Tuesday, November 30, 2010

Birth order and the Flynn effect

Studies on birth order show that first-born children tend to have IQs that are, on average, 3 points higher than those of their second-born siblings, and second-borns in turn have a point or two advantage on their younger brothers and sisters. The most popular (and most PC) explanation for this is that couples are able to invest all of their parental energy into the first-born before the next child comes along. After that, though, parental attention is split among multiple offspring. But if the explanation is at least partially one of nature (or a product of a woman's reproductive quality declining as the number of times she's given birth accumulates), might there be a causal relationship between declining birth rates and the Flynn effect (realizing that at most this would account for a 1-2 point increase in average IQ and thus constituting only a fraction of the observed effect)?


Anonymous said...

The theory of declining reproductive quality with age would predict the opposite of the Flynn effect in places where women have children later (i.e. the First World).

The theory of diluted parental investment would predict the Flynn effect. But IIRC it cuts across socieconomic strata, so perhaps the resource being diluted is something you can't buy, such as maybe facetime with the kids? Dunno.

Stopped Clock said...

It could be related to birth order directly instead of birth order as a proxy for age.

Anonymous said...

I believe the idea of 1st born kids having higher IQ's is false. This is because when you take only childs out of the data (who are from higher IQ families), the difference between 1st borns and later born kids shrinks to zero or close to zero. I think I read that in one of Judith Rich Harris' books.

ziel said...

Anonymous2 - I found that only-children are indeed smarter than others, but there appears to be a strong family-size effect - the more siblings you have, the dumber you are. See here.

Regarding birth-order effect, one explanation is the influence a precocious child has on parents' decision to have more children. A difficult child might discourage parents from having more children, while a smarter (and thus likely easier to raise) child might encourage more. This would generally skew the higher intelligence towards fewer children, as parents will reduce the child-bearing once a problem-child presents itself (the assumption is that smarter kids will be, on average, less of a burden).

I'm pretty sure I read John Hawks propose that, but I can't for the life of me find it on his blog.

Audacious Epigone said...


That's what I was thinking, or that it's not so much age but something else that deteriorates as the number of children birthed increases.


Thanks for the link.

bgc said...

What Stopped Clock said.

Strictly, to control properly, *only* families with a specific number of sibs should be analysed - each analysis being separate (eg look for birth order effects among only families with three sibs, four sibs etc).

One could control for socio-economic effects by using Mormons - which are the only group where the largest families are found among the wealthiest and most highly educated (and presumably highest IQ) sub-groups.

IF one found a birth order effect in all these separate analyses, I think it would be good evidence.

Otherwise an effect size of a couple of IQ points is more likely to be due to residual confounding (e.g. when the confounder, such as socio-economic class, is imprecisely measured, hence only partially-controlled-for), as Stopped Clock implies

silly girl said...

It would be interesting to see average IQ's based on mother's age at time of child's birth. I think I once saw that younger moms have slightly smarter kids.

Audacious Epigone said...


Seems like something someone from the University of Utah should consider.

Silly Girl,

That would be after controlling for other variables, right? I can't imagine that 20 yo single mothers have slightly smarter kids than 35 yo SWPL couples do.

Florida resident said...

Dear "Audacious Epigone"!
I have tried to find the original research paper in question (as opposed to newspaper article), and failed.
May I kindly ask you to help me with that?
Respectfully yours,
Florida resident.

Audacious Epigone said...

Florida Resident,

I don't have access to the journal Intelligence, although there are other studies with similar findings.

Florida resident said...

Thank you for the reference.

FuturePundit said...


I can imagine several mechanisms:

- Mom's nutrients are drained by each successive child.

- Dumber people have more kids. So a parent with a smart kid who has more kids is more likely to have dumber kids than the first kid.

- Mom gets older. Her womb doesn't work as well.

- Mom becomes more careless about what she eats when having the later kids.

- Mom's health habits deteriorate. She is fatter when she has the later kids. The first kid put an end to her jogging. She doesn't have as much time to keep in shape. She has less energy for making nutritious food.

- Stress is higher in Mom's body and she gets less sleep while carrying the later babies.

- Stem cells from the previous pregnancies cause more of an immune response to each successive baby.

FuturePundit said...

Hey Anonymi: Ziel wouldn't have needed to call one of you Anonymous2 if you took the trouble to give yourself a unique pseudonym.

Ajpod 7 said...

I'm wondering if the 3 point advantage in IQ of first borns has anything to do with the fact that they get first dibs on the gluteofemoral fat deposits which are rich in n-3 fatty acids that are vital for the maturation of the brain, in utero.

These fat stores are built up throughout childhood and adolescence and each successive cycle of pregnancy and lactation strips more of it from the mother permanently. This is a more "nature" (and by far, the least PC) explanation for why first borns tend to be (albeit slightly) more intelligent than their younger siblings. I'm not entirely sure where I stand on it yet.