Back in the day we worried a lot about the amount of weight a woman gained during pregnancy.  Too much weight gain increased the risk of pregnancy associated hypertension (and worse pre-eclampsia, and even worse eclampsia with fetal and even maternal death).  It also increases the likelihood of pregnancy associated diabetes, with its adverse effects on the fetus.

So when my wife was carrying our two boys, we watched her weight like a hawk (particularly since there was diabetes in her family).  Fellow med students and their wives did the same.

That may have not been so good according to a recent study of birth weight differences between identical twins [ Proc. Natl. Acad. Sci. vol. 109 pp. 11366 – 11371 ’12 ].  To cut to the chase, they found that the bigger twin had a bigger brain and was smarter (by a few IQ points).   It was a big study (139 twin pairs or which 85 were identical — monozygotic if you want to impress your friends).  They only looked at mild levels of weight disparity — the difference in birth weight had to be less than 20% of the weight of the heavier twin.

Greater birth weight resulted in greater  brain volume as shown by magnetic resonance imaging (MRI), mostly due to more cerebral cortex.  Interestingly the cortex wasn’t any thicker in the heavier twin, there was just more of it (the brain was more wrinkled).

Interestingly, the fraternal twins were smarter than the monozygotics by having full scale IQs of 112 vs. 109.  What’s going on? The average IQ should be 100.  However, they excluded twins with IQs under 80, e.g. they only looked at 1.3 standard deviations (which is 15 points) below the mean, while keeping everything above it.   It’s an interesting mathematical problem, which I don’t have time to solve, to integrate the bell curve from 1.3 standard deviations below the mean to all the way above it, and see what the average would be — my guess is that this is why the average IQs are so high.

There is also something called the Flynn effect which you should know about, as long as we’re talking IQ. [ Science vol. 323 p. 989 ’09 ] It was described  nearly 30 years about by  James Flynn of New Zealand.  He noted  that IQ scores rose steadily in the 20th century for children and adults in Western nations.  Using the late 20th century average IQ score of 100, the comparative score for the year 1900 was calculated to 60 — something clearly not true.  Actually the improvements were not in general knowledge or mathematics, but in abstract reasoning.

The conclusions of the PNAS paper seem solid, but behind your back I’ve made several inferential leaps, some of which can be checked fairly easily.  Do bigger babies have bigger brains, and if they do, does this persist throughout life.  Second do people with bigger brains have higher IQs — again the extremes must be cut off — down 2 -3 standard deviations from the average brain size, IQ is way down, and the smaller the brain down here the smaller the IQ.  Similarly with macrocephaly (large brain) — usually there is something wrong.  These are factual matters, whose results are probably in the literature already.

What’s great about the paper is that it controls for heredity, as the genomes of identical twins should be nearly identical — I’m not sure anyone has looked, and given the recent haphazard way our genomes vary between us, it should be.  For details please see https://luysii.wordpress.com/2012/07/31/how-badly-are-thy-genomes-oh-humanity/

The fourth inferential leap, is that bigger birth weight in a twin has to do with better nutrition in utero.  Clearly the bigger twin had more proliferation of cortical precursors resulting in more cerebral cortex.  So maybe your mother should have had a few more fudge cakes and ice creams when she was carrying you.

I don’t know the answer to these questions, but as Mark Twain said :”There is something fascinating about science.  One gets such wholesale returns of conjecture out of such a trifling investment in fact”.