As far as I’m concerned, the most interesting question about religion isn’t whether God exists but why so many people are religious. There are around 10,000 different religions, each of which is convinced that there’s only one Truth and that they alone possess it. Hating people with a different faith seems to be part of belief. Around the year 1500, the church reformer Martin Luther described Jews as a “brood of vipers.” Over the centuries the Christian hatred of the Jews led to pogroms and ultimately made the Holocaust possible. In 1947, over a million people were slaughtered when British India was partitioned into India for the Hindus and Pakistan for the Muslims. Nor has interfaith hatred diminished since then. Since the year 2000, 43 percent of civil wars have been of a religious nature.

Almost 64 percent of the world’s population is Catholic, Protestant, Muslim, or Hindu. And faith is extremely tenacious. For many years, Communism was the only permitted belief in China and religion was banned, being regarded, in the tradition of Karl Marx, as the opium of the masses. But in 2007, one-third of Chinese people over the age of 16 said that they were religious. Since that figure comes from a state-controlled newspaper, the China Daily, the true number of believers is likely at least that high. Around 95 percent of Americans say that they believe in God, 90 percent pray, 82 percent believe that God can perform miracles, and over 70 percent believe in life after death. It’s striking that only 50 percent believe in hell, which shows a certain lack of consistency. In the Netherlands, a much more secular country, the percentages are lower. A study carried out in April 2007 showed that in the space of 40 years, secularization had increased from 33 to 61 percent. Over half of the Dutch people doubt the existence of a higher power and are either agnostic or believe in an unspecified “something.” Only 14 percent are atheists, the same percentage as Protestants. There are slightly more Catholics (16 percent).

In 2006, during a symposium in Istanbul, Herman van Praag, a professor of biological psychiatry, taking his lead from the 95 percent of believers in the United States, tried to convince me that atheism was an “anomaly.” “That depends on who you compare yourself to,” I replied. In 1996 a poll of American scientists revealed that only 39 percent were believers, a much smaller percentage than the national average. Only 7 percent of the country’s top scientists (defined for this poll as the members of the National Academy of Sciences) professed a belief in God, while almost no Nobel laureates are religious. A mere 3 percent of the eminent scientists who are members of Britain’s Royal Society are religious. Moreover, meta-analysis has shown a correlation among atheism, education, and IQ. So there are striking differences within populations, and it’s clear that degree of atheism is linked to intelligence, education, academic achievement, and a positive interest in natural science. Scientists also differ per discipline: Biologists are less prone to believe in God and the hereafter than physicists. So it isn’t surprising that the vast majority (78 percent) of eminent evolutionary biologists polled called themselves materialists (meaning that they believe physical matter to be the only reality). Almost three quarters (72 percent) of them regarded religion as a social phenomenon that had evolved along with Homo sapiens. They saw it as part of evolution, rather than conflicting with it.