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One thing that we know from decades of twin studies is that childhood family environment explains a vanishingly small percent of long-run adult outcomes. Conte

One thing that we know from decades of twin studies is that childhood family environment explains a vanishingly small percent of long-run adult outcomes. Contextualizing this single study's result against the much larger, more established evidence from twin studies strongly suggests that one or more of the following is true. - The results are spurious and won't replicate. - The results are true, but the association is not causal, and is simply proxying correlation with an upstream factor. - Lack of touch does lead to methylation in the short-term, but by adulthood there's strong reversion to the mean. - The methylation has no significant impact on any actual metric that we care about like success, health, personality or wellbeing.

aidenn0 7y ago
There are issues with twin studies, not the least of which the number of twins raised separately, but not adopted, is tiny. However adoptive parents are a small self-selected subset of the population.
- tomrod 7y ago
  It's not vanishingly tiny, simply small. Can you think of a reason selection bias could be an issue, thus invalidating twin study cohorts?
  - anbop 7y ago
    Adoptive parents are people who have gone through a lot of effort and expense to get their children, so they're far more likely to be heavily invested in their kids. It's also expensive so they're likely to have money. If the effects of parenting are strong, but "cap out" -- i.e. above a certain level of care, parenting does little, but before that level it has a strong effect -- then twin studies will show little of that effect.
    - tomrod 7y ago
      I'm not sure I follow the causality argument here. A twin study should normalize everything genetically. Ergo epigenetic expression and environment are the only two causal levers. It sounds like you're arguing that parental involvement has a causal impact, which is the point of the studies to a degree. Perhaps my original question was unclear: is there something the twins do or have done to them that cause a latent selection bias before adoption, thus invalidating the twin studies approach as a whole?
      - aidenn0 7y ago
        We are questioning how much the parenting has an affect on the child. The parents are self-selected and so are more likely to be similar to each other.
        tomrod 7y ago
        Agreed. That was my point. Not all twins under adoption are adopted, meaning there is nothing particularly invalidating about the twin study framework.
- dcolkitt 7y ago
  Most twin studies simply compare identical and fraternal twins in non-adopted households. Since identical twins share twice the genetic covariance of fraternal twins, the impact of family environment can be backed out from the respective intra-pair correlation coefficients. In the limit case if genetics played no factor, then fraternal and identical twins should have identical pairwise correlations. For example suppose pairs of identical twins have 45% pairwise correlation for adult IQ. And say fraternal twins have 25% correlation for the same measure. That would tell us that the population level variance of adult IQ is 40% attributable to genetic heritability, 5% to environmental heritability , and 55% to non-heritable factors (i.e. not genetics and not family environment).
  - MaximumYComb 7y ago
    In case you're interested, IQ is much more heritable than that for adults. The following quote is from Wikipedia: "Twin studies of adult individuals have found a heritability of IQ between 57% and 73%[6] with the most recent studies showing heritability for IQ as high as 80%[7] and 86%.[8]. IQ goes from being weakly correlated with genetics, for children, to being strongly correlated with genetics for late teens and adults. "
  - objektif 7y ago
    Why would parents of twins pay much more attention to one than the other?
    - taneq 7y ago
      Maybe because, for whatever reason, one cries a lot more than the other?
      - objektif 7y ago
        But that would be an edge case. I would have assumed that identical twins behave similarly. How do they find enough neglected twins to make a reliable conclusion.
baxtr 7y ago
I don’t know. I have read this before. But this goes so much against my gut feeling and what I have experienced that it is hard to believe. It’s counter intuitive to say the least.
- telesilla 7y ago
  The documentary Three Identical Strangers also agrees with you, a fascinating account of triplets separated at birth to study parenting. https://en.wikipedia.org/wiki/Three_Identical_Strangers https://en.wikipedia.org/wiki/Three_Identical_Strangers

aidenn0 7y ago

There are issues with twin studies, not the least of which the number of twins raised separately, but not adopted, is tiny. However adoptive parents are a small self-selected subset of the population.

tomrod 7y ago
It's not vanishingly tiny, simply small. Can you think of a reason selection bias could be an issue, thus invalidating twin study cohorts?
- anbop 7y ago
  Adoptive parents are people who have gone through a lot of effort and expense to get their children, so they're far more likely to be heavily invested in their kids. It's also expensive so they're likely to have money. If the effects of parenting are strong, but "cap out" -- i.e. above a certain level of care, parenting does little, but before that level it has a strong effect -- then twin studies will show little of that effect.
  - tomrod 7y ago
    I'm not sure I follow the causality argument here. A twin study should normalize everything genetically. Ergo epigenetic expression and environment are the only two causal levers. It sounds like you're arguing that parental involvement has a causal impact, which is the point of the studies to a degree. Perhaps my original question was unclear: is there something the twins do or have done to them that cause a latent selection bias before adoption, thus invalidating the twin studies approach as a whole?
    - aidenn0 7y ago
      We are questioning how much the parenting has an affect on the child. The parents are self-selected and so are more likely to be similar to each other.
      - tomrod 7y ago
        Agreed. That was my point. Not all twins under adoption are adopted, meaning there is nothing particularly invalidating about the twin study framework.
dcolkitt 7y ago
Most twin studies simply compare identical and fraternal twins in non-adopted households. Since identical twins share twice the genetic covariance of fraternal twins, the impact of family environment can be backed out from the respective intra-pair correlation coefficients. In the limit case if genetics played no factor, then fraternal and identical twins should have identical pairwise correlations. For example suppose pairs of identical twins have 45% pairwise correlation for adult IQ. And say fraternal twins have 25% correlation for the same measure. That would tell us that the population level variance of adult IQ is 40% attributable to genetic heritability, 5% to environmental heritability , and 55% to non-heritable factors (i.e. not genetics and not family environment).
- MaximumYComb 7y ago
  In case you're interested, IQ is much more heritable than that for adults. The following quote is from Wikipedia: "Twin studies of adult individuals have found a heritability of IQ between 57% and 73%[6] with the most recent studies showing heritability for IQ as high as 80%[7] and 86%.[8]. IQ goes from being weakly correlated with genetics, for children, to being strongly correlated with genetics for late teens and adults. "
- objektif 7y ago
  Why would parents of twins pay much more attention to one than the other?
  - taneq 7y ago
    Maybe because, for whatever reason, one cries a lot more than the other?
    - objektif 7y ago
      But that would be an edge case. I would have assumed that identical twins behave similarly. How do they find enough neglected twins to make a reliable conclusion.

baxtr 7y ago

I don’t know. I have read this before. But this goes so much against my gut feeling and what I have experienced that it is hard to believe. It’s counter intuitive to say the least.

telesilla 7y ago
The documentary Three Identical Strangers also agrees with you, a fascinating account of triplets separated at birth to study parenting. https://en.wikipedia.org/wiki/Three_Identical_Strangers https://en.wikipedia.org/wiki/Three_Identical_Strangers