The law of intelligence is presented in test independent form. Mental abilities, physical brain size, and infant motor capacity follow the same law of growth from birth to adolescence. Mental growth is independent of race, "SES" or the Flynn effect. The vitality of the mental age scale calls for a reexamination of Wechsler's deviation IQ. This paper builds on Yen's method of standardized differences (1986). The main theoretical advance here is to put development back into intelligence testing and to show a universality among different measures of the growth of the human nervous system.