01 Mar 2011

On Those Twin/Adoption Studies…

All Posts 23 Comments

The lynchpin in Caplan’s case that “parents don’t matter” is the evidence from twin/adoption studies. I confess I haven’t delved into them yet, but my arms’ length understanding is that if you look at identical twins who are raised by different families, and then again if you look at children who are totally unrelated genetically and yet were raised by the same parents, then it looks like parenting has very little to do with how the kids turn out.

So here’s my question: On both ends of these studies, we are looking at parents who adopted a kid who wasn’t their biological child, right? I mean, if you’re studying identical twins, then at least one of them is raised by a non-biological parent, and maybe both. And then on the other end, you’re clearly studying adopted kids.

If so, then doesn’t that represent just about the biggest bias in the study you could imagine? What Caplan is saying is that if we focus on people who go out of their way to amplify their parenting on the world, AND whom the relevant authorities deem as worthy parents, then we don’t see much difference in the effects of parenting on the kids.

I would be much more interested in the results of a study that randomly took kids and assigned them to different households. Of course, I would rather remain ignorant than having someone carry out such a study.

23 Responses to “On Those Twin/Adoption Studies…”

  1. Ryan Murphy says:

    You’re assuming that the government allocates kids for adoption optimally. Haven’t you heard the horror stories?

    Besides, the psychology literature isn’t about looking at parental behavior as much as it is for looking for demonstrations that the kids differ in the same ways. According to Steven Pinker (who is in turn citing a bunch of the relevant literature in The Blank Slate), the following things are more likely to be found together in identical twins than fraternal twins (and no environmental factor is correlated with them): Autism, dyslexia, language delay, language impairment, learning disability, left-handedness, major depressions, bipolar illness, obsessive-compulsive disorder, and sexual orientation. Identical twins who meet later in life have correlated behavior in gambling, divorcing, committing crimes, getting into accidents, watching television, giggling incessantly, giving interminable answers to simply questions, and dipping buttered toast in coffee. This is found in page 46-47 of the book, which I’m not doing a very good job citing.

    This might even be more to your point. In families where there are “virtual twins” (two unrelated siblings growing up in the same household), their IQ scores are barely correlated.

  2. Anon73 says:

    Clearly you’re just insane if you don’t agree with these studies Bob. The Sabbath was made for man, not man for the Sabbath.

  3. Blackadder says:


    I believe your point is right for adoption studies, but not for twin studies (which compare identical to fraternal twins).

  4. Blackadder says:

    To clarify, the twin studies do not necessarily involve having the twins raised in different families.

  5. English Bob says:

    I think, actually, adopted children tend to underperform, despite the selection problem you mention. If Caplan is right, then this could be because people who give up kids for adoption tend to be from the low end of the societal scale.

    Also, I don’t think the study you mention in the last paragraph would be necessary. Couldn’t they just look at other kids the same family had adopted?

    For example, did you know that Steve Jobs was adopted? And did you know his biological sister, also adopted, is Mona Simpson? One more question: Have you ever heard of Patti Jobs?

    • English Bob says:

      Small correction: Mona Simpson wasn’t adopted.

  6. mario rizzo says:

    If we assume (as I imagine Caplan does) that evolutionary theory is basically correct, I wonder why parents are hard-wired to THINK that their parenting matters. Evolutionary fitness would argue that if they didn’t they would reproduce more, etc.

    • ADA says:

      Nobody says parenting doesn’t matter. Just that short of extreme deprivation of basic needs and neglect, especially in early childhood, you don’t have any meaningful influence on what kind of a human being your child turns out to be in terms of character, talent, abilities, intelligence, etc….. That is all.

      Also, It’s hard to figure out from your response what your take is on evolutionary theory, but it is obvious that you know nothing about it.

      • bobmurphy says:

        ADA, yes they do. I am 95% confident that Caplan literally has written “parents don’t matter.” That’s why I have such a bee in my bonnet about this. (Can someone find me a quote?)

