So why do conspiracy theories flourish? Though some of the conspiracy-mongers described here appear to be simple crackpots or people out to make a fast buck with one of those laughably titled books that aspire to clutter up the best-seller lists (“Henry Kissinger: Soviet Agent,” “Diana: The Killing of a Princess,” “Rule by Secrecy: The Hidden History That Connects the Trilateral Commission, the Freemasons and the Great Pyramid”), Mr. Aaronovitch tries to provide the reader with a carefully reasoned anatomy of the phenomenon in these pages.

He not only notes the appeal of narrative and causality in a frighteningly random world  something readers of Thomas Pynchon’s novels well know  but also argues that overarching theories tend to be “formulated by the politically defeated and taken up by the socially defeated.”

These losers “left behind by modernity,” he writes, “can be identified in the beached remnants of vanished European empires; the doomed bureaucrats, the White Russians and the patriotic German petit bourgeois. They are the America firsters, who got the war they didn’t want; the Midwest populists watching their small farmers go out of business; the opponents of the New Deal; the McGovern liberals in the era of Richard Nixon; British socialists and pacifists in the decade of Margaret Thatcher; the irreconcilable American right during the Clinton administration; the shattered American left in the time of the second Bush.

“If it can be proved that there has been a conspiracy, which has transformed politics and society, then their defeat is not the product of their own inherent weakness or unpopularity, let alone their mistakes; it is due to the almost demonic ruthlessness of their enemy.”

It’s not surprising, then, that conspiracy theories thrive in times of change, uncertainty and economic stress, and that the designated villains often conform to enemies in “American populist folklore.” Of the era of McCarthyism and the venom aimed at supposed Communist sympathizers, Mr. Aaronovitch writes: “They were East Coasters or Hollywooders; they were educated; they were city dwellers; they liked art and fancy music; they were separate from  and unsympathetic to  the daily travails of the American little man.”

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These days a similar sort of antipathy is directed at President Obama, the Democratic Party and the mainstream news media by the Tea Party movement and by so-called birthers, who question whether Mr. Obama was born in the United States.