Do women, on average, have a different profile of aptitudes and emotions than men? Were the events in the Bible fictitious — not just the miracles, but those involving kings and empires? Has the state of the environment improved in the last fifty years? Do most victims of sexual abuse suffer no lifelong damage? Did Native Americans engage in genocide and despoil the landscape? Do men have an innate tendency to rape? Did the crime rate go down in the 1990s because two decades earlier poor women aborted children who would have been prone to violence? Are suicide terrorists well educated, mentally healthy, and morally driven?  Are Ashkenazi Jews, on average, smarter than gentiles because their ancestors were selected for the shrewdness needed in  money lending? Would the incidence of rape go down if prostitution were legalized? Do African American men have higher levels of testosterone, on average, than white men? Is morality just a product of the evolution of our brains, with no inherent reality? Would society be better off if heroin and cocaine were legalized? Is homosexuality the symptom of an infectious disease?  Would it be consistent with our moral principles to give parents the option of euthanizing newborns with birth defects that would consign them to a life of pain and disability? Do parents have any effect on the character or intelligence of their children? Have religions killed a greater proportion of people than Nazism? Would damage from terrorism be reduced if the police could torture suspects in special circumstances? Would Africa have a better chance of rising out of poverty if it hosted more polluting industries or accepted Europe's nuclear waste? Is the average intelligence of Western nations declining because duller people are having more children than smarter people? Would unwanted children be better off if there were a market in adoption rights, with babies going to the highest bidder? Would lives be saved if we instituted a free market in organs for transplantation? Should people have the right to clone themselves, or enhance the genetic traits of their children?

Perhaps you can feel your blood pressure rise as you read these questions. Perhaps you are appalled that people can so much as think such things. Perhaps you think less of me for bringing them up. These are dangerous ideas — ideas that are denounced not because they are self-evidently false, nor because they advocate harmful action, but because they are thought to corrode the prevailing moral order. 

By "dangerous ideas" I don't have in mind harmful technologies, like those behind weapons of mass destruction, or evil ideologies, like those of racist, fascist, or other fanatical cults. I have in mind statements of fact or policy that are defended with evidence and argument by serious scientists and thinkers but which are felt to challenge the collective decency of an age. The ideas in the first paragraph, and the moral panic that each one of them has incited during the past quarter century, are examples. Writers who have raised ideas like these have been vilified, censored, fired, threatened, and in some cases physically assaulted. 

Every era has its dangerous ideas. For millennia, the monotheistic religions have persecuted countless heresies, together with nuisances from science such as geocentrism, biblical archeology,  and the theory of evolution. We can be thankful that the punishments have changed from torture and mutilation to the canceling of grants and the writing of vituperative  reviews. But intellectual intimidation, whether by sword or by pen, inevitably shapes the ideas that are taken seriously in a given era, and the rear-view mirror of history presents us with a warning. Time and again people have invested factual claims with ethical implications that today look ludicrous. The fear that the structure of our solar system has grave moral consequences is a venerable example, and the foisting of "Intelligent Design" on biology students is a contemporary one. These travesties should lead us to ask whether the contemporary intellectual mainstream might be entertaining similar moral delusions. Are we liable to be enraged by our own infidels and heretics whom history may some day vindicate? 

I suggested to John Brockman that he devote his annual Edge question to dangerous ideas because I believe that they are likely to confront us at an increasing rate and that we are ill equipped to deal with them. When done right, science (together with other truth-seeking institutions, such as history and journalism) characterizes the world as it is, without regard to whose feelings get hurt. Science in particular has always been a source of heresy, and today the galloping advances in touchy areas like genetics, evolution, and the environment sciences are bound to throw unsettling possibilities at us. Moreover, the rise of globalization and the Internet are allowing heretics to find one another and work around the barriers of traditional media and academic journals. I also suspect that a change in generational sensibilities will hasten the process. The term "political correctness" captures the 1960s conception of moral rectitude that we baby boomers brought with us as we took over academia, journalism, and government. In my experience, today's students — black and white, male and female — are bewildered by the idea, common among their parents, that certain scientific opinions are immoral or certain questions too hot to handle.

