I'm about half-way through this fascinating book, conveniently available for free online, which is at the intersection of psychiatry and evolutionary psychology.  I don't have the time to do it justice, so I'm going to post a few choice excerpts here in the hope that those who are more prolific and insightful than I am will add further analysis.

Just to make sure it's clear how this all ties in to bias, I'll start with a bias-relevant section.  The book ties delusional behavior in with the theory of consciousness as primarily existing for social intelligence purposes, and thus malfunctions in our reading of the social facts such as human intention are what cause delusions:

But some people with delusions are entirely ‘normal’ except for the false belief, and the belief itself is neither impossible nor outlandish. Any other unusual behaviors can be traced back to that false belief. For instance, a man may have the fixed, false and dominating belief that his wife is having an affair with a neighbour. This belief may be so dominating as to lead to a large program of surveillance  - spying on his wife, searching her handbag, examining her clothes etc. Yet the same man may show no evidence of irrationality in other areas of his life, being able to function normally at work and socializing easily with acquaintances, so that only close friends and family are aware of the existence of the delusion. In such instances the delusion is said to be ‘encapsulated’, ie. sealed-off from other aspects of mental life, and these people are said to have a delusional disorder.

...

Delusions are typically stated to have three major defining characteristics. Firstly that a delusional belief is false, secondly that this false belief is behaviorally dominant, and thirdly that the false belief is resistant to counter-argument. All these characteristics are shown by delusional disorders, yet they occur in a context of generally non-pathological cognitive functioning.

Humans are extremely prone to ‘false’ beliefs, or at least beliefs that strike many or most other people as false. Some of these false beliefs are strongly held and dominate behavior. It is trivially obvious that humans are imperfect logicians operating for most of the time on incomplete information, so mistakes are inevitable. But it is striking that although everyone would acknowledge the imperfections of human reasoning, many of these false beliefs are not susceptible to argument. For example, deeply cherished religious and political beliefs are nonetheless based on little or no hard evidence, vary widely, yet may dominate a person’s life, and are sometimes held with unshakeable intensity. And religious and political beliefs may strike the vast majority of other people as obviously false.