Daniel Nettle

“This clarification of the nature of intelligence predicts that there will be no relationship at all between personality and intelligence, but research in the last decade has shown that this is not quite true. There are no very strong relationships between personality and intelligence, but some relationships there are, though debate about their nature and significance goes on. Most strikingly, though, in a couple of studies where relationships between Conscientiousness and intelligence have been found, they are not, as you might imagine, positive, but weakly negative. The smarter people are, the less conscientious they are.13 The most likely explanation for this is that people who are very sharp soon learn that they can get away with not preparing things too much in advance, not being overly disciplined with their time, and so on, since their quick abilities will get them through whatever academic and professional challenges they meet. Conversely, people who are not quite so quick have to use organization and discipline to achieve what some others might achieve carelessly. Thus, a behavioural style is developed that compensates for the level of intelligence, and so ends up inversely related to it. This means that there is no intrinsic genetic connection between low Conscientiousness and high intelligence. Rather, the weak negative correlation is something that emerges through development.”
― Daniel Nettle, Personality: What makes you the way you are

“However, robust evidence shows that people systematically overestimate the probability of positive future contingencies, and underestimate the probability of negative ones — only those who are depressed or dysphoric come to accurate assessments.”
― Daniel Nettle