Nobody will be surprised to learn that these opinions are not terribly well informed. Indeed the only thing the amici seem informed by is the impress of our country’s most up-to-the-minute intellectual fashion. Inevitably they rely, innocent and wide-eyed, on the same inconclusive social science that Kass and Mansfield warn against. The amici are not a skeptical or penetrating lot. The brief makes the obvious point that science offers no evidence of the harm that gay marriage may do; they do not make the equally obvious and complementary point that science offers no evidence of the good that supporters insist gay marriage will do for its couples, its children, or the larger society.

Their brief also vigorously invokes the advantages offered by marriage, as currently defined. And these advantages are real and well documented by social scientists of all stripes. The amici attribute such benefits to the stability that state-sanctioned marriage bestows on families, which is also true, as far as it goes. But you can’t help but wonder: If stability between same-sex couples is the issue at hand—the great social good we seek—why not institute civil unions that are as binding as the marriage contract, and avoid the radical social experiment of redefining marriage?

Well, the amici say, marriage is unique. And they’re right again. Marriage is many things, all at once—much more than a simple mechanism for stability between husband and wife. The institution that social science has been studying so exhaustively for so many years is of a singular kind, with singular features. It is an ancient practice grooved by tradition and myth, shaped by social expectations as old as civilization. It arises from the natural sexual complementarity of woman and man, and formalizes the possibility of procreation and the renewal of life.

There’s no way of knowing what combination of these singular features of marriage confers which of its demonstrated advantages, culturally and psychologically. We do know, however, that if the state suddenly creates the institution of gay marriage by fiat, the result will lack most of the features that make marriage unique—and uniquely beneficial. It will not be the same institution that has won the unanimous endorsement of social scientists.