American physician whose popular treatise on birth control, the object of celebrated court actions in the United States and England, initiated the widespread use of contraceptives.

A graduate (M.D., 1824) of Dartmouth College, Hanover, N.H., Knowlton published anonymously a book advising couples of birth-control methods, The Fruits of Philosophy: or The Private Companion of Young Married People (1832), the first work of its kind in the United States. A second edition, bearing the author's name, appeared in 1833.

Although the treatise presented a temperate discussion of the medical, social, and economic aspects of birth control, it violated current conventions of taste. Prosecuted in the United States, Knowlton was fined in Taunton, Mass. (1832), and imprisoned for three months in Cambridge the same year. A second prosecution at Greenfield, Mass., resulted in his acquittal. The Fruits of Philosophy became the object of a famous test case in England, The Queen v. Charles Bradlaugh and Annie Besant, which was eventually (1877) decided in favour of the defendants. After this trial, sales of the book rose from fewer than 1,000 a year to more than 250,000 a year.