Size matters

Ten days after our baby entered the world, the latest copy of the Quarterly Journal of Economics landed in my inbox. It includes a paper I hadn’t seen before – “From the Cradle to the Labor Market? The Effect of Birth Weight on Adult Outcomes”. Here’s the abstract, from a previous working paper version:

Lower birth weight babies have worse outcomes, both short-run in terms of one-year mortality rates and longer run in terms of educational attainment and earnings. However, recent research has called into question whether birth weight itself is important or whether it simply reflects other hard-to-measure characteristics. By applying within twin techniques using a unique dataset from Norway, we examine both short-run and long-run outcomes for the same cohorts. We find that birth weight does matter; very small short-run fixed effect estimates can be misleading because longer-run effects on outcomes such as height, IQ, earnings, and education are significant and similar in magnitude to OLS estimates. Our estimates suggest that eliminating birth weight differences between socio-economic groups would have sizeable effects on the later outcomes of children from poorer families.

Our lad was about 20% above the average birthweight. The Black, Devereux and Salvanes results suggest that he will therefore earn 2% more than an average birthweight baby. What they can’t work out is the causal channel. Does this happen because of some causal effect of health on wages, or because parents and schools invest more in bigger kids?

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6 Responses to Size matters

  1. Kevin Cox says:

    Isn’t there an increase in income if the person looks attractive? If this is the case then it may be the same causal effect that people invest more in kids who appear likely to have a greater degree of success not because they are bigger or beautiful but that they believe it is the best place to invest because the return is likely to be greater. It is also the reason why we will invest more in our own kids than we will in our neighbours because our return is likely to be greater. It may also be why cultures that still expect the children to look after their parents in old age seem to invest more in their kids.

    Perhaps a way to improve the outcomes for our children is to require them to sign a declaration at birth that they will look after their mum and dad:)

  2. Wouldn’t it be that the heavier child is healthier which has a causal effect on IQ which in turn has a causal effect on income?

  3. Yobbo says:

    Was this study controlled for SES? Surely low-SES couples have lower birthweight children because of increased incidence of smoking/drinking during pregnancy, and obviously being born into a low SES family correlates strongly with low income in adulthood.

  4. Andrew Leigh says:

    Yobbo, a pair of twins have the same SES.

  5. ChrisPer says:

    But straight off, twins are in a competitive environment – against each other. The higher birth weight twin in all species has a big head start, going for more goodies and generally more confident in claiming a higher social standing (and the more productive breast). Maternal fairness in humans does not cancel this out.

    The lower birth weight twin may also correlate with other physical handicaps, both in appearance and physical competitiveness.

    SES obviously is not independent of birth weight, because those that make bad health choices including substance abuse select themselves into lower SES groups.

  6. Andrew Leigh says:

    ChrisPer, the criticisms about twins being a bad experiment are largely addressed in the article, which finds that the non-FE birthweight effects are similar in both populations.

    As to SES, the concern that Yobbo raised was selection (the child’s family SES). Whether birthweight affects your own SES is of course the question that the researchers are trying to answer.

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