One might suspect that the positive sign of the estimated coefficient for Sundays in the subsample from 1920 to 1949 may point to a misreporting of birthdays due, e.g., beliefs that children born on a Sunday are especially lucky or other birthday clusters, such as the birthday of Adolf Hitler or Kaiser Wilhelm II.3
Including two indicator variables for the birthday of Kaiser Wilhelm II and Adolf Hitler into the specifications indeed renders positive coefficients for both variables in the period from 1920 to 1949, which are statistically significant at least at the 10%-level. Note, however, that the estimated coefficients of the other explanatory variables discussed above are not affected by the inclusion of these two indicator variables.
The results for the dichotomous variables describing the month and year of birth, which are not shown in Table 1 but are available from the authors upon request, are in accordance with existing evidence. Specifically, birth frequencies in our data show the usual seasonal pattern, being more likely at the beginning of the year starting in February and less likely in midsummer and at the end of the year. Furthermore the estimated coefficients of dichotomous variables describing the year of birth reflect the overall development of birth rates in Germany, with lower rates in the Great Depression in the 1920s and the baby boom in the 1960s. The result holds even when controlling for the (lagged) German population. Although we observe a positive and significant effect of the German population on the number of births for the period 1950– 1969, the yearly dummies are positive and significantly different from zero (the results of these analyses are available from the authors upon request).
Finally, the explanatory power of our regression model decreases over time and hence, also supports our inter-pretation that advanced medical technology decrease the impact of natural factors determining the number of births. While the adjusted R2 indicates that we are able to explain more than 95% of the variation of our dependent variable