Lack of Perspective

People are not perfect. This may not come as a surprise to you.

Evolutionarily speaking, we've become creatures of habit and specialized programming that causes us to be well-adapted to pretty much any livable climate on Earth. We're made to live in societies, on our own, in small groups, or in between.

But we're not perfect.

In fact, we're not even close. And all people share certain glaring design flaws that everyone runs up against now and then. It's in your best interest to be aware of that.

Consider the fact that we were not blessed with an abundance of perspective. This is one of our many Human Flaws.

Being reasonable beings, we reason from the data we have. If all I ever see are red cars, I'm going to assume that cars are naturally red. That there is no other color possible for them to be. If someone tells me differently, I might even have a great deal of trouble believing him.

It's unlikely that you're going to be in a situation where you're completely unaware of different tints of paint, but I'm sure you can see the application.

If you trust your sample, their actions are going to inform your expectations of the population.

We can draw two conclusions from this:

Different people may have different conclusions about pretty much everything based on what they've observed.
and
The smaller your sample, the larger the chance of you being misled by it.

There's a neat little Psychological trick that proves our point, known as the False Fame Effect. If someone is exposed to a name (say, "Sarah Gavin") several times and then enough time passes so the person forgets where he heard the name, he's much more likely to think Sarah Gavin is someone famous. This is because

His "sample" being "names he hears on a regular basis"
and his "expectation" being "names I hear often* are famous names"
thus, this name must be famous.

*that don't belong to someone I know personally or peripherally

This is a subset of the Misattribution Effect, but that's another story.

Consider something more likely to occur. Say a great portion of your college crowd wears wristbands noting their favorite charities. You're likely to assume that people of college age do this everywhere; this is normal behavior.

Because your sample is "people you know of this age range"
and a great deal of that sample comes from your college
and your expectation is "most people act relatively similar to the people I know"
thus, most people of this age must wear wristbands.

When in fact, this may not be the case.
This is nothing to beat yourself up about: everyone does this. That's why it's a Human Flaw, not a personal one. So it's appropriate to inform your actions based on the assumption that everyone is committing this mental misstep at all times.

Many students don't understand how people could act so "barbaric" (in their 21st century perspective) ages ago. Well, they were human just like us. And we haven't changed much.
This is a large part of racism today: if your sample of, say, Turks is fairly minimal, then a few bad experiences may color their entire people for you. The same could be said for Gypsies, Asians, and any other group you might think of.

It's important to keep track of your own biases: you may reason perfectly from your information, but that information may not be perfectly accurate.