What makes an idea "dangerous"? One factor is an imaginable train of events in which acceptance of the idea could lead to an outcome that only recently has been recognized as harmful. In religious societies, the fear is that that if people ever stopped believing in the literal  truth of the Bible they would also stop believing in the authority of its moral commandments. That is, if today people dismiss the part about God creating the earth in six days, tomorrow they'll dismiss the part about "Thou shalt not kill." In progressive circles, the fear is that if people ever were to acknowledge any differences between races, sexes, or individuals, they would feel justified in discrimination or oppression. Other dangerous ideas set off fears that people will neglect or abuse their children, become indifferent to the environment, devalue human life, accept violence, and prematurely resign themselves to social problems that could be solved with sufficient commitment and optimism. 

All these outcomes, needless to say, would be deplorable. But none of them actually follows from the supposedly dangerous idea. Even if it turns out, for instance, that groups of people are different in their averages, the overlap is certainly so great that it would be irrational and unfair to discriminate against individuals on that basis. Likewise, even if it turns out that parents don't have the power to shape their children's personalities, it would be wrong on grounds of simple human decency to abuse or neglect one's children. And if currently popular ideas about how to improve the environment are shown to be ineffective, it only highlights the need to know what would be effective. 

Another contributor to the perception of dangerousness is the intellectual blinkers that humans tend to don when they split into factions. People have a nasty habit of clustering in coalitions, professing certain beliefs as badges of their commitment to the coalition and treating rival coalitions as intellectually unfit and morally depraved. Debates between members of the coalitions can make things even worse, because when the other side fails to capitulate to one's devastating arguments, it only proves they are immune to reason. In this regard, it's disconcerting to see the two institutions that ought to have the greatest stake in ascertaining the truth — academia and government — often blinkered by morally tinged ideologies. One ideology is that humans are blank slates and that social problems can be handled only through government programs that especially redress the perfidy of European males. Its opposite number is that morality inheres in patriotism and Christian faith and that social problems may be handled only by government policies that punish the sins of individual evildoers. New ideas, nuanced ideas, hybrid ideas — and sometimes dangerous ideas — often have trouble getting a hearing against these group-bonding convictions. 

The conviction that honest opinions can be dangerous may even arise from a feature of human nature. Philip Tetlock and Alan Fiske have argued that certain human relationships are constituted on a basis of unshakeable convictions. We love our children and parents, are faithful to our spouses, stand by our friends, contribute to our communities, and are loyal to our coalitions not because we continually question and evaluate the merits of these commitments but because we feel them in our bones. A person who spends too much time pondering whether logic and fact really justify a commitment to one of these relationships is seen as just not "getting it." Decent people don't carefully weigh the advantages and disadvantages of selling their children or selling out their friends or their spouses or their colleagues or their country. They reject these possibilities outright; they "don't go there." So the taboo on questioning sacred values make sense in the context of personal relationships. It makes far less sense in the context of discovering how the world works or running a country. 

Should we treat some ideas as dangerous? Let's exclude outright lies, deceptive propaganda, incendiary conspiracy theories from malevolent crackpots, and  technological recipes for wanton destruction. Consider only ideas about the truth of empirical claims or the effectiveness of policies that, if they turned out to be true, would require a significant rethinking of our moral sensibilities. And consider ideas that, if they turn out to be false, could lead to harm if people believed them to be true. In either case, we don't know whether they are true or false a priori, so only by examining and debating them can we find out. Finally, let's assume that we're not talking about burning people at the stake or cutting out their tongues but about discouraging their research and giving their ideas as little publicity as possible. 