        It’s true, when you press Caplan, he backpedals. “Oh sure, if you lock your kid in a closet or feed him just Gummy bears for the first 18 years, that will affect his SAT scores. But I’m talking about in the range of ‘good parent’ behaviors, little tweaks don’t do anything.”

        So my post is reinforcing (at least it’s possible) this fact, that we are looking not among all parents, but among those from whom many of the “bad parents” have been weeded out already. So it shouldn’t be as shocking that we don’t see huge differences in their outcomes.

        • Dan says:

          I was able to find this quote from Caplan,

          “Traits run in families primarily due to heredity, not upbringing.  The mighty effects that people ascribe to parenting are largely imaginary.”

          I didn’t find one where he specifically said they don’t matter at all but most of his comments are that they have very little impact.

          So I guess the government is wasting their time with all the propaganda. We would have the same society with a free market in education anyways, according to Caplan’s view.

        • Dan says:

          Let me know when they compare one twin that is home schooled by a libertarian and the other twin taught by a socialist. Why study a group of kids that all go to brain wash camp from k-12? I would agree that if you take all the kids and brainwash them that parents don’t matter.

  7. Sam says:

    Commonsense would say: parenting matters a lot; but super-involved “helicopter” parenting does not add anything beyond laid-back but good parenting.

    Also, parenting matters more in cases of special needs children.

    Whatever the case, the thesis of this book–that parenting doesn’t matter so have more kids–is very unlikely to convince the target audience of intellectuals to get pregnant! 🙂

    • bobmurphy says:

      Mr. Torode apparently thinks “commonsense” has something to do with economists. His mistake. (And I say that as an economist.)

  8. geoih says:

    “I would be much more interested in the results of a study that randomly took kids and assigned them to different households.”

    How about a study with a real control: children that grow up completely without parents.

  9. ADA says:


    You are reminding me of all the type of objections statists usually have to libertarian theory and economics when they just get some glimpses into them here and there There is always a strong motivation to resist the possibility of getting out of your comfort zone, at least for a while.

    I told you before. For starters, read just the chapter on Children in Steven Pinker’s “The Blank State”. Get familiar with the argument and the full scope of the research. Then take it from there….

    • bobmurphy says:

      ADA, back at you. If you want to defend Caplan by saying, “Yes he at times oversimplifies in order to be provocative,” OK fine. But you are wrong when you flat out declare, “Nobody says parenting doesn’t matter.”

      And your arguments about Pinker are all suspect, if my claim is true. You are taking it at face value that they have shown, e.g., that genetics explains speech delays much better than parenting styles. But if my point here is valid, then that could be because we’re only looking at the subset of parents who aren’t nearly as likely to be contributing to their kids’ speech delays. (NB I am not saying I think parenting style necessarily causes speech delays, I’m just pointing out that you are unable to even step outside of your view on the matter and see why it might be wrong. I.e. the very thing of which you are accusing me.)

      • ADA says:

        I never said you were totally wrong or wrong about everything. I said that from what you’e been criticizing, thus far, you are obviously not very familiar with what the studies of behavioral genetics actually tell us and what they don’t.

        For example, they certainly don’t tell us that it’s all about genetics, but only about half. Second, they specifically tell us that what they call the “shared environment” (parenting being in that category) matters very little or practically nothing. However, all of these studies, and there have been many of them, have been conducted on subjects belonging to more or less similar (middle class) socioeconomic status. Nevertheless, the difference in cultures and environments varied substantially between the subjects.

        What is suspect about Pinker? I merely suggested a sort of 1 hour crash course on the subject. That is all.

      • ADA says:

        And I never had any intention of defending Caplan.

  10. mario rizzo says:


    You are rude.

    • bobmurphy says:

      He lacks the courtesy gene.

      • Daniel Hewitt says:

        ..that or he’s a product of bad parenting (depends on how this argument shakes out).

      • ADA says:

        So it’s not my fault then.

        It’s not all in the genes. You really should not learn about this stuff from Caplan.

      • ADA says:

        So it’s not my fault then.
        It’s not all in the genes. You really should not learn about this stuff from Caplan.