There is a good discussion going at my original post on Mercier & Sperber's new paper on why reasoning may have evolved to support argument. Mercier himself is responding in the comments. I want to raise one additional point. I'm no evolutionary biologist or evolutionary psychologist--but I know something about basic issues in the field. And my problem is, I just don't know how this "argumentative theory of reasoning" fits into the whole debate over group vs. individual (or gene) selection. My piece on "motivated reasoning" assumed that our biases are ego protective, and a kind of self-defense mechanism. I even likened them to fight-or-flight at one point. The idea is that you rapidly apply what you think you know about the world to new situations, before even thinking consciously about it, because what you think you know is reliable and can protect you. This would presumably have once favored the fitness/survival of the individual. (Whether it does any more is an open question.) But Mercier & Sperber are saying that reasoning leads the individual into problems (no doubt about that) but can serve groups nicely. Are they thus proposing a group selection theory?