In the comments on this SlateStarCodex post, someone is questioning my claim that sharing the chorion (what is that?) is unimportant:
Emil:
Uterine environment stuff is not important. We know this because of this study
link.springer.com/article/10.1007/s10519-015-9745-3
and because ordinary siblings are about as similar as DZ twins who did share the uterus at the same time. They have the same genetic relatedness of .50 by descent.
NN:
Uterine environment stuff is not important. We know this because of this study
link.springer.com/article/10.1007/s10519-015-9745-3
and because ordinary siblings are about as similar as DZ twins who did share the uterus at the same time. They have the same genetic relatedness of .50 by descent.
First, the paper that you linked doesn’t seem to be as strong as you claim. I quote from page 7: “Intra-uterine environmental factors do influence the intra-pair similarity of MCMZ and DCMZ pairs for birth weight, weight during the first years of life, achieving motor milestone standing alone, externalizing behaviors at age 3, internalizing behaviors and anxiety at age 12, and autistic behavior at age 7.” I’d be very reluctant to wave all of that away as unimportant.
The abstract claims that these results would only have “small” effects on heritability estimates, but the numbers brought up in the Discussion don’t sound that small to me: “For internalizing behavior at age 12 years, with a difference of 0.11, the heritability will be 50 % [2*(0.71–0.46)] when including both types of twins and 38 % [2*(0.65–0.46)] when including only DC twins. For anxiety at age 12, the heritability will be 70 % [2*(0.71–0.36)] when both types were included and 52 % [2*(0.62–0.36)] when only DCMCs were included.” Maybe these results are due to chance and as such these effects aren’t real, but we’d need to look at more studies to determine whether or not that is the case.
Regardless, even if we were to assume that research generally finds uterine environment to be unimportant, isn’t the (as far as I can tell, let me know if there is something that I missed) very well established Older Brother Effect – that is, having more biological older brothers makes a man more likely to be gay – a glaring exception to that? And if it is, doesn’t that raise the question of what other effects we might have missed?
The first point is to mistake a detection of an effect for an important effect. A large study can find unimportant effects and a study that looks for a lot of effects, will find some with p<a (p value less than alpha aka “significant”) by chance. It is important to focus on effect sizes, not p<a.
So, the proper strategy is to extract the data of the effects and analyze them. I did this by extracting them from the provided supplementary materials. I note that the authors were silly since they not only provided an oddly formatted table, but also provided it in .DOCX instead of a spreadsheet. Anyway, I extracted the data in R and analyzed it. The analysis is simply calculating MC – DC. This is the difference between the relatedness of the persons by whether they shared the chorion or not. A null + sampling error model predicts a distribution of effects around 0 with a few outlying values. What do we see?
The mean is 0.00, tightly clustered around 0. In fact, we can go further. A null model + sampling error model also predicts that the larger effects in either direction should be the less precisely measured effects. So, we calculate the standard errors of the deltas and plot this against the absolute effect size.
We do see a large effect: the less precise estimates tend to be further away from 0. So, it really does seem that chorion effects are either nonexistent or very weak indeed. Of course, one could invoke the contextual defense: but it’s possible that sharing the chorion is important for some traits and unimportant for others, and you haven’t disproven that. This is the same defense usually adopted by authors whose pet hypothesized effects fail to replicate, so color me skeptical.
Data and code