There is a good case for exploring all ideas relevant to our current concerns, no matter where they lead. The very act of engaging in rational discourse presupposes a commitment to evaluating ideas on their intellectual warrant alone. Otherwise how could one even make the case that dangerous ideas should be discouraged, in the face of someone else arguing (as Dan Gilbert does in this volume) that the idea of discouraging ideas is itself morally dangerous? Should proponents of keeping dangerous ideas private then be forced to keep that idea private, because their opponents deem it to be dangerous? If not, why should the proponents' judgment about dangerousness and nondangerousness be granted a privilege they deny to others? The idea that ideas should be discouraged a priori is inherently self-refuting. Indeed, it is the ultimate arrogance, as it assumes that one can be so certain about the goodness and truth of one's own ideas that one is entitled to discourage other people's opinions from even being examined.  

Also, it's hard to imagine any aspect of public life where ignorance or delusion is better than an awareness of the truth, even an unpleasant one. Only children and madmen engage in "magical thinking," the fallacy that good things can come true by believing in them or bad things will disappear by ignoring them or wishing them away. Rational adults want to know the truth, because any action based on false premises will not have the effects they desire. Worse, logicians tell us that a system of ideas containing a contradiction can be used to deduce any statement whatsoever, no matter how absurd. Since ideas are connected to other ideas, sometimes in circuitous and unpredictable ways, choosing to believe something that may not be true, or even maintaining walls of ignorance around some topic, can corrupt all of intellectual life, proliferating error far and wide. In our everyday lives, would we want to be lied to, or kept in the dark by paternalistic "protectors," when it comes to our health or finances or even the weather? In public life, imagine someone saying that we should not do research into global warming or energy shortages because if it found that they were serious the consequences for the economy would be extremely unpleasant. Today's leaders who tacitly take this position are rightly condemned by intellectually responsible people. But why should other unpleasant ideas be treated differently? 

There is another argument against treating ideas as dangerous. Many of our moral and political policies are designed to pre-empt what we know to be the worst features of human nature. The checks and balances in a democracy, for instance, were invented in explicit recognition of the fact that human leaders will always be tempted to arrogate power to themselves. Likewise, our sensitivity to racism comes from an awareness that groups of humans, left to their own devices, are apt to discriminate and oppress other groups, often in ugly ways. History also tells us that a desire to enforce dogma and suppress heretics is a recurring human weakness, one that has led to recurring waves of gruesome oppression and violence. A recognition that there is a bit of Torquemada in everyone should make us wary of any attempt to enforce a consensus or demonize those who challenge it. 

"Sunlight is the best disinfectant," according to Justice Louis Brandeis's famous case for freedom of thought and expression. If an idea really is false, only by examining it openly can we determine that it is false. At that point we will be in a better position to convince others that it is false than if we had let it fester in private, since our very avoidance of the issue serves as a tacit acknowledgment that it may be true. And if an idea is true, we had better accommodate our moral sensibilities to it, since no good can come from sanctifying a delusion. This might even be easier than the ideaphobes fear. The moral order did not collapse when the earth was shown not to be at the center of the solar system, and so it will survive other revisions of our understanding of how the world works. 

In the best Talmudic tradition of arguing a position as forcefully as possible and then switching sides, let me now present the case for discouragingcertain lines of intellectual inquiry. Two of the contributors to this volume (Gopnik and Hillis) offer as their "dangerous idea" the exact opposite of Gilbert's: They say that it's a dangerous idea for thinkers to air their dangerous ideas. How might such an argument play out? 

First, one can  remind people that we are all responsible for the foreseeable consequences of our actions, and that includes the consequences of our public statements. Freedom of inquiry may be an important value, according to this argument, but it is not an absolute value, one that overrides all others. We know that the world is full of malevolent and callous people who will use  any pretext to justify their bigotry or destructiveness. We must expect that they will seize on the broaching of a topic that seems in sympathy with their beliefs as a vindication of their agenda.

Not only can the imprimatur of scientific debate add legitimacy to toxic ideas, but the mere act of making an idea common knowledge can change its effects. Individuals, for instance,  may harbor a private opinion on differences between genders or among ethnic groups but keep it to themselves because of its opprobrium. But once the opinion is aired in public, they may be emboldened to act on their prejudice — not just because it has been publicly ratified but because they must anticipate thateveryone else will act on the information. Some people, for example, might discriminate against the members of an ethnic group despite having no pejorative opinion about them, in the expectation that their customers or colleagues will have such opinions and that defying them would be costly. And then there are the effects of these debates on the confidence of the members of the stigmatized groups themselves. 

