You argue that we deceive ourselves all the time, but why do we do it?
One reason is to better deceive others. Deceiving consciously is cognitively demanding. I've got to invent a false story while being aware of the truth, it's got to be plausible, it cannot contradict anything you already know or are going to find out and I've got to be able to remember it so that I don't contradict myself.
This takes concentration and I may give off cues that I'm lying. If I try to slip something by you I may not be able to meet your gaze. For linguistic cues, there are more pauses and fillers while I try to come up with my story. I'll choose simple action words and avoid qualifiers. Another thing that gives us away us is the effort to control ourselves. Let's say I'm coming to a key word in a lie. I tense up, but tensing up automatically raises my voice. That's a very hard thing to fight.
So believing the lie yourself can help with this cognitive burden?
Yes. If I can render all or part of the lie unconscious I can remove the cues that I'm deceiving you. So that's one kind of general reason to practice self-deception: to render the lie unconscious, the better to hide it.
In your new book you get into some quite serious stuff about how self-deception fuels warfare and other evils...
Regarding warfare, if you can get the group believing the same deception, you have a powerful force to impose group unity. And if you've sold the population a false historical narrative, say "the German people need room in which to live", then it's relatively easy to couple marching orders to the delusion.
Tell me about the relationship between self-deception and religion.
It's complex. At one extreme you could say religion is complete nonsense, so the whole thing is an exercise in self-deception. I was raised as a Presbyterian and I occasionally attend. I stand back and I read the creed that I was taught as a child and it's utter, utter nonsense. But could it have spread so far by self-deception alone? Religion has been selected for. It has given many benefits to people - health benefits, cooperative benefits. So I take an intermediate position.