IQ:

"IQ has more behavioral correlates than any other psychological measurement."

"There are marked and highly reliable individual differences, for example, in fingerprints and form of the outer ear, but, because these features show little correlations with hardly anything else, they are of no interest to anyone, except as a reliable means of identification. The great and persistent interest in IQ, on the other hand, is a direct result of the readily perceived and undeniable fact that IQ is correlated with so many other variables that are deemed important in life by almost everyone."

Arthur Jensen