We like to think of ourselves as special because we can reason and we like to think that this ability expresses the essence of what it is to be human. In many ways this belief has formed our civilization; throughout history, we have used supposed differences in rationality to justify moral and political distinctions between different races, genders, and species, as well as between “healthy” and “diseased” individuals. Even to this day, people often associate mental disorder with irrationality and this has very real effects on people living with mental disorders.
But are we really that rational? And is rationality really what distinguishes people who live with mental illness from those who do not? It seems not. After decades of research, there is compelling evidence that we are not as rational as we think we are and that, rather than irrationality being the exception, it is part of who we normally are.
So what does it mean to be rational? We usually distinguish between two kinds of rationality. Epistemic rationality, which is involved in acquiring true beliefs about the world and which sets the standard for what we ought to believe, and instrumental rationality which is involved in decision-making and behavior and is the standard for how we ought to act.
More here – Scientific American