The correspondence bias is the tendency to draw inferences about a person's unique and enduring dispositions from behaviors that can be entirely explained by the situations in which they occur.
     —Gilbert and Malone

We tend to see far too direct a correspondence between others' actions and personalities.  When we see someone else kick a vending machine for no visible reason, we assume they are "an angry person".  But when you yourself kick the vending machine, it's because the bus was late, the train was early, your report is overdue, and now the damned vending machine has eaten your lunch money for the second day in a row.  Surely, you think to yourself, anyone would kick the vending machine, in that situation.

We attribute our own actions to our situations, seeing our behaviors as perfectly normal responses to experience. But when someone else kicks a vending machine, we don't see their past history trailing behind them in the air.  We just see the kick, for no reason we know about, and we think this must be a naturally angry person—since they lashed out without any provocation.

Yet consider the prior probabilities.  There are more late buses in the world, than mutants born with unnaturally high anger levels that cause them to sometimes spontaneously kick vending machines.  Now the average human is, in fact, a mutant.  If I recall correctly, an average individual has 2-10 somatically expressed mutations.  But any given DNA location is very unlikely to be affected.  Similarly, any given aspect of someone's disposition is probably not very far from average.  To suggest otherwise is to shoulder a burden of improbability.

Even when people are informed explicitly of situational causes, they don't seem to properly discount the observed behavior.  When subjects are told that a pro-abortion or anti-abortion speaker was randomly assigned to give a speech on that position, subjects still think the speakers harbor leanings in the direction randomly assigned.  (Jones and Harris 1967, "The attribution of attitudes.)

It seems quite intuitive to explain rain by water spirits; explain fire by a fire-stuff (phlogiston) escaping from burning matter; explain the soporific effect of a medication by saying that it contains a "dormitive potency".  Reality usually involves more complicated mechanisms: an evaporation and condensation cycle underlying rain, oxidizing combustion underlying fire, chemical interactions with the nervous system for soporifics.  But mechanisms sound more complicated than essences; they are harder to think of, less available.  So when someone kicks a vending machine, we think they have an innate vending-machine-kicking-tendency.

Unless the "someone" who kicks the machine is us—in which case we're behaving perfectly normally, given our situations; surely anyone else would do the same.  Indeed, we overestimate how likely others are to respond the same way we do—the "false consensus effect".  Drinking students considerably overestimate the fraction of fellow students who drink, but nondrinkers considerably underestimate the fraction.  The "fundamental attribution error" refers to our tendency to overattribute others' behaviors to their dispositions, while reversing this tendency for ourselves.

To understand why people act the way they do, we must first realize that everyone sees themselves as behaving normally.  Don't ask what strange, mutant disposition they were born with, which directly corresponds to their surface behavior.  Rather, ask what situations people see themselves as being in.  Yes, people do have dispositions—but there are not enough heritable quirks of disposition to directly account for all the surface behaviors you see.

Suppose I gave you a control with two buttons, a red button and a green button.  The red button destroys the world, and the green button stops the red button from being pressed.  Which button would you press?  The green one.  Anyone who gives a different answer is probably overcomplicating the question.

And yet people sometimes ask me why I want to save the world.  Like I must have had a traumatic childhood or something.  Really, it seems like a pretty obvious decision... if you see the situation in those terms.

I may have non-average views which call for explanation—why do I believe such things, when most people don't?—but given those beliefs, my reaction doesn't seem to call forth an exceptional explanation.  Perhaps I am a victim of false consensus; perhaps I overestimate how many people would press the green button if they saw the situation in those terms.  But y'know, I'd still bet there'd be at least a substantial minority.

Most people see themselves as perfectly normal, from the inside.  Even people you hate, people who do terrible things, are not exceptional mutants.  No mutations are required, alas.  When you understand this, you are ready to stop being surprised by human events.