Richard Thaler nudges the broccoli argument in the ACA litigation down a slippery slope:

One pernicious category of imaginary risks involves those created by users of the dreaded “slippery slope” arguments. Such arguments are dangerous because they are popular, versatile and often convincing, yet completely fallacious. Worse, they are creeping into an arena that should be above this sort of thing: the Supreme Court, in its deliberations on health care reform. . . .

Of course, it sometimes happens that society takes successive steps in one direction. Although women don’t control the government, there is thankfully much more gender — and racial — equality now than a century ago. But I think that few would be tempted to compare the pace of that progress with that of an skier on a downhill training run.

Eugene Volokh, a legal scholar at the University of California, Los Angeles, has written extensively on the theoretical mechanisms that could cause such effects. But he rightly notes that the topic “cries out for empirical research” to determine whether slope risk is predictable. Such research does not exist, he says, perhaps because it is so hard to do. . . .

Given how flimsy slippery-slope arguments can be, it is downright scary that they might play an important role in the Supreme Court decision on the new health care law. The case before the court is whether it is constitutional for the federal government to penalize people who fail to buy health insurance.

As everyone concedes, we can’t include the popular rule that forbids insurance companies from discriminating against people with pre-existing conditions unless we encourage nearly everyone to buy health insurance. Although most legal scholars seem to think that the law is constitutional, there is considerable question about whether the Supreme Court will rule that way. And the slippery-slope arguments being used here are just wacky. . . .

More generally, we would be better off as a society if we could collectively agree to ignore all slippery-slope arguments that aren’t accompanied by evidence that said slope exists. If you are opposed to a policy, state your case based on the merits — not on the imagined risk of what else might happen down the road. The path of that road is so unpredictable that it may even produce a U-turn.

And Thaler takes on head-long the broccoli argument advanced by Justice Scalia:

Consider these now-famous comments about broccoli from Justice Antonin G. Scalia during the oral arguments. “Everybody has to buy food sooner or later, so you define the market as food,” he said. “Therefore, everybody is in the market. Therefore, you can make people buy broccoli.” Showing remarkable restraint, he did not mention anything about ending up in a roadside ditch.

Justice Scalia is arguing that if the court lets Congress create a mandate to buy health insurance, nothing could stop Congress from passing laws requiring everyone to buy broccoli and to join a gym. He and Chief Justice John G. Roberts Jr. were asking the solicitor general to explain what the principle would be to stop the government from going so far. If the law stands, Justice Roberts suggested, “it seems to me that we can’t say there are limitations on what Congress can do under its commerce power.” He added, “Given the significant deference we accord to Congress in this area, all bets are off, and you could regulate that market in any rational way.”

Please stop! The very fact that a slippery slope is being cited as grounds for declaring the law unconstitutional — despite that “significant deference” usually given to laws passed by Congress — tells you all that you need to know about the argument’s validity. Can anyone imagine Congress passing a broccoli mandate law, much less the court allowing it to take effect?

The irony is that Justices Roberts and Scalia are warning of a risk that they and their colleagues have the power to prevent. Surely, the justices have the conceptual resources to draw a distinction between the health care market and the market for broccoli. And even if they don’t, then all the briefs, the zillions of blog posts and a generation’s worth of economic literature can help them.

Totally as an aside, Thaler uses the example of the DirectTV commercials as an example of slippery slope logic–I compared it to Rational Basis Analysis:

There is a DirecTV ad that humorously illustrates the basic form of the slippery-slope argument. A foreboding announcer intones a list of syllogisms that are enacted on screen: “When your cable company puts you on hold, you get angry. When you get angry, you go blow off steam. When you go blow off steam, accidents happen.” Later, we reach the finale: “You wake up in a roadside ditch. Don’t wake up in a roadside ditch.”

Although this ad is intended to be funny, arguments that make no more sense can and do affect public policy. The idea is that while Policy X may be acceptable, it will inevitably lead to the terrible Outcome Y, so it is vital that we prevent Policy X from ever being enacted. The problem is that such arguments are often made without any evidence that doing X makes Y more likely, much less inevitable. What percentage of people who are left on hold on the telephone end up in a roadside ditch?