexual behavior in economic terms and discusses a range of controversial subjects related to sex, proposing reforms in American laws.

The book received mixed reviews. The work was described as ambitious and Posner was credited with providing a learned discussion of, and a valuable overview of scholarly literature about, sex. It was noted that Posner's discussion of homosexuality played a central role in his work. Some reviewers praised Posner's treatment of gay rights issues, including service in the American military by gay people, but others criticized his treatment of homosexuality. Posner was also criticized for his treatment of women's sexual behavior and preferences, feminism, female infanticide, welfare, contraception, rape, prostitution, pornography, and abortion, his use of sociobiology, the authorities he relied upon, and his approach to morality. Reviewers considered some of Posner's conclusions speculative. Posner subsequently reevaluated his view of gay rights, and abandoned the opposition to same-sex marriage he had expressed in the work.

Posner discusses human sexuality from a multidisciplinary perspective, aiming to summarize the principal findings of scientific literature on the subject and explain their relevance to law. He considers controversial topics such as the AIDS epidemic, abortion, the gay rights movement, the sexual revolution, surrogate motherhood, marital rape, date rape, sexual harassment, sexual abuse, and pornography. According to Posner, he decided to write about sex because of his "belated discovery that judges know next to nothing about the subject beyond their own personal experience", despite being responsible for the interpretation and application of laws regulating sex. He describes his reading of the philosopher Plato's 4th century BC dialogue the Symposium, which he describes as a "highly interesting and articulate" defense of homosexual love, as one of the events that inspired him to begin the research for his book. Posner writes that his "larger ambition is to present a theory of sexuality that both explains the principal regularities in the practice of sex and in its social, including legal, regulation and points the way toward reforms in that regulation—thus a theory at once positive (descriptive) and normative (ethical)." He refers to this approach to the study of sexual behavior and its social regulation as "the economic theory of sexuality", describing it as, "Functional, secular, instrumental" and "utilitarian".

Authors whose work on sex Posner discusses include Sigmund Freud, the founder of psychoanalysis, the biologist Alfred Kinsey, and the philosopher Michel Foucault. He writes that Kinsey was "picked by the university authorities to head up the newly created Institute for Sex Research" and that "when their limitations are understood and respected" the Kinsey Reports "are a vast mine of useful information, have been repeatedly corroborated by other studies, and appear to be generally accurate, at least for the sample interviewed, because of the extraordinary lengths to which the interviewers went to elicit truthful answers." According to Posner, sociobiology has "advanced striking hypotheses concerning aspects of human sexuality such as courting, the double standard, polygamy, and homosexual preference." He refers to the anthropologist Donald Symons's The Evolution of Human Sexuality (1979) as the "best single book on the sociobiology of sex".

Posner criticizes the philosopher Herbert Marcuse, maintaining that Marcuse's Eros and Civilization (1955) contains "political and economic absurdities" but also interesting observations about sex and art. He credits Marcuse with providing arguments that made his work a critique of conventional sexual morality superior to Bertrand Russell's Marriage and Morals (1929), but accuses Marcuse of wrongly believing that polymorphous perversity would help to create a utopia and that sex has the potential to be a politically subversive force. He considers Marcuse's argument that capitalism has the ability to neutralize the subversive potential of "forces such as sex and art" interesting, though clearly true only in the case of art. He argues that while Marcuse believed that American popular culture had trivialized sexual love, sex had not had a subversive effect in societies not dominated by American popular culture.

Sex and Reason was first published in 1992 by Harvard University Press.

Sex and Reason received a positive review from A. W. B. Simpson in The Times Literary Supplement, mixed reviews from the philosopher Martha Nussbaum in The New Republic, Marek Kohn in New Statesman and Society, the lawyer Tony Honoré in the London Review of Books, and the legal scholar Jedediah Purdy in The American Prospect, and negative reviews from Elizabeth Kristol in The American Spectator and the philosopher Roger Scruton in National Review. Other discussions include those from Don Herzog in The New York Times Book Review, M. Gordon in Choice, The Chronicle of Higher Education, and the legal scholar Patricia J. Williams in The Nation.

Simpson described the book as "highly readable" and wrote that it was "rich in its ingenuity and humane in its orientation" and "based on a genuine attempt to derive rational guidance for social policy from an immense literature". He credited Posner with being able to "pursue his analysis without using any weak buttressing arguments" and with resisting "the temptation to push the analysis too far" and with being "aware of the limits of economic analysis".