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Cyclical theory is any theory which purports to identify repeating cycles in the economy, the stock market, politics, social trends, or other things.
Several writers who have used cyclical theories in the past to write best-sellers making rash predictions now leave those books off their current résumés, for obvious reasons. This should tell you something about the track record of these theories.
- The "Kondratiev wave" or "K-wave" is a theory of Soviet economist Nikolai Kondratiev which claims to identify cycles of inflation and depression in capitalist economies. The USSR's communist government liked his theory, for obvious reasons (it lent itself to anti-capitalist critiques — although Kondratiev later ran afoul of Stalin's paranoia and was executed in one of Stalin's purges of the USSR Communist Party), but oddly, the K-wave has since seen "waves" of popularity among right-wing capitalist authors of "get rich quick" investment books purporting to show you how to get rich by anticipating future economic cycles.
- The "Elliot wave" is another claimed cyclical analysis of the stock market, this one invented during the Great Depression by Ralph N. Elliot, an American accountant.
- Many financial bestsellers are based on one or both of the Kondratiev or Elliot waves: The Great Depression of 1990 by Ravi Batra (1985), and Conquer the Crash by Robert Prechter (2002) are two examples.
- William Strauss and Neil Howe have a theory that societal trends run in four repeating cycles of about 15-20 years each (High, Awakening, Unraveling, and Crisis) which correspond to four generational types which also repeat. (The most recent "Awakening" (1960s-1970s) happened when the Baby Boomers were coming of age; the "Unraveling" (1980s-1990s) happened when Generation X slackers came of age while the thirty-and-forty something Baby Boomers were indulging in yuppiedom; the current "Crisis" is happening because Baby Boomers are now the elders in charge of society and ready for a crusade while a new younger generation has been brought up (like the G.I.s of World War II, the last "Crisis"), to work in teams and be joiners and civic do-gooders.)
- Edward R. Dewey founded the "Foundation for the Study of Cycles" in 1931 at the behest of then-President Herbert Hoover, in an attempt to determine the cause of the Great Depression. Dewey wrote a number of books on the subject claiming that both economics and wars happened in cycles. This foundation still exists today as a non-profit organization.
- R.E. McMaster overlaid the cyclical theories of Dewey, Elliot, Kondratiev and several others and noted these otherwise contradicting cycles all converged at "bad" peaks in 1982 in his 1978 book Cycles of War: The Next Six Years. He predicted this meant a major world war around 1982. McMaster is better known as an investment writer specializing in the commodities market, and should have stuck with his area of expertise since his world war prediction was, shall we say, a bit premature. (Not to be confused with the white supremacist book with a similar title by Richard Kelly Hoskins, see below.)
- Global warming denialists will often cite claims that world climate runs in hot and cold cycles, and use this to discredit anthropogenic global warming.
- Astrology and biorhythms are both forms of cyclical theory. They are claimed to be able to predict a person's mood, social life, love life, and whatnot.
- Arthur Schlesinger, Jr., an otherwise sensible advocate of political moderation and centrism, wrote The Cycles of American History (1986) in which he analyzed American politics in terms of repeating cycles of liberal progress followed by a conservative backlash. In the context of cyclical theory, this book isn't as bad as most and avoids most of the pitfalls of the genre, but still…
- War Cycles, Peace Cycles is a book by the white supremacist Richard Kelly Hoskins, which claims cycles in warfare and the economy exist and are caused by you-know-who.
 Examples of real cycles which are not pseudoscience
- The Earth rotates on its axis about once every 24 hours.
- Daily tides caused by the Moon's orbit around Earth
- The seasons of the year.
- The repeating cycles of an internal combustion engine, compact disc player, tire, etc. in operation.
- The water cycle.
The difference between the above and most pseudoscientific cyclical theories is that politics, economics, and sociology are by their nature subjective, unlike physics. Observations derived from these disciplines are rooted in opinion, not in empirical reality; hence, it is possible to construct just about any cyclical theory one wants from them.