Kings and Emperors were rich, but not all the Rich were Kings and Emperors. So, it follows that historical evidence showing Kings and Emperors had lots of kids doesn’t tell us much about whether or not the Rich had lots of kids.
Laura Betzig makes this error in her critique of Farewell to Alms (pdf). To counter the claim, falsely attributed to Clark, that England was the only place where there was a relationship between fertility and economic status, she cites a long list of Kings and their concubines in various other countries and eras ((Clark only conjectures the connection between fertility and economic status was stronger in England. He doesn’t provide any cross-country evidence support this conjecture, though.)). Its an interesting historical fact that some men in the past had hundreds (and some thousands) of wives and children, but this says nothing about what was happening in the society at large. While Nebuchadnezzar was stealing other Kings’ wives and I’m sure having many sons by them, who knows how reproductive his underlings and other members of the upper classes were.
By definition, Kings are different (they’re Kings!) so they simply can’t be a representative sample. Clark’s wills are as close to a representative sample of the whole population as one can hope for in historical data. Using this data, Clark shows English nobility, on average, had fewer children than their merchant-class counter-parts and he shows that while there’s a relationship between wealth and fertility no such relationship exists between social class and fertility. The data from England should make us even more suspect of previous studies that use the fact of a few highly fecund royalty to suggest links between fertility and economic status in the society as a whole. These guys were just outliers and we usually throw outliers out of the analysis.
This is a common mistake people make when they talk about Clark’s book. I think its caused by history being biased towards Kings. Pre-modern history, as it is taught, is populated by Kings and their loyal subjects. It is full of stories of their conquests (usurpations, wars and treaties). Most of history though, as it was actually lived, is populated with normal every day folk and their stories are more mundane ((Approximately 0% of the total humans that ever lived were Kings, but approximately 100% of pre-1800 history is about Kings.)). This means most of our historical data, and much of our historical thinking, is biased towards describing Kings rather than every day folk. If we want to test theories about every day folk, like Darwinian theories of the Industrial Revolution, we have to remove this bias.