Endogenous parenting and twin adoption studies

by on March 1, 2011 at 4:41 am in Education | Permalink

I'm not sure how convincing I find this piece, by Alessandro Lizzeri and Marciano Siniscalchi, but it's worth a read if you've been following the debates over twin adoption studies.  In the model, parents both expose their children to learning and protect them.  Parents also judge a successful child by how much that child has their abilities, so in equilibrium the more a child differs from a parent, the more the parent intervenes to direct the path of the child's development.

Again, in the model children of "better" parents have better outcomes on average, above and beyond genetic transmission as a mechanism.  When it comes to adopted children, parents intervene more and parents also bring more similar interventions to bear on identical twins than on fraternal twins.

In twin adoption studies it will appear that parenting does not matter when in fact it does. Here is a neat passage from the paper:

Thus, differential sheltering by biological and adoptive parents provides a countervailing force to common rearing…We can also reinterpret this countervailing force…Differential sheltering by biological and adoptive parents implies that, despite being reared apart, adopted twins are in fact subject to a shared environmental influence–namely, adoption itself. This intuitively leads to greater phenotypic correlation by compensating for the lack of the direct commonrearing effect.

The piece ended up being published in the QJE 2008.

For the pointer I thank a loyal MR writer.

Steve Sailer March 1, 2011 at 1:51 am

The question you brought up in the previous post is quite interesting: What is the effect of sibling rivalry on identical twins raised together?

For example, in the pair of teenage identical twins I know best, one has decided that he wants to be an actor and the other has decided that he wants to be an engineer.

It seems quite plausible to me that if they were raised separately, they would have less dissimilar ambitions. But raised together, minute differences in talent and the urge to assert individuality have led to highly different ambitions.

This could imply that studies of twins raised apart might underestimate the effect of parents and home environment because the twins raised apart are not subject to the repulsive effects of sibling rivalry.

Steve Sailer March 1, 2011 at 3:07 am

My guess is that being raised by your biological parents is better for specialization. Under my model, John Stuart Mill would not have been as good an economist if he hadn't been raised by James Mill and instead have been raised by some random couple who had been checked out as likely good parents by the adoption agency.

If J.S. Mill had been adopted by some average middle class couple (assuming they were wealthy enough to provide for his nutrition and education, which is a big assumption for the early 19th Century), he still would have been very bright, but he wouldn't have grown up plugged into his father's network of the most advanced thinkers in England. If he had been adopted by somebody else, maybe Mill wouldn't have become the kind of historical figure who is referred to just by his last name.

On the other hand, John Stuart Mill might have grown up happier if he had been adopted. We hear a lot of sad tales from sad adoptees searching for their biological parents, but we don't hear much from contented adoptees because they don't have much to complain about.

For example, my upbringing as an adopted child was pretty idyllic. The professionals at the adoption agency had made sure my (adoptive) parents had no major flaws such as drunkenness, violence, or marital instability.

More subtly, being adopted means that your parents are unlikely to share your own peculiar minor flaws. For example, I am, by nature, prone to procrastination. I probably inherited that from one or both of my biological parents. Yet, my adoptive parents were, by nature, anti-procrastinators. This made for a good balance. So, during my low-stress youth, I was never late for anything. In contrast, in my own household today, nature and nurture pile up and thus procrastination is a constant problem.

J.S. Mill's intellectually intensive upbringing, while no doubt effective in cultivating his genetic capabilities to the fullest, seems like too much of a good thing.

TGGP March 1, 2011 at 5:23 am

I first heard of the idea that parenting is endogenous to the child from Judith Harris in "The Nurture Assumption".

Tracy W March 1, 2011 at 7:27 am

An interesting paper. So what test could tell the difference between the two hypotheses? One that comes to mind is the difference between true identical twins, and twins that are actually fraternal but their families believed to be identical, until genetic testing got involved.

Bill March 1, 2011 at 8:22 am

Are there any adoption studies in which the twins are adopted by the grandparents. It would seem that the adoption effect might be mitigated.

Steve Sailer March 1, 2011 at 3:08 pm

"So what test could tell the difference between the two hypotheses? One that comes to mind is the difference between true identical twins, and twins that are actually fraternal but their families believed to be identical, until genetic testing got involved."

Generally, the opposite case is more common: parents notice tiny differences between their twins and decide that therefore they must be fraternal rather than identical. For example, the Hamm Brothers, who were the top two American men's all-around gymnasts at the 2004 Olympics were always told by their parents that they weren't identical twins because their hair twirls in opposite directions. To television viewers, however, they look and act identical.

TGGP March 1, 2011 at 7:58 pm

Bill, I don't know about identifying twin-status, but there are studies of biracial children which take into account the perception of race on the part of parents. Can't remember where I heard about them though. The one factoid I remember is that it makes a difference whether your biological parents were BFWM or WFBM (though we can't conclude whether that's a selection effect or result of pre-natal environment).

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