President Barack Obama was dead wrong last month in suggesting that law school educations should be only two years. The third year is not an expensive frill but a crucial resource in training lawyers for 21st-century challenges.

U.S. law is in the midst of an intellectual revolution. Supreme Court Justice Oliver Wendell Holmes saw it coming more than a century ago: “For the rational study of the law the blackletter man [who focuses on existing legal rules] may be the man of the present, but the man of the future is the man of statistics and the master of economics.”

Holmes’ future is our present. Today’s law school casebooks highlight opinions that display new patterns of legal argument based on economics and statistics as well as psychology and other social sciences. These pathbreaking decisions are written by judges of different political persuasions — liberals such as Stephen Breyer and Guido Calabresi and conservatives such as Frank Easterbrook and Richard Posner. They do not represent a passing political fad but are a central tendency of modern law.

If students are to engage with these opinions, they can’t merely repeat the blackletter rules announced by judges. They must confront fundamental issues: When do free markets fail the test of economic efficiency? When should efficiency be trumped by justice? When do impressive-looking statistics amount to fancy ways of lying?

Law schools already are taking these questions seriously. Yale University, for example, offers courses on the use and abuse of statistics, the implications of behavioral and financial economics for regulation, the significance of social psychology in the criminal justice system, the potential of political science in designing better decision-making institutions and the ways different contemporary theories of justice constrain the use of cost-benefit analysis. These discussions combine theory with concrete examples and provide a context for similar themes when they are encountered in more doctrinal courses. Cutting back to two years of study will put an end to these evolving trends and effectively push legal education back more than 75 years.

If Obama’s “cost-cutting” measure were adopted, it would impoverish American public life. Once two-year graduates move into practice, they won’t be able to deal adequately with bread-and-butter issues of antitrust, intellectual property or corporate law, let alone with the challenges of civil rights or environmental law.

It is frivolous to suppose that these lawyers would pick up the key skills on the job. Social science and statistics require systematic training, not a crash course in response to particular problems.

The predictable outcome will be massive professional retreat. Increasingly, lawyers will become secondary figures who prepare the way for “experts” to present the crucial arguments before administrative agencies, courts and legislatures. Decision-makers with two-year law degrees will proceed to rubber-stamp the expert testimony that seems most impressive because they aren’t prepared to test it in a serious way.

In contrast, if law schools redeem the promise of a three-year curriculum, their graduates will have something valuable to contribute to the larger conversation. They will never rival experts in their command of statistics and social science, but so long as they understand the basics they will be in a position to integrate technical insights into a broader understanding of the fundamental values of the American legal tradition.

Rigorous doctorate programs in economics or statistics — or even political science or public policy — increasingly focus on formal models and big data, pushing the lawyer’s emphasis on concrete problem-solving to the periphery. There is a big question, then, concealed by Obama’s modest proposal: Should the future of U.S. law be shaped through a conversation between lawyers and technocrats, or should it be dominated by technocrats alone?

Obama’s answer is surprising, given some of his personnel choices. In naming Cass Sunstein the chief White House official to guide his administration’s regulatory policies, Obama selected a preeminent spokesman for an integrated approach to law and social science. After Sunstein came Howard Shelanski — another bridge figure who complements his doctorate in economics with a law degree that introduced him to the complexity of the evolving tradition. This sensibility will be lost if a truncated curriculum satisfies itself with black-letter law.

Developments in Washington and the federal courts are only indicators of a general movement throughout the country. If courts and legislatures are to confront “stop-and-frisk” laws or No Child Left Behind legislation, it is imperative for them to deal with big data. But will they also root their response in a thoughtful understanding of legal principles?

We have come a long way since Alexis de Tocqueville emphasized how American lawyers dominated lawmaking in the 19th century. The days of lawyerly monopoly have passed, but modern law schools can help sustain the distinctive values of the legal tradition in a different world. It would be tragic if short-term cost-cutting makes it impossible to succeed in this long-term project.

Bruce Ackerman is a professor of law and political science at Yale University.