I think this is a bit more controversial and complicated than you think. There could be many confounding factors to take into account trying to measure this sort of relationship.
We can take a group of people, and measure their IQ directly or find some variable which normally correlates with IQ, and then compare that with their fertility. But, if the group is below the age at which they can still produce children, we're only getting a partial picture of lifetime fertility. If low intelligence correlates with early fertility, but not with overall fertility, this could easily be obscured. Joe may have fathered three kids by the time he's 25 with low economic prospects and little hope of social success, while Dave waits till his forties when he can provide for his three kids with vast business empire he'd been building up which Joe was sowing his oats.
And the relative success of Joe and Dave's children brings us to the other problem - mentioned in the wikipedia article. Having babies isn't the only issue - it's how well those babies do at the reproductive game themselves. The same as you could easily miss later-life fertility through your survey, if you pick people of an age where you expect most of their child-bearing days to be over, you will miss earlier mortality. This is brought into starkest view when you look at the international comparisons. Yes, Angola has a much higher fertility rate than any industrialised nation, but many of these children will die before they ever reach maturity.
This isn't a subject I've looked into very deeply, but I remain sceptical that low IQ correlates with fewer great-grandchildren - which is the sort of measure that's important if we're thinking about the human race evolving to be more or less intelligent.
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.
I've heard of it but never seen it. I'll check it out.
So are you admiting there's a problem? Is Bender correct:
No, I don't think so.
I had a read through this meta-analysis from Demographic Research, which looks at trends in the relation between status and fertility. The overall picture seems to show that before about 1750 in Europe and North America, and before the 20th century in the rest of the world, high status was associated with high fertility. After this, the picture reverses, but the differentials are much less than previously.
Two points to bear in mind. Firstly, lower status groups have higher mortality and morbidity than higher status ones, so a slightly higher fertility rate (and it is only slight) will not necessarily lead to a shift in relative proportions in the population.
Secondly, not all - probably not most - of the differences in social status are genetically inheritable factors. The biggest driver of decreased fertility is higher education, and education levels have much more to do with the opportunities avaiable to you than they do with any genetically inherited intelligence.