What you think is “average” or “normal” is a function of what’s normal for you. Both because we tend to generalize from the self, and because birds of a feather flock together.
In terms of generalizing from the self, people have a tendency to consider themselves dead average in a lot of categories, and don’t think the range is that wide. Even if you know you’re above- or below-average at something, you still don’t think that the far ends of the spectrum are that different from you.
But this is also a factor of what your life looks like. If you’re a pro athlete, you probably know a lot more other pro athletes than the average person who isn’t one. As a result, most people you know are probably in much better shape than the average, which further skews your view of what “the average” is. “Sure, I think most normal people can bench 300 lbs.,” you might say – because you can, and lots of your friends can, too. But that’s not a representative sample!
And you are never a representative sample all by yourself. Even if you take one of those tests like in grade school and it says “You’re in the 80th percentile,” that still only means you’re smarter than 80% of people who took the test. That by itself is a skewed metric!
The point is this – when you’re trying to guess what a group of people is going to be like on average, never use yourself or the people you know as your measuring stick!
The Opportunity Machine 