I would have scarcely believed it possible, but a few short hours after teaching the naturalistic fallacy in my Philosophy of Biology class, I was exposed to an argument–from a professional philosopher–that, roughly, same-sex marriage is problematic because a) marriage is all about procreation and the raising of children and because b) evolution tell us that reproductive success is important, therefore: Gay marriage should be frowned upon. This resistance then, has nothing to do with religion, God, or the divine sanctification. Rather, it is the scientific thing to do: resist gay marriage because it is against evolutionary demands made on us as a species. This means that active disapproval of homosexuality–societal and legal discrimination for instance–is an expression of a biological instinct and should not be condemned as a moral failing.
The outlines of this argument should be familiar to most folks. It has been made time and again and despite having been spectacularly debunked, it rises again and again, like a zombie, or your favorite refusing-to-die cinematic ghoul.
What this argument attempts–and fails–to do is derive a proposition with normative import from a set of propositions that are purely descriptive. This–as David Hume pointed out a long time ago in his A Treatise of Human Nature—is an instance of the naturalistic fallacy, an attempt to bridge the is-ought gap:
In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary ways of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when all of a sudden I am surprised to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, ’tis necessary that it should be observed and explained; and at the same time that a reason should be given; for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it.
This fallacy manifests itself in the current situation as follows. There are biological facts about us: We reproduce, we pass on our genes, various reproductive strategies are adopted, some work better than the others (in securing more offspring to whom we can pass on our genes). This much can be ascertained by observation and measurement. But what should we do on noting these observations? The proponent of the argument noted above, wants to derive the following: Those reproductive strategies that work ‘better’ are ‘good’, and therefore should be encouraged, should be praised. The rest should be condemned. (Marriage, it will be noted, has been admitted as a successful reproductive strategy; this is a matter of empirical assessment and could well turn out to be false.)
But whence ‘better’, whence ‘good’? Why is ‘reproductive success’ a moral good to be sought? What is the source of that valuation and why is it allowed to override other values in the derivation above? Might we be allowed to admit other values in arriving at an alternative conclusion? Like, for instance, a more tolerant society is a ‘better’ society than one that isn’t? But then, we would be opening up a debate–conducted within some broad ethical and moral frameworks–on valuation, which is precisely what our protagonist didn’t want. He merely wanted the straightforward elevation of reproductive success to the preeminent moral value without further debate.
The tireless proponents of the so-called evolutionary arguments against same-sex marriage forget that efforts to read normative judgments off the historical workings out of the evolutionary process have as much difficulty in bridging the is-ought gap as any other species of argument. Calling upon biology here is not the scientifically sophisticated thing to do; it is merely to reveal one’s ignorance of the limitations of evolutionary explanation.