Emotions and social interactions are perhaps the most human part of being a human.

What controls whether people are happy? You might expect that you would feel good when there are lots of good things in your life and feel bad when there are lots of bad things in your life, but happiness is actually quite relative. People who have something very good happen to them (like winning the lottery) or have something very bad happen to them (like becoming paraplegic) feel very strongly about it at first, but a year later they’re no more happy or unhappy than the average person.

Your feelings can have physical effects on your body - like smiling when you’re amused or sweating when you’re nervous - but the effect can also go the other way around. One experiment found that if people positioned their face muscles in a way that looks like smiling, they judged cartoon videos to be more positive. Another experiment found that when people perceived that their heart was beating faster they rated pictures of faces as more attractive (even though it wasn’t their actual heartbeat they were listening to, the experimenters just told them it was).

Social behaviors, just like other behaviors, aren’t always motivated by the reasons you think they are. If you ask people what they would do in a particular situation, they will often say that they would do the thing that seems socially “right” (and they actually believe that’s what they would do), but in reality they would often behave differently. Your willingness to stop and do considerate things for others depends partly on how nice you are, but it also depends a lot on whether you’re in a hurry to go do something else.

People are strongly influenced by the behavior of the other people around them. This is usually a good thing, and in many cases if you act too different from other people you will be perceived as rude or crazy. But in some cases, people conform in ways that seem inappropriate. If many other people agree on the answer to a question, sometimes you will act like you agree with them even if their answer is obviously wrong. And people are much more likely to do violent or illegal things if they know other people are doing them.

A related problem that happens in real life is called the bystander effect: when a lot of people see someone who needs help (such as someone who is being robbed or has fainted), the more people there are, the less likely each person is to try to help, partly because they see that other people around aren’t helping either.

People also have a tendency to obey authority figures, which, similarly, is usually good but can sometimes be dangerous. In one famous experiment, people were told by an experimenter to give increasingly large electric shocks to another person, and many of them obeyed even when they thought the other person was in extreme pain and possibly dying. In another famous experiment, people were told to act as “prisoners” and “guards”, and they played their roles for several days even when it involved degradation and humiliation.

If determining the motivations of your own feelings and behaviors is hard, it’s even harder to determine those of other people. Generally you are familiar with yourself and want to think positively of yourself, so when you do something “bad”, you’re likely to think of it as being caused by circumstances beyond your control, whereas if someone else does something “bad”, you’re more likely to think they did it on purpose and that’s just their personality.

Your impression of a person based on one attribute can affect your perception of their other attributes too - this is called the halo effect. The most common example of this is physical attractiveness: if someone is attractive, you will be more likely to think they have other positive qualities such as friendliness and intelligence.

When judging behavior, there are some things that people perceive as inherently “right” or “wrong”, even when they can’t explain why. To some extent these things vary across people and cultures (for example, whether it’s good or bad to lie in order to help a friend), but to some extent people generally agree about them (for example, pretty much everyone has a negative reaction to the idea of killing people for no reason or the idea of marrying your sister). Here are some examples of cases where people’s instinctive reactions are not straightforwardly explained by logic:

  • Suppose someone is driving drunk, and accidentally swerves off the road, killing a pedestrian. What should their punishment be? Now suppose that instead of a pedestrian, the person had hit a tree. What should their pubishment be in that case? Many people will say the driver who killed the pedestrian should be punished more, even though the driver’s behavior in the two cases was exactly the same.

  • Suppose you are a train controller and you know that a train’s brakes have failed, and if you don’t do anything it will run over five people who are on the tracks ahead; but there’s a lever you can pull to make the train take a different fork of the track so that it will only run over one person. Do you pull the lever? Now suppose that instead the lever will not divert the train but will open a trapdoor in a bridge above the track, causing a person to fall onto the track and get run over, but the force of hitting that person will prevent the train from hitting the other five people. Do you pull the lever in that case? Many people will say yes in the first case and no in the second, even though both involve choosing one person to die instead of five.

  • Suppose I give someone $10, and they have to offer you part of the $10, but they get to choose how much; if you accept the offer, you get to keep your part and they get to keep their part, but if you reject the offer, I take back the whole $10. If they offer you $1 (meaning they would keep $9), would you accept the offer? Many people will say no - you’re turning down a free dollar, but in exchange you’re preventing an unfair person from receiving $9.