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: Males tend to be more variable in intelligence. It is well known that males are far more likely to have intellectual deficits, including mental subnormality, learning disorders, and behavior problems. This book also presents evidence that males are more likely to be exceptionally high in cognitive abilities (other than memory), and in such areas as advanced mathematics, spatial perception, and creative music. Partial or total reversions to the aboriginal level of intelligence can account for virtually all cases of non-specific mental subnormality. These conditions are now identified by such terms as Renpenning syndrome, Martin-Bell syndrome, Fragile-X syndrome, and cultural-familial mental retardation. Because of the probability of offsetting genes, females are less likely to be severely affected by these conditions. 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. Some are precocious, but many are delayed, and not ready for the three R's at the traditional age of 6. This is the basis for almost all cases of learning disability. 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. This invalidates earlier studies of parent-child transmission of IQ, which have included father-son correlations. In effect, earlier studies have come up with estimates of the heritabilityof intelligence that are too low.