Francis Galton, Darwin’s cousin, best known for promoting eugenics, invented the mathematical concept of correlation. Since then, many mistakes have been made relating correlation and causality, which sometimes coincide, but can also be completely different.
Two variables are said to be correlated when increases in one resemble increases in the other, and decreases in one resemble decreases in the other. But not all correlations are the same: to distinguish them, Galton devised the correlation coefficient, a number between -1 and 1.
