All of the article that I have read just mention IQ and fertility which is a measure of live births; however, I believe that the fertility measure looks at all age groups so children born later in life would be counted. I think the low fertility numbers accurately reflect people that never had children at any age.
I'm not sure how that could be done. I can't read most of the articles cited in the Wikipedia article, since they don't seem to be up online, but I'm just looking at the descriptions in the article. Let's go through the cites of the recent studies one-by-one:
Jinks, J. L., & Fulker, D. W. (1970). Comparison of the biometrical, genetical, MAVA and classical approaches to the analysis of human behavior. Psychological Bulletin, 73, 311−349. -
the description says that this looks at 9,000 high school graduates. I don't know how they were selected, and what age they were, but they measured the intelligence of these people, which means they were still alive. Unless they were all very old, they cannot have measured whole-life fertility, since men can often have children very late in life. And their study necessarily excludes those who died before being measured (and before graduating high school), which is significant since we know that low IQ correlates with low income, and low income correlates with early mortality.
Bachu, Amara. 1991. Fertility of American Women: June 1990. U.S. Bureau of the Census. Current Population Report Series P-20, No. 454. Washington, D.C.: Government Printing Office.
This one is taken from census data, and purports to show that high-school dropouts have higher fertility than those who completed high school. Now, whilst I cannot find the exact report quoted, I did find the equivalent report from the next census
, with data from June 1994. Assuming the data is arranged the same in the two reports, this is not a lifetime study of people. It's a count of those whon had a baby in the previous year only, so from this it is a big leap to make any sweeping generalisation about lifetime fertility coupled with survival of offspring.
Incidentally, the data from 1994, contra 1990, show a higher fertility for high school graduates than for high-school dropouts. Whilst college atendees have lower fertilty overall than high-school dropouts, those with Bachelor's degrees are again more likely to have had a child than high-school dropouts - it's college dropouts and post-grads who bring the average down. Clearly, the pattern doesn't always hold that clearly.
Furthermore - there is some very important data in the census study about what I suggested earlier - low IQ may correlate more with early fertility than overall fertility. 76.4% of high-school dropouts aged under 30 had had a child in the previous year, compared to 65.6% of those with a bachelor's degree and 53.4% of those with a graduate degree. However, only 45.5% of dropouts aged 30 or over had had a child in the previous year, compared with 73.1% of those with a bachelor's degree and 52% of those with a graduate degree.
People with higher IQ and/or education have children later, whether because they use their intelligence better to avoid mistakes or, more likely to me, because they spend more of their youth acheiving this higher educational status and economic success.
I can look into this more later, but I really am supposed to be working at the minute.
Edited by caffeine, : No reason given.