Birth Order Effects Needn't Be Biological

And mutational load probably doesn't explain them

Birth order effects—where siblings systematically differ based on the order they were born in—are a subject of perennial interest and equally frequent consternation. Most people have siblings, so there’s a reason they’re interested; the source of consternation is less obvious and has to do with the fact that birth order effects are often hard to replicate. There are two big reasons for their replication issues:

1. Parents are inequality averse and;

2. Parents learn how to parent as they age.

Some examples will suffice.

Terskaya had access to large-scale census data from Mexico circa 2010. She used the variation in family size and child disability status to show that parents are averse to inequality. The method behind this is simple: There’s an education deficit associated with being disabled. In families with only one child, parents can’t show inequality aversion, but when there are multiple children, they can.

In multi-child families, it turns out the disability-associated education deficit is reduced. There are many more robustness tests and the power of unobservables to explain this finding was calculated and the conclusion is clear—it seems that parents do something to help their disabled kids catch up to their non-disabled siblings.

In a later publication, Sanz-de-Galdeano and Terskaya showed evidence of inequality aversion again, this time more directly. Using data on parent-child interactions, they showed that parents invested more time in their kids who had lower educational attainment polygenic scores—evidence that parents could detect a difference in endowments and invest accordingly.

This has implications for interventions that target lower-performing children outside the home because, if those children have their performance boosted by an intervention and their observed endowments relative to their sibling are made more equal, the inequality aversion parents act on could become less potent. Parents might let up from their compensatory efforts, investing less in the children whose endowments they were compensating for. As a result, compensatory intervention effect sizes could wind up attenuated.¹

A major reason why early work on birth order effects delivered ambiguous and inconsistent results had to do with the effects of family size and parental age.² Abdellaoui et al. explained this like so:

Within a family, later children have older parents by definition. Older parents have more life experience and may have higher income, which may help later children. Kantarevic and Mechoulan show[ed] that mother’s age at childbirth indeed mechanically offset the negative effect of birth order. Including parents’ age means we can separate the effect of parental age from birth order.

Tarkiainen et al. used Finnish register data and found something similar: modeled together, the effect of being later in the birth order was significant, sizable, and negative within families, whereas it was marginally significant, still negative, and much smaller between families.³

---

With a combination of decent methodology and large samples, birth order effects reliably show up for several different phenotypes. Contrary findings seem to have cropped up primarily in cases where the methods were poor or there wasn’t much power. So the question remains: why?

One answer is “mutational load”—later-born siblings have more mutations due to older fathers’ gametes having had time to accumulate additional mutations.⁴ Another is cultural preference—in some cultures, earlier-born children receive greater shares of bequests, favored roles in the family, etc. If resources given to kids matter for their development, then simple resource dilution could explain birth order effects.

There are additional, more specific theories to be sure, but I want to contrast biological and nonbiological theories. I suspect that when it comes to birth order effects, biology isn’t dominant.

---

Immigrant Acculturation