Worth reading all of the posts Jayman links to. He has written on this extensively.Instead of guessing, I should do the requisite data delving to evaluate that assertion about political orientation being a stronger predictor of fertility than partisan affiliation for whites. The results, using parameters similar to Hail's*, are as follows (with % of white female population in parentheses):
It is also worth considering how much the political orientation gap among white Republicans and white Democrats has widened over time. In the mid-seventies, white Republicans were only slightly more likely to self-identify as politically conservative than white Democrats were. That difference has trebled in the last four decades to the point that white Republicans are now far more likely to identify as conservative than white Democrats are.
Political orientation is probably more heritable than partisan affiliation. I'd guess the gap will appear wider on the liberal-moderate-conservative spectrum than on the Democrat-independent-Republican one.
Two variables that are stronger predictors of fertility than political orientation or party affiliation are educational attainment (inversely correlated, especially for women) and religiosity (positively correlated--to the extent that high IQ people who attend religious services regularly outbreed the irreligious at every level of intelligence, social class, race/ethnicity, etc.
Parenthetically, educational attainment looks to be the driving force, not intelligence. Fertility by wordsum score varies little once educational attainment is controlled for, but educational attainment is a strong predictor even after wordsum score is controlled for--put more clearly, educational attainment is 5x as strong a predictor of fertility as IQ is.
The strong inverse relationship between education and fertility shows up strongly on the international level as well.
Extremely liberal (2.5%) -- 1.61
Liberal (12.7%) -- 1.72
Slightly liberal (11.3%) -- 1.58
Moderate (39.9%) -- 2.14
Slightly conservative (15.6%) -- 2.03
Conservative (14.3%) -- 2.34
Extremely conservative (3.7%) -- 2.62
Indeed, the fertility gap appears wider on the liberal-conservative spectrum than it does on the Democrat-Republican one. Hail's results show a white Republican:white Democrat total fertility rate ratio of 2.07:1.76. The white conservative:white liberal total fertility rate ratio is twice as large, at 2.23:1.65.
GSS variables used: POLVIEWS(1-3)(4)(5-7), RACECEN1(1), SEX(2), COHORT(1944-1974), CHILDS, AGE(40-50)
* I used the COHORT variable to pull data from those born between 1944-1974 while being able to exclude survey results from 1994, 1996, and 1998 since in those years "white" includes Hispanic. From 2000 onward, non-Hispanic whites and Hispanics are separated into distinct categories. Consequently, his numbers include some Hispanics, while mine are only comprised of non-Hispanic whites. Also, what Hail terms "political orientation" I regularly describe as "partisan affiliation". I use the phrase "political orientation" to refer to the liberal-moderate-conservative spectrum rather than the Democrat-independent-Republican one.