Let us assume that people usually do what they think is best, given what they know. But how do people come to know what they know? Russell Hardin's recent book addresses this question in light of an “economic theory of knowledge.” He sketches the theory in the first chapter and then applies it to a series of epistemic domains: science, voting, government, morality, institutions, religion, culture, and extremism, very few of which can be addressed here. Hardin's core argument is that ignorance is often not irrational, as many assume, because it may reflect a rational response to the high costs of acquiring new knowledge. Hardin's focus is the everyday knowledge of ordinary people, not the public knowledge certified by experts. He is primarily concerned with cognitive propositional knowledge (“knowing that”), rather than embodied knowledge associated with skills (“knowing how”). And he is interested in “the ways people come to hold their beliefs” (p. 2), rather than whether their beliefs are justified or true. In these respects, How Do You Know? echoes the sociology of knowledge, social epistemology, and science studies, traditions that he mentions only in passing (pp. 4, 152).

Hardin's economic theory of ordinary knowledge has two key elements, neither of which can be reduced to a narrow view of “economic” as related to prices or money. First, knowledge is a resource, and its value rests on “how it would matter to us and our behavior, with consequences broadly defined to include the full range of costs and benefits of coming to know that bit of knowledge and of putting it to use” (p. 5). When asking why someone knows something, “[f]or ordinary knowledge, the answer is generally that it is useful” (p. 32). Although the author understands “useful” quite broadly, and associates his approach with John Dewey's pragmatism (pp. 5, 26, 31), his emphasis on individual rational decisions clearly differs from Dewey's concern with constitutive social-epistemic relations.

Second, Hardin's economic approach “implies essentially that there are choices to be made” (p. 6), involving constraints of time, mental capacity, and other resources. These constraints mean that whenever we acquire some bit of knowledge, rather than either remain ignorant or acquire some other bit of knowledge, we inevitably encounter trade-offs. Whenever I learn something, I forgo the opportunity to learn or do something else. Knowledge is path dependent, and what seems to be a person's irrational refusal to learn may actually be a rational response to the fact that it is “mentally cheaper to question a bit of new knowledge than to jettison a lot of old knowledge” (p. 8).

Despite this focus on the usefulness of “a choosing agent's knowledge” (p. 26), one of Hardin's key arguments is that much of our knowledge was not chosen at all. Sometimes we may intentionally seek knowledge, but sometimes knowledge is imposed on us as indoctrination, and we also learn many things by accident; knowledge “rains on us while we are engaged in some other enterprise” (p. 7). Moreover, most of what we know we accept on the authority of others, because it would be impossible to personally investigate and verify all of our beliefs (pp. 11–12): “We sweep vast quantities of putative knowledge into the maw of our minds with hardly a second glance” (p. 13). Even natural scientists necessarily rely on the authoritative testimony of others (p. 28). Moreover, Hardin argues, most people do not sharply distinguish moral and factual knowledge with regard to their relative certainty or objectivity, and most of our moral knowledge also rests on authority (pp. 108–13, 186–87).

With regard to popular knowledge of science, Hardin's economic theory offers a distinctive way of defending ordinary knowledge against the dominance of scientific experts. An account of rational action should not compare ordinary people's actions with expert beliefs but, rather, with ordinary people's own beliefs. In the 2005 Kitzmiller decision, for example, a federal court declared unconstitutional a school board requirement to present intelligent design as an alternative to evolution, and many observers expressed dismay at the school board's scientific ignorance. But Hardin argues that the school-board members were “not blameworthy for not knowing more science” because they had little incentive to know it (p. 54). Rather, their fault was to cloak a religious agenda in scientific language.

Regarding ordinary knowledge of politics, Hardin challenges Anthony Downs's famous argument that it is rational for people not to vote because the costs of voting outweigh the benefits. For Hardin, most voters lack sufficient incentive to acquire knowledge of Downs's theorem, and so they do not know it is irrational for them to vote. Indeed, more than half generally do vote, and many feel regret if they do not vote and their party loses: “To feel regret about not voting because one's party loses makes no sense unless one supposes one might actually have made a difference” (p. 73). The author acknowledges but sets aside the possibility that people vote out of a sense of civic membership or moral obligation, rather than to “make a difference” in a narrowly instrumental sense (p. 64). He goes on to argue that even though people mistakenly think they have an incentive to vote, they know they lack an incentive to acquire the knowledge required to vote their interests (pp. 74–76, 81–82). He dismisses research purporting to show that voters use various proxy indicators to vote in accord with their interests, despite low political knowledge (pp. 67–68).

Hardin reinforces his skeptical account of popular political knowledge with a minimalist theory of representative government. Bringing together Madisonian constitutionalism, Millian liberty, and the “Austrian social theory” of Friedrich Hayek, as well as James Scott's critique of authoritarian state planning in Seeing Like a State (1998), Hardin argues for narrowing the agenda and scope of the state. Modern states seek to make citizens “legible” through the bureaucratic collection of population statistics: “Representative democracy and the census go hand in hand” (p. 88). But liberty and innovation depend on widely distributed knowledge and decentralized decision making. He thus argues that representation should be restricted to “fairly abstract, general policies” (p. 89), thereby increasing private liberty and reducing the information needs of voters. It is notable that Hardin contrasts his liberal constitutionalism with Soviet totalitarianism and Islamic fundamentalism, and not with European welfare states. Within this context, he argues that recent history confirms his view. Whereas Madison and Hayek lacked a real-world test for the decentralization they advocated, “the decentralist vision has been and is being tested, and it seems to be doing very well” (p. 99). One wonders whether Hardin penned these words prior to the current economic crisis.

For a book on the economics of knowledge, there is little discussion here of how economic power shapes popular knowledge. Nor does Hardin devote much attention to the politics of knowledge. For example, he does not discuss how commercial incentives and the politics of publicly funded science affect the kinds of research that get done (e.g., pharmaceuticals, cancer, or malaria), and thus how science shapes ordinary knowledge. Nonetheless, he offers an insightful lens on popular knowledge in society and politics.