A hundred years ago — and that's when this picture was taken, in 1912 — men didn't leave home without a hat. Boys wore caps. This is a socialist political rally in Union Square in Manhattan. There may be a bare head or two in this crowd, but I think those heads are women's.
Here's another rally, Union Square again. This time it's an Occupy Wall Street demonstration. A hundred years have passed. Same place. Same kind of crowd. But this time: hardly a hat.
For decades, teachers, managers and parents have assumed that the performance of students and employees fits what's known as the bell curve — in most activities, we expect a few people to be very good, a few people to be very bad and most people to be average.
The bell curve powerfully shapes how we think of human performance: If lots of students or employees happen to show up as extreme outliers — they're either very good or very bad — we assume they must represent a skewed sample, because only a few people in a truly random sample are supposed to be outliers.