Are birth order effects due to SES?

As early as 1874, Sir Francis Galton noted that first-borns and only children were overrepresented in English men of science, making birth order effects one of the earliest constructs studied in psychology. Over a thousand studies has since been conducted, most of them contradicting the rest. As is typical in psychology, the arguments tend to center around the correct way of measuring birth order.

A common counterargument is that birth order is also an index of SES. Poor families have more children. Just from that, you would expect that there would be few scientists with ten older siblings (Galton actually pointed this out himself, noting that among the wealthy, first-borns tend to have large inheritances, and thus are freed to pursue whatever they like). That said, many birth-order effects have held up even when holding SES constant.

The typical pro-birth-order position is that children are shaped by their environment, and their environment is shaped by the age of their siblings. Here is a quote from Alfred Adler, who developed the first full-blown theory of birth order effects:

It is a common fallacy to imagine that children of the same family are formed in the same environment. Of course there is much which is the same for all children in the same home, but the psychological situation of each child is individual and differs from that of others, because of the order of their succession.
Then, this weekend, I read what was meant to be a counter-argument by Wichman, Rodgers and MacCullum (2006), who are adamantly on the side of no birth order effects:

For example, as parents age, they typically increase in SES level and also may spend more time at work and less time with their children. Thus, later-born children may, on average, mature in a slightly higher SES environment than their earlier-born siblings, but one in which parents spend less time with them and which therefore may negatively affect their intellectual development. If later-born children have lower IQs, are we observing an effect of being a later-born child (a real birth order effect) or an indirect effect of SES?

It's hard to see why they think this is explains away birth order effects. It's simply a different explanation of birth order effects.

Anyway, I expect to finish my review of the birth order literature today or tomorrow (no, I'm not reading all 1000+ articles; I'm at 25 and counting, though), so I'll update soon with what I've learned.

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