Westermarck was born into an upper-class Finnish-Swedish family in Helsinki, Finland, on November 20, 1862. However, he lived much of his adult life in England, where he held an appointment as a professor in sociology at the London School of Economics (1907-1930), and in Morocco where he did extensive fieldwork in the 1890s and 1900s. He also had an academic career in Finland, as a professor of philosophy at the University of Helsinki (1906-1918) and at the Åbo Akademi University (1918-1932). After a long and distinguished career, he died on September 3, 1939.

Westermarck studied religions and religious rituals, in Morocco in particular, but his fame came to rest on his encyclopedic studies of the history of marriage, the cultural patterns of sexual conduct, sexual mores, and the nature of moral judgment. The most renowned of these studies are The History of Human Marriage (1891) and The Origin and Development of Moral Ideas, I-II (1906-1908).

Like evolutionary psychologists today, Westermarck was intrigued by the question as to how social, psychological, and biological dimensions interconnect and mold human life, in particular sexual life. He very much appreciated Havelock Ellis's research on sexual psychology, and he supported the emancipatory reforms attempted by Magnus Hirschfeld. However, he seriously doubted Sigmund Freud's theories of the Oedipus complex and the incest taboo.

It appears to have been an open secret at least among his colleagues that Westermarck had homosexual preferences, and he discussed the issue of homosexuality extensively in his studies of sexuality and moral norms. With wide-ranging data from history and anthropology, he was able to show that homosexuality had occurred throughout the ages and around the world.

In Westermarck's view, the institutions of Christianity were largely responsible for the oppression of homosexuality in the Western world. A convinced atheist who fought for the rights of free-thinkers, he welcomed the decreasing power of religion in the modern world.

Westermarck considered homosexuality a legitimate mode of human sexuality. He suggested that in some people a homosexual preference may well be congenital and should be regarded not as abnormal but as "only a feature in the ordinary sexual constitution of man."

Westermarck also regarded sexual desire as potentially malleable, at least to the extent that sexual orientation could become transformed by social factors such as habit and practice. Based on his studies in Morocco, he argued that sexual preferences may be more fluid and dependent on social scripting than previously thought.

Westermarck's studies attracted considerable attention in the early twentieth century, but subsequently became regarded as outdated and marginal to anthropological scholarship. He was a critical evolutionist who aspired to explain how certain social forms such as marriage had developed, but his conceptualizations of sexuality, society, and culture were rather static.

In the 1970s and 1980s, however, the proponents of constructionist views of homosexuality cited Westermarck among the scholars who, along with Ruth Benedict and Margaret Mead, paved the way for constructionist interpretations and more liberal views of homosexuality. In the last two decades there has been a revival of interest in his work, in particular among evolutionary psychologists.