regression to the mean

Regression to the mean is the statistical tendency for extreme measurements to be followed by less extreme ones.

A student who scores unusually high on one test will likely score closer to average on the next—not because performance declined, but because the first score included some luck.

The sophomore slump is often just regression to the mean.

The phenomenon appears everywhere: sports, medicine, gambling.

It explains why the tallest parents tend to have shorter children, and why miracle cures often seem to work—patients seek treatment when symptoms peak, then naturally improve.