 Data shows that only in few countries there was significant and persistent increase in the marital fertility index during the baby boom, which suggests that most of the increase in fertility was driven by the increase in marriage rates.Jona Schellekens claims that the rise in male earnings that started in the late 1930s accounts for most of the rise in marriage rates and that Richard Easterlin's hypothesis according to which a relatively small birth cohort entering the labor market caused the marriage boom is not consistent with data from the United States.Matthias Doepke, Moshe Hazan, and Yishay Maoz all argued that the baby boom was mainly caused by the alleged crowding out from the labor force of females who reached adulthood during the 1950s by females who started to work during the Second World War and did not quit their jobs after the economy recovered.