The author presents a theory that major genes controlling the growth of human intelligence, both left- and right-brain attributes, are on the X-chromosome. The more significant of the implications of such X-linkage include: BL Males tend to be more variable in intelligence. BL Partial or total reversions to the aboriginal level of intelligence can account for virtually all cases of non-specific mental subnormality. BL Since the X-linked genes control a pattern of growth, boys are more variable in the age of readiness for the skills required for progress in school. BL Being on the X-chromosome, these genes, favorable or unfavorable, are not passed on from father to son, although they are passed on from father to daughter. In effect, earlier studies have come up with estimates of the heritability of intelligence that are too low.
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