Arnold Kling comments about my assertion that until recently cities were genetic black holes:
Today,. we think of cities as places where people come to thrive. Wealth is higher in cities than in small towns and rural areas. Richard Florida tells us that the creative class is to be found in cities.
I wonder: who came to cities? Was it people without land? Were cities like an awful lottery that people would play when they had no other choice? A bunch of landless people gathered together to prey on one another, with the winners thriving (moving to the country as soon as they could afford it) and the losers enduring a Hobbesian existence, where life was nasty, brutish and short?
My comment was a contention in relation to reproductive fitness; not quality of life or satisfaction (H/T to Greg Cochran for the observation). In the comments I cite a paper which suggests that city and rural divide in mortality favored the rural until around 1900 in the United States. The divide does not exist in large part due to proactive public health measures. Needless to say, though there were variations (e.g., compare 18th century London to Tokyo/Edo), in the pre-modern context public health was much more primitive.
So why move to the city? I think it is likely correct that city air makes one free. We can see this today as social change is occurring in Developing World megalopolises. In the ancient cities there were clear benefits for the poor, Rome and Constantinople had doles for the urban proletariat (though these doles were of course simply viable due to rents derived from their Empire). After the wars of the middle to late republic many impoverished peasants migrated to Rome to escape famine (the famine was exacerbated by the fact that many men were called up as soldiers to serve in foreign wars, and so their labor was missing). On the other hand, despite the dole there was often no regular employment. From the data I have seen modern urban-life worldwide tends to correlate with a lower fertility; and I see no reason this would not be so in the pre-modern world (I assume that the marginal return on “extra hands” provided by more children would likely be lower for a sporadically employed urban laborer than a farmer). But the main difference I suspect is disease load over time. Plagues regularly killed on the order of 50% of the population of ancient cities. After the population declines the cities would bounce back, but not through natural increase, but further waves of rural migrants. Rome’s population declined to tens of thousands in the medieval period, from on an order of 1 million in antiquity. I am skeptical of the idea that most modern Romans are descended from a demographic expansion of the medieval deme as opposed to migrants from the hinterland.
All the population genetic negatives are not to deny that civilization and the city are to a large extent identical (Sumer). Who says that cultural creativity or innovation has to track Darwinian fitness? Look at our own modern societies, the least successful by accepted measures are often the most “fit” in Darwinian terms.
Addendum: A shorthand way of thinking what I’m asserting, imagine two brothers who are farmers. One moves to the city to get on the bread dole, while the other attempts to make do on the margins. The former might have a higher chance of surviving, but because of the greater power of disease in the urban context the city brother is likely to have far fewer descendants than the country brother as periodically all of his descendants come under threat of dying in a plague (again, I also believe that fertility will be lower for urban descendants).