Gay marriage and the null hypothesis

Two controversial topics in one post!

Orin Kerr this week on Perry vs. Schwarzenegger:

Several of the key factual findings in Judge Walker’s opinion are in the form of predictions, not facts. For example, Judge Walker finds that “permitting same-sex couples to marry will not . . . otherwise affect the stability of opposite-sex marriages.” But real predictions have confidence levels. You might think you’re going to get an “A” on an exam next week, but that’s not a fact. It’s just a prediction, and there’s a hidden confidence level: Maybe there’s an 80% chance you’ll get that grade, or a 60% chance. Judge Walker’s prediction-facts have no confidence levels, however. He doesn’t say that there is an 87% chance that permitting same-sex marriage will not affect the stability of opposite-sex marriages. He says that it is now a fact — with 100% certainty — that that will happen.

I think Kerr is incorrect about Walker’s meaning.  When we say, for instance, that a clinical trial shows that a treatment “has no effect” on a disease, we are certainly not saying that, with 100% certainty, the treatment will not change a patient’s condition in any way.  How could we be?  We’re saying, instead, that the evidence before us gives us no compelling reason to rule out “the null hypothesis” that the drug has no effect.  Elliott Sober writes well about this in Evidence and Evolution.  It’s unsettling at first — the meat and potatoes of statistical analysis is deciding whether or not to rule out the null hypothesis, which as a literal assertion is certainly false!  It’s not the case that not a single opposite-sex marriage, potential or actual, will be affected by the legality of same-sex marriage; Walker is making the more modest claim that the evidence we have doesn’t provide us any ability to meaningfully predict the size of that effect, or whether it will on the whole be positive or negative.

This doesn’t speak to Kerr’s larger point, which is that Walker’s finding of fact might not be relevant to the case — California can outlaw whatever it wants without any evidence that the outlawed thing causes any harm, as long as it has a “rational basis” for doing so.  The key ruling here seems to be Justice Kennedy’s in Heller v. Doe, which says:

A State, moreover, has no obligation to produce evidence to sustain the rationality of a statutory classification. “[A] legislative choice is not subject to courtroom factfinding and may be based on rational speculation unsupported by evidence or empirical data.”

and later:

True, even the standard of rationality as we so often have defined it must find some footing in the realities of the subject addressed by the legislation.

I’m in the dark about what Kennedy can mean here.  If speculation is unsupported by evidence, in what sense is it rational?  And what “footing in the realities of the subject” can it be said to have?

More confusing still:   in the present case, the legislation at issue comes from a referendum, not the legislature.  So we have no record to tell us what kind of speculation, rational or not, lies behind it — or, for that matter, whether the law is intended to serve a legitimate government interest at all.  Maybe there is no choice under the circumstances but for the “rational basis” test to be no test at all, and for the courts to defer completely to referenda, however irrational they may seem to the judge?

(Good, long discussion of related points, esp. “to what extend should judges try to read voters’ minds,” at Crooked Timber.)