I have argued in various places that self-deception is not an adaptation evolved by natural selection to serve some function. Rather, I have said self-deception is a spandrel, which means it’s a structural byproduct of other features of the human organism. My view has been that features of mind that are necessary for rational cognition in a finite being with urgent needs yield a capacity for self-deception as a byproduct. On this view, self-deception wasn’t selected for, but it also couldn’t be selected out, on pain of losing some of the beneficial features of which it’s a byproduct.

Neil Van Leuween, Why Self-Deception Research Hasn’t Made Much Progress