Kluge: The Haphazard Construction of the Human Mind (Houghton Mifflin; 2008)

Given that humans are ten times [more] different than one another [than expected], it would seem that a four percentage point difference between the chimpanzee and the human genome could mean hundreds of times differences between each individual human and each individual chimpanzee. And this difference would demolish any reasonable defense of evolution .. . the more scientists find, the more the Bible is proven.

If there had been less genetic variation, the argument might well have run, "We are all made in God's image; therefore it's no surprise that our DNA is all the same," but in the quoted passage there seems to be no discernible logic relating the premise (that there is an unexpectedly large amount of genetic variation) to its rather antiscientific conclusion.

When people in power indulge in motivated reasoning, dismissing important signs of their own error, the results can be catastrophic. Such was probably the case, for example, in one of the great blunders in modern military history, in the spring of 1944, when Hitler, on the advice of his leading field marshal, Gerd von Rundstedt, chose to protect Calais rather than Normandy, despite the prescient lobbying of a lesser-ranked general, Erwin Rommel. Von Rundstedfs bad advice, born of undue attachment to his own plans, cost Hitler France, and possibly the entire Western Front.*

Why does motivated reasoning exist in the first place? Here, the problem is not one of evolutionary inertia but a simple lack of foresight. While evolution gave us the gift of deliberate reasoning, it lacked the vision to make sure we used it wisely: nothing forces us to be evenhanded because there was no one there to foresee the dangers inherent in pairing powerful tools of reasoning with the risky temptations of self-deception. In consequence, by leaving it up to our conscious self to decide how much to use our mechanism of deliberate reasoning, evolution freed us — for better or for worse — to be as biased as we want to be.

Even when we have little at stake, what we already know — or think we know — often further contaminates our capacity to reason and form new beliefs. Take, for example, the classic form of logic known as the syllogism: a formal deductive argument consisting of major premise, minor premise, and conclusion — as stylized as a sonnet:

All men are mortal.

Socrates was a man.

Therefore, Socrates was mortal.

*Why didn't von Rundstedt listen? He was too attached to his own strategy, an elaborate but ultimately pointless plan for defending Calais. Hitler, for his part, trusted von Rundstedt so much that he spent the morning of D-Day asleep, evidently untroubled by Rommel's fear that Normandy might be invaded.

Nobody has trouble with this form of logic; we understand the abstract form and realize that it generalizes freely:

All glorks are frum.

Skeezer is a glork.

Therefore, Skeezer is frum.

Presto — a new way for forming beliefs: take what you know (the minor and major premises), insert them into the inferential schema (all X's are Y, Q is an X, therefore Q is a Y), and deduce new beliefs. The beauty of the scheme is the way in which true premises are guaranteed, by the rules of logic, to lead to true conclusions.

The good news is that humans can do this sort of thing at all; the bad news is that, without a lot of training, we don't do it particularly well. If the capacity to reason logically is the product of natural selection, it is also a very recent adaptation with some serious bugs yet to be worked out.

Consider, for example, this syllogism, which has a slight but important difference from the previous one:

All living things need water.

Roses need water.

Therefore, roses are living things.