Since the momentous events of Sept. 11, 2001, countless news stories, TV commentaries, and books have speculated on the causes of terrorism, the attitudes of Muslims, and a purported clash of civilizations between Islamic societies and the West.

What has not been available is any reliable measure of the viewpoints of ordinary Muslims, who constitute 20 percent of the global population.

That is no longer the case. Through an ambitious six-year project that involved hour-long, face-to-face interviews with residents in nearly 40 nations, Gallup has plumbed the perspectives of Muslim men and women – urban and rural, educated and illiterate, young and old.

The Gallup Poll of the Muslim World surveyed a representative sample of 90 percent of the world's 1.3 billion Muslims, the most comprehensive study ever done. The findings are explored in the new book "Who Speaks for Islam?" by John Esposito, Islamic studies professor at Georgetown University; and Dalia Mogahed, executive director of the Gallup Center for Muslim Studies in Washington.

Here are some of the key results, which frequently counter conventional wisdom: