Seymour Martin Lipset, who ignored family pressure to be a dentist and instead became a pre-eminent sociologist, political scientist and incisive theorist on American uniqueness, died on Dec. 31 in Arlington, Va. He was 84.

The cause was a stroke, his wife, Sydnee, said.

Mr. Lipset’s convictions were shaped early, in the cauldron of leftist politics in New York City in the 1930s, a time when his poor immigrant family urged him to study dentistry in order to take over his uncle’s lucrative practice. Instead, as a young Trotskyist at City College, he became fascinated with the question of why the United States never had a major socialist party.

As he metamorphosed from political partisan to social scientist, his quest for an answer to that question — as well as the many others that logically followed — resulted in dozens of books and hundreds of articles. He became known for his argument that America’s ideology of individualism precluded socialism in the European form.

Francis Fukuyama, the political philosopher, wrote in 1997 in The New York Times Book Review that Mr. Lipset’s insights into ways that America was different from other nations made him “the most thoughtful contemporary authority on American exceptionalism.”