Prenatal and early postnatal influences on measured intelligence.

In larger mammals natural selection has led to reduction of litter size and prolongation of the period of gestation. Even in human pregnancy, which is normally single, gestation is prolonged beyond the time when the growth of most fetuses can be fully supported. This conclusion is based on comparison of prenatal and postnatal growth rates. The lengthening of pregnancy may be related to the selective advantages of maturity at birth, particularly, in man, the development of immunity to infectious disease.Verbal reasoning scores recorded in the 11-plus examination provide no evidence that the retarded prenatal growth associated with prolongation of gestation affects measured intelligence. There are considerable score differences in a general population of births in relation to birth weight and duration of gestation; but, like the larger differences related to maternal age and order of birth, they are not found in sibs. Still more suggestive is the finding that twins raised singly from birth have scores higher than twins raised together and little lower than those of single births. This indicates that even the pronounced retardation of fetal growth which occurs in twin pregnancy has little effect on verbal reasoning, the large differences between twins and single births being due to postnatal experience.