Commenter Hail notes that white women who identify more or less as Republicans have 18% more children than white women who identity more or less as Democrats:
According to GSS: White women born 1944-1974 (sample size=5,343) who were between ages of 40-50 at the time of data collection (in 1994-2014, i.e., having reached their final lifetime completed fertilities) had the following fertility rates by political orientation:
[Political Identification] (% of Pop.): Final Lifetime TFR (Whites age 40+) (GSS variables: childs, age, race, 1994-2014)
I think this is actually Completed Fertility, which is not exactly the same as Total Fertility Rate, but pretty close. Also, if you are looking at women 40 to 50, a few will have more children, so these numbers are slight understatements.
1. [Strong Democrat] (10%): 1.61 Final TFR
2. [Not Strong Democrat] (16%): 1.83
3. [Independent, Near Democrat] (11%): 1.79
4. [Independent] (18%): 2.00
5. [Independent, Near Republican] (10%): 1.86
6. [Not Strong Republican] (19%): 2.12
7. [Strong Republican] (13%): 2.14
8. [Other Party] (2%): 1.89
The difference in final fertility (acc. to GSS) between White Democrats (categories 1,2,3 above) and White Republicans (5,6,7) is 2.07 : 1.76, or given equal starting numbers, a Republican child generation 118% as large as a Democrat child generation.
Another way of looking at it (given replacement fertility = 2.1) is that both groups are shrinking, but White Democrats are shrinking somewhat faster.
White Republicans, base population 100, at 2.07 TFR
First child generation: 98
Second child generation: 96
Third child generation: 94 (Starting today as Year Zero, this will be past year 2100)
White Democrats, base population 100, at 1.76 TFR
First child generation: 84
Second child generation: 70
Third child generation: 59
New ratio, given equal starting numbers, in third child generation: 94 : 59, or 160 White Republicans for 100 White Democrats (assuming that political ideology is perfectly inherited).
Since it’s not 100% perfectly inherited, the changes in number are likely to be slower due to random remixing. But I may be getting the logic wrong, so your comments are, as usual, welcome.
It would be interesting to know if this gap is growing and how far back into the past it’s discernible. If you go back a half century into the past, the stereotype was that Irish Catholic Democrats had the biggest families.