This paper focuses attention on an economic explanation of the birth rate. Although abstracting from many social, psychological and economic hypotheses which have been adduced as explanations of birth rate trends, there is no intention to convey the idea that only the factors which it considers in the analysis are of significance. It is well known that in the developed countries, the secular trend in the birth rate has been downward during the past 100 years. This has occurred in the face of rising average family income. In this paper an economic model is proposed which suggests that the rise in family income contributed to the decline in family size.