People, as a species, seem to be prone to superstition - an irrational belief that something mundane has supernaturally significance. We often see it with athletes, who tend to develop elaborate superstitions about what will help them win. Examples include eating specific things, wearing specific objects or jewelry, not shaving, etc.
While reporters talk about such superstitions leading up to big games, nobody considers them particularly odd or unusual. Sure, they may make little sense and have no evidence to support them, but we understand because we all have had some kind of superstition at some point in our lives.
Two things strike me about superstitions: they illustrate the fact that all of us sometimes believe irrational things, and beg the question of how we distinguish between "normal" irrationality, and "abnormal" irrationality - i.e., mental illness.
First the "normal" side: I think people tend to revert to superstition, and other forms of irrationality and magical thinking, when they can't make sense of events using more rational explanations. For example, superstitions may be a way to cope with the reality that something is unpredictable, or beyond one's control, by imposing a system of logic and control not based in reality. The difference between such thinking and delusion is that most of us know that we're being superstitious or irrational - we know, for example, that not shaving during the playoffs probably has very little bearing on the outcome. However, nobody wants to be the one to break the superstition, just in case it somehow changes the outcome. After all, we know from placebos that believing something can cause real change.
In contrast, mental illness tends to be characterized by the lack of awareness of irrationality, or belief that the irrational is actually rational. While most of us accept and respond to data that contradicts our irrational thinking, delusions tend to be impervious to contradictory data, and distort facts so that they are perceived as supporting evidence.
Another difference is the degree of upset caused by threats to an irrational system of thought or behavior: challenging delusions or interfering with compulsive behavior may produce intense anxiety or anger, in contrast to mild anxiety or irritation when we're prevented from following a superstition. Similarly, while superstitions tend not to take up much of our time and energy, the irrational beliefs characterizing mental illness tend to take up a great deal of one's time, energy, and thinking, thereby interfering with functioning.
Lastly, it's worth noting that what is considered irrational, and what distinguishes "normal" versus "abnormal" irrationality, is influenced by one's cultural context and related worldview. Just because we consider something false doesn't mean it's not widely believed in another culture, and definitely doesn't make it delusional!
How do you think about "normal" irrationality, and how would you differentiate between "normal" and "abnormal" degrees of irrationality?