Possible genetic consequences of family planning

Abstract

The recent demographic transition in Japan must have resulted in some reduction in the frequencies of a variety of congenital defects correlated with parental age and birth order. These defects are classified into 2 categories; new mutations and the other determined by both genetic and environmental factors. The decrease in the diseases of the latter category is dysgenic in the sense that selection against the genes is relaxed, but, so long as the pattern of small family size continues the manifestation of those diseases would be prevented. The same transition should in the near future result in a significant reduction in consanguineous marriages and therefore in a reduction in mortality and morbidity. Again, this effect would be dysgenic, because of the increase in the carriers of rare detrimental genes. The variations in the distribution of family planning practice, as well as variations in fertility among some social strata, are rapidly decreasing, but because of lack of knowledge it is difficult to evaluate the extent of the assumed dysgenic effect upon some mental traits correlated with social stratification. The reduction in childhood mortality correlated with smaller family size must have resulted in relaxation of selection against most, if not all, of the detrimental genes affecting viability. The rate of infertility has slowly decreased, but the net intensity of selection due to infertility seems unlikely to undergo relaxation. The loss of variation in the number of children among potentially fertile women is obvious, but this reflects social conformity more than genetic homogeneity, so that the fecundity of future population may scarcely be affected. The possible effects upon the immediate or near future must be distinguished from those taking place in the long run. The increased opportunities of using birth-control means for eugenic purpose, if guided by proper genetic counselling, could counterbalance the presumed dysgenic effects.