One asshole can ruin a birthday party, but a single optimist doesn't have much of a shot at warming up a group of feuding friends. A conversation between a depressed person and a cheery one is more likely to leave both parties sad than happy. Why is that? Why is it so much harder to spread goodwill than negativity?

A cynic might argue that a pessimist spreads their viewpoint more easily because they're simply exposing the undeniable truth that the world is an awful place. I don't really buy that. Regardless of whether the world is awful or not, I feel like our tendency to take on the misery of jerks and reject the high-spirits of the merry runs deeper than any rational assessment of the world.

I was discussing this with a friend, and a result from game theory came to mind.

It has to do with evolution of strategies in the Prisoner's Dilemma. I'll let Wikipedia summarize the game:
Two suspects are arrested by the police. The police have insufficient evidence for a conviction, and, having separated the prisoners, visit each of them to offer the same deal. If one testifies for the prosecution against the other (defects) and the other remains silent (cooperates), the defector goes free and the silent accomplice receives the full 10-year sentence. If both remain silent, both prisoners are sentenced to only six months in jail for a minor charge. If each betrays the other, each receives a five-year sentence. Each prisoner must choose to betray the other or to remain silent.[...] How should the prisoners act?
The game is interesting because it's conceptually similar to many kinds of interpersonal interactions: if two people cooperate, they both do well; if one cooperates and the other defects, the defector does even better, and the other guy gets screwed; but if no one cooperates, nothing good ever comes of it. By studying the abstract game, we can learn something about relationships in the concrete world. So the question is, if a bunch of individuals play this game over and over, what strategy works the best?

The answer is (of course) that it depends on what everyone else is doing. In a room full of constant defectors, being cooperative isn't going to get you very far. Conversely, if everyone you meet is going to cooperate with you no matter what, defecting seems like a pretty appealing idea.

In fact, in a scheme where folks change their strategies over time, if you start with everyone cooperating, pretty soon someone will figure out to always defect, and before you know it, you've got a room full of back-stabbers, all a lot worse off than when they were cooperating. Although everyone will agree things were better when they cooperated, each individual became infected by their neighbors' refusal to play nice.

What about the reverse? In a population full of defectors, is there any hope of rediscovering cooperation? One strategy a would-be cooperator can employ is to initially cooperate, but start defecting as soon as their opponent defects on them. This is a very hard road to take - assuming you're the only one who ever cooperates, you'll simply get screwed the first time you play with someone, and after that things go exactly as if you'd defected to begin with. In this case, initially cooperating is strictly worse.

However, if two such idealists persevere long enough to meet, they'll start by cooperating and will continue to reap the substantial rewards of cooperation every time they interact. If a few members of the population take this leap of faith to tentatively play nice, the payoff can more than make up for occasionally being the sucker. The defectors can then see how well the others are doing, and slowly adapt to their strategy until everyone is cooperating again. Then, of course, it all starts over.

The point is that when everyone cooperates, it's both completely rational and obvious to start defecting, and once one person goes bad, everyone follows suit. To be hopeful in an overwhelmingly negative setting is both irrational and quite painful in the short term, and even with a small handful of cooperators in the room it's not necessarily worth it to switch strategies. Building back that mutual trust is a slow process, if it happens at all.

Something strikes me as similar between this evolution of strategy and our tendency to absorb the negative and to deflect the positive. An individual's bad day can spread to friends, family, and coworkers almost immediately, but a fighting couple can spend a week stonewalling each other when internally all they really want is to get along - but it's really hard to do so. It's fascinating how much more difficult restoring that state of cooperation is compared to disrupting it in the first place.