Of course, academics can warn against these abuses, but the qualifications and nitpicking they do for a living may not catch up with the simpler formulations that run on swifter legs. Even if they did, their qualifications might be lost on the masses.    We shouldn't count on ordinary people to engage in the clear thinking — some would say the hair-splitting — that would be needed to accept a dangerous idea but not its terrible consequence. Our overriding precept, in intellectual life as in medicine, should be "First, do no harm."

We must be especially suspicious when the danger in a dangerous idea is to someone other than its advocate.  Scientists, scholars, and writers are members of a privileged elite. They may have  an interest in promulgating ideas that justify their privileges, that blame or make light of society's victims, or that earn them attention for cleverness and iconoclasm. Even if one has little sympathy for the cynical Marxist argument that ideas are always advanced to serve the interest of the ruling class, the ordinary skepticism of a tough-minded intellectual (the mindset that leads us to blind review, open debate, and statements of possible conflicts of interest) should make one wary of "dangerous" hypotheses that are no skin off the nose of their hypothesizers. 

But don't the demands of rationality always compel us to seek the complete truth? Not necessarily. Rational agents often choose to be ignorant. They may decide not to be in a position where they can receive a threat or be exposed to a sensitive secret. They may choose to avoid being asked an incriminating question, where one answer is damaging, another is dishonest, and a failure to answer is grounds for the questioner to assume the worst (hence the Fifth Amendment protection against being forced to testify against oneself). Scientists test drugs in double-blind studies in which they keep themselves from knowing who got the drug and who got the placebo, and they referee manuscripts anonymously for the same reason. Many people rationally choose not to know the gender of their unborn child, or whether they carry a gene for Huntington's disease, or whether their nominal father is genetically related to them. Perhaps a similar logic would call for keeping socially harmful information out of the public sphere. 

As for restrictions on inquiry, every scientist already lives with them. They accede, for example, to the decisions of committees for the protection of human subjects and to policies on the confidentiality of personal information. In 1975 biologists imposed a moratorium on research on recombinant DNA pending the development of safeguards against the release of dangerous microorganisms. The notion that intellectuals havecarte blanche in conducting their inquiry is a myth.

Though I am more sympathetic to the argument that important ideas be aired than to the argument that they should sometimes be suppressed,  I think it is a debate we need to have. Whether we like it or not, science has a habit of turning up discomfiting thoughts, and the Internet has a habit of blowing their cover. Tragically, there are few signs that the debates will happen in the place where we might most expect it: academia. Though academics owe the extraordinary perquisite of tenure to the ideal of encouraging free inquiry and the evaluation of unpopular ideas, all too often academics are the first to try to quash them. The most famous recent example is the outburst of fury and disinformation that resulted when Harvard president Lawrence Summers gave a measured analysis of the multiple causes of women's underrepresentation in science and math departments in elite universities and tentatively broached the possibility that discrimination and hidden barriers were not the only cause. But intolerance of unpopular ideas among academics is an old story. Books like Morton Hunt's The New Know-Nothings and Alan Kors and Harvey Silverglate's The Shadow University have depressingly shown that universities cannot be counted on to defend  the rights of their own heretics and that it's often the court system or the press that has to drag them into policies of tolerance. In government, the intolerance is even more frightening, because the ideas considered there are not just matters of intellectual sport but have immediate and sweeping consequences. Chris Mooney, in The Republican War on Science, joins Hunt in showing how corrupt and demagogic legislators are increasingly stifling research findings they find inconvenient to their interests. 

The essays in the present volume offer a startling variety of stimulating thoughts. Some are frankly speculative, others are ideas about an unrecognized danger, and many are versions of Copernicus's original dangerous idea — that we are not the center of the universe, literally or metaphorically. Whether you agree or disagree, are shocked or blasé, I hope that these essays provoke you to ponder what makes ideas dangerous and what we should do about them.