In my most recent post and others—and in chats with George Johnson and Robert Wright on—I rail against biological determinism and defend free will. Some critics accuse me of letting wishful thinking cloud my judgment when it comes to these issues. They say that objective reality is objective reality, regardless of our subjective attitudes toward it. "The man wants scientific results to conform to his notion of the way the world should be," the evolutionary biologist and free-will denier Jerry Coyne scolds me, "and that's always been a terrible mind-set for understanding nature." Actually, science itself demonstrates that our hopes and fears about reality often shape it. A few examples:

*Physicians have known for centuries that patients' expectations about a treatment can become self-fulfilling. This is the well-known placebo effect and its dark converse, the "nocebo" effect. Patients' emotions cannot cure cancer or schizophrenia, but they nonetheless produce robust, measurable effects. In "The Placebo Effect," published in Scientific American in 1998, the psychiatrist Walter Brown cites a study in which asthmatic patients breathed more easily after inhaling what they were told was a medicinal mist, which was actually just saltwater. Their breathing became labored and their lungs measurably constricted if they were told that the mist contained allergens that might exacerbate their asthma. Many psychiatrists acknowledge that the placebo effect accounts for most of the benefits of antidepressants and other psychiatric drugs. In fact, Brown recommended that placebo pills be prescribed for treating some depressed patients, because placebos are often just as effective as pharmaceuticals such as SSRIs (selective serotonin reuptake inhibitors), cost less and have fewer side effects.

*One of the reasons why racist theories of intelligence—which are the most repugnant manifestations of biological determinism—are so insidious is that they can become self-fulfilling when widely believed. As I mention in my previous post, 19th-century scientific claims that black Africans are inferior bolstered slavery, which of course prevented blacks from becoming educated and corroborated whites' racism. Today, racist and sexist stereotypes work in a more insidious way. According to the "stereotype threat" theory of the social psychologist Claude Steele, negative self-image can affect cognitive performance. He has shown that women score higher on mathematics tests after being informed that both genders perform equally well on the test. Blacks perform better on tests that they believe are unrelated to academic achievement. White students perform worse on a test after being told that Asian-Americans aced it.

*I've been hard on scientists—notably the anthropologist Richard Wrangham—who claim that war stems from innate male urges. I have two concerns about this theory: first, it doesn't have much empirical support; second, it may help perpetuate war. In the 1980s the psychologist David Adams sought to document the ill effects of biological theories of war. Adams and a colleague asked 126 students at Wesleyan University and other schools whether they thought "wars are inevitable because human beings are naturally aggressive." Thirty-three percent of the respondents replied "yes," and 40 percent agreed with the statement that "war is intrinsic to human nature." Adams found that students with these views were less likely to participate in antiwar and disarmament activities. In the same way, I suspect, politicians who believe that war is inevitable are more likely to support hawkish policies.

*A recent experiment shows that belief in free will has measurable consequences. The psychologists Kathleen Vohs and Jonathan Schooler asked subjects to read a passage by Francis Crick, co-discoverer of the double helix, that casts doubt on free will. Crick wrote in The Astonishing Hypothesis (Charles Schribner's Sons, 1993) that "although we appear to have free will, in fact, our choices have already been predetermined for us and we cannot change that." Subjects who read this passage were more likely to cheat on a test than control subjects who read a passage about brain science that did not mention free will. Mere exposure to the idea that we are not really responsible for our actions, it seems, can make us behave badly.

This finding supports a sensible defense of free will mounted by the philosopher Daniel Dennett in his 2003 book Freedom Evolves (Viking Adult, 2003). Dennett argues, first, that free will is "not what tradition declares it to be: a God-like power to exempt oneself from the causal fabric of the physical world." Free will, he contends, is an emergent property of the brain, like consciousness, that allows us to perceive, mull over and act on choices; in fact, choice, or even freedom, are reasonable synonyms for free will. Dennett calls free will "an evolved creation of human activity and beliefs" that humanity acquired recently as a consequence of language and culture as well as consciousness. Our free will grows along with our knowledge, material well-being and political freedom. Dennett's most subtle, profound point is that free will is both an "objective phenomenon" and dependent on our belief in and perception of it.

In other words, the more we value and believe in free will, freedom and choices, the more we actually have. This is both wishful thinking and an objective, empirical truth. Wishful thinking works!

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