Almost all traits run in families. But why? People have literally debated the question for thousands of years. Is the cause nature/heredity/genes, nurture/upbringing/parenting, or some mixture of the two?
Until a few decades ago, the debaters basically just chased their own tails. And then... enlightenment happened. Social scientists finally discovered a Rosetta Stone to disentangle nature from nurture. Or to be precise, they discovered two Rosetta Stones. The first was the twin study: comparing identical to fraternal twins. The second was the adoption study: comparing adoptees to their adopted families - and occasionally their biological families as well.
Since then, researchers have used these Rosetta Stones to decipher a massive list of mysteries. As I recount in my forthcoming book, twin and adoption researchers studied human health, intelligence, happiness, success, character, values, appreciation, and more. Their answers are beyond surprising. With a few important exceptions, they learned that nature handily wins its ancient cage match with nurture, especially in the long-run. Traits run in families primarily due to heredity, not upbringing. The mighty effects that people ascribe to parenting are largely imaginary.
Faced with these achievements, you'd expect almost any social scientist to be impressed, even awed. But not Tyler Cowen. His reaction, instead, is to complain that twin and adoption methods don't contribute more.* Who cares if you've solved the ancient nature/nurture debate? He wants a Theory of Everything.
I submit that this is both unreasonable and ungrateful. Behavioral geneticists don't have a Theory of Everything. No one does. But behavioral geneticists have answered many important, age-old questions. How many other sub-disciplines in social science can say the same?
Now I'll reply to Tyler point-by-point. Tyler's in blockquotes, I'm not.
"Culture" and "genes" are two major factors determining individual
outcomes, toss in parenting, and if you wish call parenting and
culture two parts of "environment." It is obvious that culture matters
a great deal, and this comes from knowledge which existed prior to
rigorous behavioral genetic studies.
... "The culture word" may be overused and abused,
but still the power of culture is evident.
If "culture" just means "everything besides heredity and upbringing," then Tyler's clearly right. Identical twins raised together are hardly ever literally identical, therefore other stuff matters. A lot. But if you define culture more falsifiably, things get complicated very quickly. Sure, there are traits like accent that clearly stem from humans' tendency to copy each other. And yes, you can't be "bookish" unless your society has books. Nevertheless, many allegedly cultural traits could easily be genetic, and we don't yet have a Rosetta Stone to disentangle the two.
If twin adoption studies seem to show that parenting does not matter much, I think:
1. Matter for what and for whom? Parenting matters a lot for
language and religion and obedience and also one's sense of "how the
world works," and those factors matter to parents even if they don't
always matter to researchers and economists. The word "matters" is
going to carry real weight here; in my admittedly extreme pluralist
view, "doesn't affect adult income" does not translate into "does not
Sigh. In my book, which Tyler not only read but blurbed, I cover the twin and adoption evidence not just for income, but for an entire Parental Wish List: health, intelligence, happiness, success, character, values, and appreciation. Religion falls under "values," and the punchline is that parenting has a big but superficial effect. Parents strongly affect what you say your religion is, but have little long-run effect on your intrinsic religiosity or observance. I don't discuss language, but it's pretty clear how a twin or adoption study would play out: You can make your kid semi-fluent in another language with a lot of effort.
2. We already know that culture matters a great deal in shaping what
kind of adults children become, but often individual families cannot
much affect the broader culture a child is raised in. It's sometimes
the individual family which is impotent, not the surrounding culture as
Plausible. I've made such arguments myself. But twin and adoption methods are poorly designed to test such claims, and it isn't reasonable to expect them to.
3. Most parents are deep conformists. There isn't always a lot of
cross-sectional variation in adoption studies. Even if most parenting
strategies don't matter (if only because they are not varying much), if
a child is raised in an Orthodox Jewish family, or in a strict
American-Chinese family, or among the Amish, that probably matters,
even adjusting for genes.
The adoption studies can be showing that a) most parents don't so
much shape a child's culture at the margin, or b) that environment
doesn't much matter in light of the power of genes.
Twin and adoption studies measure the effects of the kinds of parenting that people in the First World frequently use. I say this repeatedly in my book. If you want to do social policy or weigh whether to join the Amish, it's an important limitation. If you want to answer the kinds of questions that most parents in the First World are actually asking, it's not. And if you want to call the vast majority of Western parents a bunch of "conformists" and claim that their parenting is all basically the same, give credit where credit is due. On the surface, parenting styles seem to vary widely. The only reason anyone would conclude that these diverse approaches are roughly equivalent is by reasoning backwards from their effects. And the only reason anyone would conclude that these effects are small is twin and adoption evidence itself.
* Tyler singles out "twin adoption studies," which is normally a synonym for "separated twin studies." But his critique applies to ordinary twin and adoption studies as well.