I'd go so far as to say that anyone who advocates cooperating in a one-shot prisoners' dilemma simply doesn't understand the setting. By definition, defecting gives you a better outcome than cooperating. Anyone who claims otherwise is changing the definition of the prisoners' dilemma.

I think this is correct. I think the reason to cooperate is not to get the best personal outcome, but because you care about the other person. I think we have evolved to cooperate, or perhaps that should be stated as we have evolved to want to cooperate. We have evolved to value cooperating. Our values come from our genes and our memes, and both are subject to evolution, to natural selection. But we want to cooperate.

So if I am in a prisoner's dilemma against another human, if I perceive that other human as "one of us," I will choose cooperation. Essentially, I care about their outcome. But in a one-shot PD defecting is the "better" strategy. The problem is that with genetic and/or memetic evolution of cooperation, we are not playing in a one-shot PD. We are playing with a set of values that developed over many shots.

Of course we don't always cooperate. But when we do cooperate in one-shot PD's, it is because, in some sense, there are so darn many one-shot PD's, especially in the universe of hypotheticals, that we effectively know there is no such thing as a one-shot PD. This should not be too hard to accept around here where people semi-routinely accept simulations of themselves or clones of themselves as somehow just as important as their actual selves. I.e. we don't even accept the "one-shottedness" of ourselves.