Book excerpt: Defenders of marijuana use may seize on the ambiguity or absence of evidence for such damage and ignore any other effects on education or safety; those opposed to marijuana use may emphasize the possibility of chronic disease that is suggested by some laboratory findings and ignore the social, political, and economic costs of fighting a well-established custom. The Committee wishes to make clear what it regards as the limits of this report for the selection of policy alteratives. Scientific judgment can estimate the prevalence of different kinds of use, risks to health, economic costs, and the like under current policies and can try to project such estimates for new policies. It can come to some conclusions based on those estimates. But selection of an alternative is always a value-governed choice, which can ultimately be made only by the political process.