How much dumber are big families?

We know that dumb people tend to have larger families, so I was curious about how much IQ drops with each additional child. Here's the mean IQ by family size (sample size = 26,388, immigrants excluded):

Mean offspring IQ by number of children

1    101.4
2    101.2
3    100.2
4      98.4
5      97.1
6      95.2
7      94.2
8      93.0
9      91.7
10    90.8
11    89.6
12+  88.8

If we do an OLS regression, we see that for each additional child, the predicted drop in IQ is 1.13 points. This doesn't mean, of course, that having more kids causes IQs to fall. It simply means that the dumber the couple, the bigger the number of children. As shown in the movie "Idiocracy," smart people see every reason to delay having kids, while the dull ones do what comes naturally.