There are few subjects more timely than the one tackled by Susan Jacoby in her new book, “The Age of American Unreason,” in which she asserts that “America is now ill with a powerful mutant strain of intertwined ignorance, anti-rationalism and anti-intellectualism.”

For more than a decade there have been growing symptoms of this affliction, from fundamentalist assaults on the teaching of evolution to the Bush administration’s willful disavowal of expert opinion on global warming and strategies for prosecuting the war in Iraq. Conservatives have turned the term “intellectual,” like the term “ liberal,” into a dirty word in politics (even though neo-conservative intellectuals played a formative role in making the case for war against Iraq); policy positions tend to get less attention than personality and tactics in the current presidential campaign; and the democratizing influence of the Internet is working to banish expertise altogether, making everyone an authority on everything. Traditional policy channels involving careful analysis and debate have been circumvented by the Bush White House in favor of bold, gut-level calls, and reasoned public discussions have increasingly given way to noisy partisan warfare among politicians, commentators and bloggers alike.

Meanwhile, studies show that American students are falling behind students from other developed countries in science and math, and that ignorance of basic civics class fundamentals, not to mention basic liberal arts concepts, is widespread. Ms. Jacoby notes that two-thirds of Americans cannot name the three branches of government or come up with the name of a single Supreme Court justice. She cites one survey finding that American 15-year-olds rank 24th out of those from 29 countries in mathematical literacy, and another indicating that only 57 percent of adult Americans had read a nonfiction book in a year.

In “American Unreason” Ms. Jacoby, the author of earlier books like “Freethinkers: A History of American Secularism,” proposes to anatomize this dismaying phenomenon, while situating it in historical context. Her book is smart, well researched and frequently cogent  particularly in looking at the causes of American anti-intellectualism, past and present  but just as often the material is overly familiar, blandly reprising arguments made by Richard Hofstadter in “Anti-Intellectualism in American Life,” by Neil Postman in “Amusing Ourselves to Death,” and other, more recent studies, while failing to pull these observations together into a coherent, new argument.