After observing that the IQ gains associated with breast-feeding vanish when we control for the IQ of mothers, Sam Wang and Sandra Aamodt observe that good parenting skills are particularly important for children who grow up in poor households:
In one recent study, parents’ IQ influenced the child’s IQ at 2 years of age — but not if the household was low-status. In 7-year- olds, only 10 percent of IQ variability in low-status children was attributable to genes. In other words, a poor environment can wipe out genetic advantages.
Low social status often means economic insecurity, unsafe neighborhoods and chaotic households. And it means increased secretion of stress hormones. At age 10, low-status children have twice as much stress hormone in their blood as high-status children do. Chronic stress reduces the volume of the hippocampus, a brain region involved in learning, and leads to memory problems. It also interferes with development of the frontal cortex, which is important for planning and organizing behavior and for self-control.
What Wang and Aamodt associate with socioeconomic status might also be attributed to the robustness of social networks. Affluent people tend to have stronger, more extensive social networks — it is one of the reasons why these people are affluent. So one interesting question is whether the children of non-affluent people with strong, extensive social networks more closely resemble affluent people or non-affluent people who have very limited social networks in educational and health outcomes. Income can certainly insulate families and children from stress. Yet it seems at least possible that having a supportive network can do the same.
Consider, for example, the phenomenon of low levels of infant mortality and high life expectancy among Latinos, which Razib Khan has discussed in detail. The so-called “Hispanic paradox” draws attention because Latinos are poorer on average than the larger U.S. population and they tend to have less access to insurance coverage, yet they seem to have superior outcomes on infant mortality and longevity. As Razib notes, however, this applies to a number of other U.S. groups as well:
[T]here are broad swaths of the rural north where whites are relatively poor, but long-lived, in contrast to the South. What you see on the map on the broad purple swaths are the echoes of the Yankee Empire, and the New England Diaspora, which includes the Mormons of Utah. Yankee probity seems to have attracted Scandinavians, Germans, Irish, and Italians, of like mindset. Or the children of immigrants were acculturated to Yankee values.
What is the moral here? Economic development is broadly indicative of life expectancy, but in modern developed societies culture and social milieu matters on the margin, and can swamp the effects of economics. Just as heritability for height is higher in developed societies, so variation in life expectancy within developed societies may be more likely to track cultural categories than economics.
So what is going on? I suspect that the strength of social networks is a key driver. Controlling for income, members of robust communities will flourish while members of atomized communities will not.
But income is far more intellectually tractable, and so we focus on income. If all you have is a hammer, every problem looks like a nail.