Questioning the "evolution of an underclass"

2 minute read

A little life history theory can be a dangerous thing. Case in point: “Die young, live fast: The evolution of an underclass.” The article discusses correlations among longevity, health, income, and age at first birth within industrialized societies and cross-culturally worldwide.

Evolutionary theory predicts that if you are a mammal growing up in a harsh, unpredictable environment where you are susceptible to disease and might die young, then you should follow a "fast" reproductive strategy - grow up quickly, and have offspring early and close together so you can ensure leaving some viable progeny before you become ill or die. For a range of animal species there is evidence that this does happen. Now research suggests that humans are no exception.

The cross-mammal generalization is true, and the article discusses correlations within human populations that run the same way. Women who live in communities with shorter life expectancies and shorter expected healthy lives also tend to have their children younger and with shorter birth intervals.

It should be obvious but I’ll point it out anyway: The use of the word “evolution” is misleading. The comparisons among human groups discussed in the article are not cases of biological evolution; they are mostly social effects of industrialization. I wouldn’t rule out long-run changes in gene frequencies coming from such systematic fitness differences, but calling it the “evolution of an underclass” is alarmism.

The article goes on to quote public health researchers and sociologists who seem to confuse correlation with causation. They assume that making communities live longer will cause young women to wait longer to reproduce.

That could happen, sure. The article suggests a plausible causal hypothesis: Young women decide that they’d better reproduce in a hurry so that they’ll have healthy older relatives around to help them. Or they decide they’ve got nothing else to wait for, so they’d better have kids.

But if you dig a little further into life history theory, there are alternatives. The cross-species comparison reflects the outcomes after a long process of adaptation, and appears simple mainly when you compare species across several orders of magnitude of longevity. Looking at any set of closely related species, the story is more complex.

The dynamics by which a population may pass from one value to another can also be described mathematically, and interestingly that math may lead to different ideas about how age at first birth might respond to differences in longevity. Hamilton (following Fisher) noted that in a growing population, a reproductive bonus goes to younger mothers; in a shrinking population it’s the opposite. A rational person might conclude that longer healthy lifespans would cause the population to grow (as indeed happened during the early twentieth century) and therefore decide that younger births will increase fitness. That is, if fitness outcomes were relevant.

I don’t think people generally care about fitness outcomes, and so I don’t think there’s much reason to expect the correlations in today’s population to remain as strong as health improves. It seems pernicious to argue that we should invest in health because it will make women put off reproducing. Why not just recognize that health is an intrinsic good?