If you had eaten great meals at a local restaurant every week for ten years, and then, shockingly, last week you were served a salad with a long, curly hair in it, would you go back this week? Probably not. Why can one bad experience outweigh so many good experiences? Because you, like all human beings, are subject to the psychological phenomenon known as the negativity bias. We unconsciously pay more attention to and give more weight to negative experiences than positive ones, because our brains react more quickly and more powerfully to negative information than they do to positive information.
Daniel Kahneman explains why this is: “The brains of humans and other animals contain a mechanism that is designed to give priority to bad news. By shaving a few hundredths of a second from the time needed to detect a predator, this circuit improves the animal’s odds of living long enough to reproduce.” In other words, it was more important for our ancestors to be able to avoid a threat quickly than to gain a reward. If they missed a reward (say, a tasty rabbit), it wasn't too big a deal; there would always be more rabbits. But if they weren't able to avoid a threat, they might end up dead. Natural selection slowly shaped us to be on alert at all times, hyper-aware of anything that might cause us harm.
In a 1998 study, researcher John Cacioppo set out to test how negative and positive stimuli are processed differently by the brain. He showed participants pictures that would arouse in them positive feelings (e.g. a picture of a Ferrari), negative feelings (e.g. a mutilated face), and neutral feelings (e.g. an electrical outlet). Meanwhile, he and his team recorded electrical activity in the brain’s cerebral cortex to show the magnitude of the information processing taking place. The results noted that the participants' brains showed greater electrical activity toward the negative stimuli than they did to the positive or neutral stimuli. This finding demonstrates that negativity bias isn't a conscious choice but, as Kahneman says above, is instead something that happens deep in our unconscious mind.
In order to stay alive and reproduce, our ancestors evolved to focus on the negative, and that legacy remains with us today. And, like many of the other cognitive biases, the negativity bias that helped us in the past can often lead us to trouble today. After all, our ancestors on the savanna might have had to contend with predators and rival bands, but they didn't have to worry about the stock market, international politics, or other potential threats to our wellbeing—all frequently blown far out of proportion by the negativity bias.