Updated 8/28/12: See below!
As I’ve discussed previously, the low White fertility rate seen in America today stems mostly from liberals. This is because liberals contain a higher share of secular individuals and because liberal women pursue more education on average, which leads them to delay child birth (and often marriage as well).
I have posited that these differences are magnified in America with respect to the European countries of origin of White Americans thanks to evolutionary forces during the colonial era that acted upon White settlers. This is because—as was the case during the European colonization of the country—when habitable land is not a constraining factor, those who breed fastest have a distinct selective advantage. Religious and politically conservative individuals often have “pro-natalist” attitude, likely both because they are less materialistically oriented and because religious and social conservatism helps to curb sexually impulsive behavior (through the strict social mores around sex; for conservatives, sex is more coupled with procreation than it is for liberals). Overall, the behavior of conservatives leads more reliably to reproduction than does the behavior of liberals.
But have conservatives always had a fertility edge? I decided to take a look back into the GSS to find out:
Edit: [I realized after I posted this that for the older cohorts, I needed not limit myself to the 1990 onward GSS data, so as to maximize sample sizes, I looked at earlier datasets, 1972-2010 datasets for the 1899-1919 cohorts, and 1983-2010 datasets for the 1920-1939 cohort.]
This chart is the average number of children had by non-Hispanic White Americans, by political views and by year of birth, from the 1990-2010 GSS data (the 1960-1972 cohort is drawn from the 2000-2010 data only here, however). The error bars are the 95% confidence intervals, to give an idea of the sample sizes.
As can be seen, in every cohort, the pattern is fairly consistent, with conservatives reliably being more fertile than their liberal counterparts. This is even true for the pre-birth control cohorts, even though the pattern is not as pronounced there. [With the increased sample sizes in the pre-1920 cohorts, I found that the liberal-conservative skew in fertility seems to have disappeared. I noticed that the 1900-1919 cohort was the one who spent their early childbearing years during the Great Depression, so I separated them from the 1883-1899 cohort. It is possible that everyone’s fertility was suppressed going through the bottleneck of the Depression. It’s unclear what’s going on in the 1800s cohort, as sample sizes are small. The implications of this for my hypothesis is not clear, as the earliest cohort may be exhibiting the overall pattern, but also may not.] This is another piece of evidence [edit: that may support] a key point of my Pioneer Hypothesis: that liberals are just, on the whole, slow-breeders while conservatives are, overall, fast-breeders.