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The consistent findings about cognitive ability and job performance that apply most directly to group differences in cognitive ability are these:

• Measures of cognitive ability and job performance are always positively correlated.

• The size of the correlation goes up as the job becomes more cognitively complex.

• Even for low-skill occupations, job experience does not lead to convergence in performance among persons with different cognitive ability.

• For intellectually demanding jobs, there is no point at which more cognitive ability doesn’t make a difference. Increases in IQ scores are statistically associated with increases in productivity at every level of cognitive ability. "

― Charles Murray , Facing Reality: Two Truths about Race in America