Regression toward the mean is a property of any scatterplot where the linear relationship is less than perfect. Under such circumstances the regression of Y on X will have a slope that is less than 45 degrees.

As a result, the mean of the values of Y for a given value of X will be closer to the mean of all values of Y than that value of X is to the mean of all the values of X.

In the scatterplot shown below, each dot represents the height of a father and the height of his son plotted together.