In this paper we investigate the reasons for the growth in the female labor force in America during the twentieth century. In our research, we study the longer-term trends since 1900 and conduct a more intensive examination of developments after the Second World War. On the basis of our work, we conclude that rising real wages accounted for 60% of the total growth in the female labor force. Half of this wage effect in expanding labor supply was the fertility-reducing consequence of a higher female wage.