According to conventional economic theory, growing up in a large family predicts less income as an adult because parents were unable to invest as much in each child. By contrast, genetic theory would predict that that family size would not matter for how much income you earn as an adult; rather, income would be predicted by one's IQ. Let's test these two competing hypotheses using General Social Survey data.
Respondents were asked how much income they earned in the past year, and they were also given a ten-word vocabulary quiz, which makes a decent proxy for IQ (N = 19,902).
This table shows the estimates for a regression model that
includes personal income in constant dollars as the dependent variable and the
number of siblings as the predictor. You can see that each additional sibling results
in a predicted reduction in one's income of $1,139.
Looking at this table above, we can see that IQ is positively related to income, and the beta indicates that the relationship is of considerable magnitude. It is predicted that each additional IQ point will result in an additional $423 in income.
The unstandardized coefficient for number of siblings has
dropped from $1,139 down to only $586. In other words, much of the
reason why a large family predicts a smaller income is due to the correlation between
having many siblings and having a lower IQ. This finding supports,
to some extent, the genetic hypothesis. On the other hand, we see that the
sibling coefficient is still statistically significant, so even after
controlling the influence of IQ, the number of siblings is still negatively
correlated with income.
With a little back on the envelope math I conclude:
ReplyDelete1) every sibling is worth 3 IQ points.
2) if we could increase newborns IQ by 20 points, they could have 6 siblings and earn the same money than before.
Might also be because in large high-IQ families, siblings are more inclined to support one another and are less likely to strive to their maximum limits. So a form of group promotion at the expense of the individual.
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