The tendency for a variable to revert towards its mean over time or in repeated measurements. Many physical and economic variables are mean-reverting. Examples include height (the children of tall parents tend to be closer to average height than their parents) and the beta coefficient of stock (stock with a high beta in one time period tend to have lower beta in the subsequent period). The use of the word ‘regression’ for the statistical tool was introduced by Sir Francis Galton, who first observed reversion, or regression, to the mean in his studies of heredity.