Commenter Lion of the Judah-Sphere informed us of a post by JayMan claiming birth order has no real effect on IQ.

I don’t know whether birth order and has a causal effect on IQ or whether it’s just a proxy for other variables, but in my humble opinion, there is indeed a negative correlation between IQ and birth order.

Using unpublished data on 209 Canadians (aged 19 to 76; IQ 57 to 137) I obtained the following regression equation:

IQ = 104.056 – 1.650(BIRTHORDER)

Assuming a linear relationship, this predicts the following:

Firstborns: average IQ 102
Secondborns: average IQ 101
Thirdborns: average IQ 99
Fourthborns: average IQ 97
Fifthborns: average IQ 96
Sixthborns: average IQ 94
Seventhborns: average IQ 93
Eighthborns: average IQ 91
Ninthborns: average IQ 89
Tenthborns: average IQ 88
Eleventhborns: average IQ 86
Twelfthborns: average IQ 84

Causation

I suspect later-borns average low IQ for the following reasons:

1) r genotype: As the late scholar J.P. Rushton, observed, primitive genotypes are not intelligent or caring enough to have high survival rates for themselves or their kids, so they evolved high birth rates to compensate. Low IQ people are more likely to have these primitive genes, and thus come from families with lots of kids.

2) Prenatal environment: I suspect that each child a woman gives birth to makes subtle changes to her womb. For example, research shows that for males, the more male fetuses have been in your mother’s womb before you, the higher the odds of being gay. Could a similar effect increase the odds of low IQ?

3) Mutation load: Older parents pass on more genetic mutations to their kids, some of which might impair IQ. Later borns are obviously concieved by older parents than newer borns, on average.