Understanding some key facts about twin and adoption studies helps make their results seem less counter intuitive.
The data discussed here is primarily on children and parents in first world countries who are non-poor. This data does not help answer questions about parenting effects that are very different from typical first world non-poor parenting styles. The data does not help address the effect of growing up in malnourished or without access to education. Indeed, twin and adoption studies with adopted kids in extremely poor households show that nutrition is an important predictor of life outcomes (link)
It also doesn’t address extreme parenting styles. Not many people raise their kids in the woods cut off from the rest of society and this kind of variable is not included in the regressions, so the data has little to say about this kind of parenting.
If adopting parents treat their adopted children with “less intense” parenting than their biological children, then adoption studies will understimate the effect of parenting. In the extreme case, if all adopting parents treat their adopted children the same as other adopting parents but vary in how they treat their biological children, we will measure a zero effect size even if parenting has important effects.
Parenting could have a big effect on combinations of outcomes while only having small measured effects on each individual outcome. For example, parenting could have a large effect on “having a major drug or alcohol or gambling problem” but the measured effect of parenting on each of these individually could still be small because the adoptees and their adopted sibblings can have different symptoms (one has a drug problem and another has a gambling problem).
It’s also the case that adopting parents are probably systematically different from non-adopting parents. It could be that non-adopting parents tend to have parenting styles that do have important effects on long term child outcomes, while adopting parents have parenting styles that have very small effects.
When an adoption study finds that parenting does not affect outcome X, it does not mean that parenting cannot affect it, just that the parenting styles in the data set did not affect it.
As I understand it, it means that they did a regression between some aggregate measure of family environment (like total family income) and the outcome of interest and it put a small regression coefficient on the aggregate measure and/or that the differences in outcomes between adopted children in the same home were not smaller than the differences between adopted children in different comes. In the later case, I think you can compare adopted children both to adopted and unadopted siblings. Have I explained that clearly?
Thanks! It seems to say that how well you age is more determined by genetics than who raised you, which is unsurprising given that you’ve had decades of other environmental influences by that time.
I see at least two twin-adoption studies which deal with adult outcomes (in Ch 2. of Caplan’s book), but I haven’t searched very thoroughly:
(I’ve requested this paper, but don’t yet have it Edit: here it is ). Age differences in genetic and environmental influences for health from the Swedish Adoption/Twin Study of Agings this one has adult outcomes, but Jennifer Harris et al., “Age Differences in Genetic and Environmental Influences for Health from the Swedish Adoption/Twin Study of Aging,” Journal of Geroontology 47 (3) (May 1992), pp. 213-220.
I haven’t gone though all the citations thoroughly though, so there might be more there. You’re right that there doesn’t seem to be that many.
Interpreting Adoption Studies
This is supplementary.
Understanding some key facts about twin and adoption studies helps make their results seem less counter intuitive.
The data discussed here is primarily on children and parents in first world countries who are non-poor. This data does not help answer questions about parenting effects that are very different from typical first world non-poor parenting styles. The data does not help address the effect of growing up in malnourished or without access to education. Indeed, twin and adoption studies with adopted kids in extremely poor households show that nutrition is an important predictor of life outcomes (link)
It also doesn’t address extreme parenting styles. Not many people raise their kids in the woods cut off from the rest of society and this kind of variable is not included in the regressions, so the data has little to say about this kind of parenting.
If adopting parents treat their adopted children with “less intense” parenting than their biological children, then adoption studies will understimate the effect of parenting. In the extreme case, if all adopting parents treat their adopted children the same as other adopting parents but vary in how they treat their biological children, we will measure a zero effect size even if parenting has important effects.
Parenting could have a big effect on combinations of outcomes while only having small measured effects on each individual outcome. For example, parenting could have a large effect on “having a major drug or alcohol or gambling problem” but the measured effect of parenting on each of these individually could still be small because the adoptees and their adopted sibblings can have different symptoms (one has a drug problem and another has a gambling problem).
It’s also the case that adopting parents are probably systematically different from non-adopting parents. It could be that non-adopting parents tend to have parenting styles that do have important effects on long term child outcomes, while adopting parents have parenting styles that have very small effects.
When an adoption study finds that parenting does not affect outcome X, it does not mean that parenting cannot affect it, just that the parenting styles in the data set did not affect it.
Nitpick: Probably did not affect it differently.
Maybe I should know this, but what does it formally mean when a study claims “parenting does not affect outcome X”?
As I understand it, it means that they did a regression between some aggregate measure of family environment (like total family income) and the outcome of interest and it put a small regression coefficient on the aggregate measure and/or that the differences in outcomes between adopted children in the same home were not smaller than the differences between adopted children in different comes. In the later case, I think you can compare adopted children both to adopted and unadopted siblings. Have I explained that clearly?
Here’s an arbitrary adoption paper cited in the book: Sacerdote, B. (January 01, 2007). How Large Are the Effects from Changes in Family Environment? A Study of Korean American Adoptees *. Quarterly Journal of Economics, 122, 1, 119-157.
Note that the Sacerdote is the only study I’ve been able to find on adult outcomes of adoptees. There’s not much out there.
If you’re interested, I posted the swedish study.
Thanks! It seems to say that how well you age is more determined by genetics than who raised you, which is unsurprising given that you’ve had decades of other environmental influences by that time.
I see at least two twin-adoption studies which deal with adult outcomes (in Ch 2. of Caplan’s book), but I haven’t searched very thoroughly:
(I’ve requested this paper, but don’t yet have it Edit: here it is ). Age differences in genetic and environmental influences for health from the Swedish Adoption/Twin Study of Agings this one has adult outcomes, but Jennifer Harris et al., “Age Differences in Genetic and Environmental Influences for Health from the Swedish Adoption/Twin Study of Aging,” Journal of Geroontology 47 (3) (May 1992), pp. 213-220.
I haven’t gone though all the citations thoroughly though, so there might be more there. You’re right that there doesn’t seem to be that many.
This one talks about adult outcomes, but I can’t find out what ages they’re actually talking about, so it may be that most of them are early in life. Thomas Bouchard et al., “sources of Human Psuchological Differences: The Minnesota Study of Twins Reared Apart,” Science 250 (4978) (October 1990), p. 223.
I want to say “That can’t be right”, but I don’t have Caplan’s book with me to check his references. I’ll check this evening.
That study pretty much changed my mind about adopting, so if you find anything else, I’d like to know.