Over the past 15 years, Michael Shermer, 56, has created a cottage industry out of debunking everything from UFO sightings and paranormal experiences to religious faith.

In his new book, The Believing Brain: From Ghosts and Gods to Politics and Conspiracies—How We Construct Beliefs and Reinforce Them as Truths (Times Books), this "Apostle of Disbelief" goes a little further and tackles belief itself, as in, why do we believe in anything at all? It is, he says, a way of sorting and understanding the world, of trying to put meaning to patterns we encounter to explain the nature of life and mortality, to assess risks, and to reduce the natural world into a comprehensible template. And there is a fundamental conflict, he says, between the human predisposition to believe and what he sees as the more reliable scientific skepticism.

"What skepticism is, is science, and science is to assume that whatever claim someone is making is not true until proven otherwise," says Shermer, the founding publisher of Skeptic magazine and executive director of the Skeptics Society. "Unfortunately, that's the opposite of the way the brain works."

The human mind's default position is belief, he says, because that is the safest and least challenging approach.

"In a religious worldview in which everything happens for a reason, there are no random patterns, and that's comforting, in a way," Shermer says. "It's easier, faster, and more efficient to just assume that everything you encounter is real, until proven otherwise. Science is counterintuitive. It takes extra effort and energy. It's uncomfortable to challenge your preconceptions."

There was a time in his life when Shermer saw the invisible hand of God guiding human events. Raised in a nonreligious family, he became a born-again Christian fundamentalist in the early '70s as a teenager. "All of my friends were doing it," Shermer says. "It was kind of a peer group–influenced thing... once I was into it, I started getting into it intellectually."

His family treated his religious turn with amusement—"My sister called me a ‘Jesus freak' "—but everyone was happy when he veered into skepticism, he says, "because I quit evangelizing to them." But Shermer and theism also began parting ways as he left the reinforcing circle of fellow believers and entered grad school, where dissent and questions were welcomed as signs of intellectual rigor, not as the beginnings of apostasy.

Shermer, who eventually earned a Ph.D. in the history of science from Claremont Graduate University and has taught at several Los Angeles area colleges, began to embrace scientific inquiry as the best method with which to weigh the big questions of existence and behavior. That pursuit has led to a series of bestselling books, including Why People Believe Weird Things: Pseudoscience, Superstition, and Other Confusions of Our Time (W.H. Freeman, 1997), How We Believe: The Search for God in an Age of Science (W.H. Freeman, 2000), and others challenging what people believe.

Now an atheist, Shermer prefers to call himself a skeptic. "The problem with atheism is that it isn't a thing. There isn't anything to be, there's no set of tenets. Skeptics, we believe in science as a set of methods to understand the world. It's tried and true."

Beliefs, whether they are based on science or religion, determine how individuals and societies view and engage the world. Getting people to understand the roots of their own beliefs, he says, is crucial to building tolerance for followers of other, often contradictory, beliefs.

"I hope to get people introspective about their own worldviews, and get them to realize that they have the same cognitive biases as everyone else," Shermer says. "I hope, this way, to teach tolerance of other worldviews. Liberals and conservatives, Christians and Muslims—everybody. I'm trying to reach everybody."

Scott Martelle is the author of The Fear Within: Spies, Commies, and American Democracy on Trial (Rutgers University Press, 2011).