Reason Is For Winning Arguments, Not For Finding Truth

And finally, why is reasoning so biased and motivated whenever self-interest or self-presentation are at stake? Wouldn't it be adaptive to know the truth in social situations, before you then try to manipulate?

The answer, according to Mercier and Sperber, is that reasoning was not designed to pursue the truth. Reasoning was designed by evolution to help us win arguments. That's why they call it The Argumentative Theory of Reasoning. So, as they put it, and it's here on your handout, "The evidence reviewed here shows not only that reasoning falls quite short of reliably delivering rational beliefs and rational decisions. It may even be, in a variety of cases, detrimental to rationality. Reasoning can lead to poor outcomes, not because humans are bad at it, but because they systematically strive for arguments that justify their beliefs or their actions. This explains the confirmation bias, motivated reasoning, and reason-based choice, among other things."

Another "reason" (ha!) why it's not enough to be smart. Via Edge: JONATHAN HAIDT: The New Science of Morality.