Regression toward the mean
In 1886, Galton published a paper  called Regression towards mediocrity in hereditary stature.  In the paper, he observed that extreme characteristics (e.g., height) in parents are not passed on completely to their offspring. Rather, the characteristics in the offspring regress towards a mediocre point. Today, this point is called  the mean. By measuring the heights of hundreds of people, he was able to quantify regression to the mean, and estimate the size of the effect. Galton wrote that, “the average regression of the offspring is a constant fraction of their respective mid-parental deviations”. This means that the difference between a child and its parents for some characteristic is proportional to its parents' deviation from typical people in the population. If its parents are each two inches taller than the averages for men and women, on average, the child will be shorter than its parents by some factor times two inches. Today, this factor has been calculated to be  one minus the regression coefficient.  For height, Galton estimated this coefficient to be about two thirds: the height of an individual will measure around a midpoint that is two thirds of the parents’ deviation from the population average.