Modern science originated from an attempt to weed out such subjective lapses—what that great 17th century theorist of the scientific method, Francis Bacon, dubbed the "idols of the mind." Even if individual researchers are prone to falling in love with their own theories, the broader processes of peer review and institutionalized skepticism are designed to ensure that, eventually, the best ideas prevail.
Scientific evidence is highly susceptible to misinterpretation. Giving ideologues scientific data that's relevant to their beliefs is like unleashing them in the motivated-reasoning equivalent of a candy store.
Our individual responses to the conclusions that science reaches, however, are quite another matter. Ironically, in part because researchers employ so much nuance and strive to disclose all remaining sources of uncertainty, scientific evidence is highly susceptible to selective reading and misinterpretation. Giving ideologues or partisans scientific data that's relevant to their beliefs is like unleashing them in the motivated-reasoning equivalent of a candy store.
Sure enough, a large number of psychological studies have shown that people respond to scientific or technical evidence in ways that justify their preexisting beliefs. In a classic 1979 experiment (PDF), pro- and anti-death penalty advocates were exposed to descriptions of two fake scientific studies: one supporting and one undermining the notion that capital punishment deters violent crime and, in particular, murder. They were also shown detailed methodological critiques of the fake studies—and in a scientific sense, neither study was stronger than the other. Yet in each case, advocates more heavily criticized the study whose conclusions disagreed with their own, while describing the study that was more ideologically congenial as more "convincing."
Since then, similar results have been found for how people respond to "evidence" about affirmative action, gun control, the accuracy of gay stereotypes, and much else. Even when study subjects are explicitly instructed to be unbiased and even-handed about the evidence, they often fail.
And it's not just that people twist or selectively read scientific evidence to support their preexisting views. According to research by Yale Law School professor Dan Kahan and his colleagues, people's deep-seated views about morality, and about the way society should be ordered, strongly predict whom they consider to be a legitimate scientific expert in the first place—and thus where they consider "scientific consensus" to lie on contested issues.
In Kahan's research (PDF), individuals are classified, based on their cultural values, as either "individualists" or "communitarians," and as either "hierarchical" or "egalitarian" in outlook. (Somewhat oversimplifying, you can think of hierarchical individualists as akin to conservative Republicans, and egalitarian communitarians as liberal Democrats.) In one study, subjects in the different groups were asked to help a close friend determine the risks associated with climate change, sequestering nuclear waste, or concealed carry laws: "The friend tells you that he or she is planning to read a book about the issue but would like to get your opinion on whether the author seems like a knowledgeable and trustworthy expert." A subject was then presented with the résumé of a fake expert "depicted as a member of the National Academy of Sciences who had earned a Ph.D. in a pertinent field from one elite university and who was now on the faculty of another." The subject was then shown a book excerpt by that "expert," in which the risk of the issue at hand was portrayed as high or low, well-founded or speculative. The results were stark: When the scientist's position stated that global warming is real and human-caused, for instance, only 23 percent of hierarchical individualists agreed the person was a "trustworthy and knowledgeable expert." Yet 88 percent of egalitarian communitarians accepted the same scientist's expertise. Similar divides were observed on whether nuclear waste can be safely stored underground and whether letting people carry guns deters crime. (The alliances did not always hold. In another study (PDF), hierarchs and communitarians were in favor of laws that would compel the mentally ill to accept treatment, whereas individualists and egalitarians were opposed.)
Head-on attempts to persuade can sometimes trigger a backfire effect, where people not only fail to change their minds when confronted with the facts—they may hold their wrong views more tenaciously than ever.
In other words, people rejected the validity of a scientific source because its conclusion contradicted their deeply held views—and thus the relative risks inherent in each scenario. A hierarchal individualist finds it difficult to believe that the things he prizes (commerce, industry, a man's freedom to possess a gun to defend his family) (PDF) could lead to outcomes deleterious to society. Whereas egalitarian communitarians tend to think that the free market causes harm, that patriarchal families mess up kids, and that people can't handle their guns. The study subjects weren't "anti-science"—not in their own minds, anyway. It's just that "science" was whatever they wanted it to be. "We've come to a misadventure, a bad situation where diverse citizens, who rely on diverse systems of cultural certification, are in conflict," says Kahan.
And that undercuts the standard notion that the way to persuade people is via evidence and argument. In fact, head-on attempts to persuade can sometimes trigger a backfire effect, where people not only fail to change their minds when confronted with the facts—they may hold their wrong views more tenaciously than ever.
Take, for instance, the question of whether Saddam Hussein possessed hidden weapons of mass destruction just before the US invasion of Iraq in 2003. When political scientists Brendan Nyhan and Jason Reifler showed subjects fake newspaper articles (PDF) in which this was first suggested (in a 2004 quote from President Bush) and then refuted (with the findings of the Bush-commissioned Iraq Survey Group report, which found no evidence of active WMD programs in pre-invasion Iraq), they found that conservatives were more likely than before to believe the claim. (The researchers also tested how liberals responded when shown that Bush did not actually "ban" embryonic stem-cell research. Liberals weren't particularly amenable to persuasion, either, but no backfire effect was observed.)