The most well-understood and agreed upon personality trait in the FFM is neuroticism, or the tendency to experience negative emotions. The word is more popular as the name of a mental illness or irrational tendency and, to be honest, the FFM description of it is hard to distinguish from this. While we don’t know exactly how many negative emotions one should feel and so it’s not clear that neuroticism is an irrational or diseased response to the world, many other indicators point to it being a trait with no discernible upside. It’s hard to give a plausible reason why nature would select for neurotics, the trait is a good predictor of heart disease, and having no standard for how many negative emotions a person should feel seems to be an argument for never having them at all. Positive feelings are better than negative ones when all else is equal, and the absence of any standard makes this the case.
My N-levels are on the higher side of average while my wife’s are very low. I’m the one cursing at traffic while she tells me that it’ll all be fine and I’ll make the flight; I’m the one who talks to others by telling jokes about what went wrong that day in while she smiles about all the fun and interesting things she did and all the problems she solved; my insomnia gets caused by all the things that could go wrong or the things I did wrong and hers gets caused by thinking about all the things she has yet to plan or do. Since both of us are equally sincere and rational, it’s hard to see what the upside in my approach to the world is, and the more I monitor negative emotion the more it seems irrational and even an absence of insight. After all, things normally work out, most of what you think you botched are things that other people thought were either great or that they didn’t even notice, and even if neuroticism gave some insight it would be hard to argue that it was worth an increased risk of heart attacks.
So neuroticism is a confused and maladaptive response to the world – a cross to bear and (we can hope) to be retrained. It was a tolerated mistake in our evolutionary history, like our bad backs, relatively narrow hips, or our propensity to any other disease. All the ancient ethical systems sought to train us out of neuroticism: Stoicism insists that our responses are up to us and that we should reject the disheartening, Epicureanism saw the whole point of ethical habituation as minimizing psychic pain, and Christianity divinized the command to not worry, revealed that all works out for the good of those who love God, that and commands that we should rejoice always.