Sex has been in decline for more than 20 years in the pages of academic journals.
The fall has been matched by the steady rise of the word "gender", which now dominates social sciences, arts and humanities publications and is making ground in physical sciences, even biology.
Prudishness, political correctness and fashion may lie behind the trend, uncovered in a survey of the titles of more than 30 million journal articles published between 1945 and 2001.
David Haig, professor of biology at Harvard University, said that, in most cases, gender was now used as a synonym for sex. Yet before 1955, it appeared in scholarly texts only as a grammatical term. "It shows how broader social changes can influence the terminology academics use," he said.
The rise of "gender" owes its origins to a study of human hermaphrodites by John Money, a leading "sexologist" based at Johns Hopkins University in the US. He used the term in a paper published in 1955 to distinguish between the sex role and biological status of an individual.
Dr Money noted some years later: "The majority of people who contributed to this new meaning of gender were hermaphrodites or intersexes."
Between the mid-1960s and early 1980s, the term was adopted by feminists to distinguish between social and biological differences between the sexes.
It then soared in popularity in the 1980s, overtaking "sex" in arts and humanities articles by 1987 and conquering social sciences in 1990. It has made inroads into the natural sciences, though in 2000, "sex" was still twice as prevalent.
Professor Haig said: "Within some areas of academe, 'gender' became the politically correct term to use. But in the natural sciences, many did not know the connotations it had within feminist discourse and have just used it as a synonym for sex."
He added that "sex's" suggestion of sexual intercourse may also have contributed to its decline.
The study is published in the Archives of Sexual Behavior .