The idea of using twins to study the heritability of traits was the brainchild of the 19th-century British intellectual Sir Francis Galton. He's not exactly the progenitor you might want for your scientific methods. Galton coined the term "eugenics" and was the inspiration for the push to manipulate human evolution through selective breeding. ...

Over the last few decades, Galton's older half-cousin Charles Darwin has been promoted from secular sainthood to his current roles as the Jesus of atheists. But, the rise of Darwininsm in the world has not been an unmitigated blessing, so Galton has come to play the role of scapegoat, or devil. Since Darwin, the secular redeemer, is, by definition, above sin, all sins associated with Darwinims must be the fault of the designated devil, Galton.

Nearly five decades after Galton published "The History of Twins"—and more than 10 years after the word "gene" entered the lexicon—researchers in the 1920s "perfected" Galton's methods by comparing identical and fraternal twins and inferring heritability from the differences between the two. The twin study today is based on the same assumptions that were made back then. (As you may be aware, a lot has changed in the field of genetics over that time.) And despite numerous indications that these assumptions are deeply flawed, researchers continue to crank out new papers, probably in response to a public demand—both insatiable and inexplicable—for evidence that we're just like our parents. (If only Freud were alive today.) ... 

Twin studies rest on two fundamental assumptions: 1) Monozygotic twins are genetically identical, and 2) the world treats monozygotic and dizygotic twins equivalently (the so-called "equal environments assumption"). The first is demonstrably and absolutely untrue, while the second has never been proven. 

That identical twins do not, in fact, have identical DNA has been known for some time. The most well-studied difference between monozygotic twins derives from a genetic phenomenon known as copy number variations. Certain, lengthy strands of nucleotides appear more than once in the genome, and the frequency of these repetitions can vary from one twin to another. By some estimates, copy number variations compose nearly 30 percent of a person's genetic code. 

These repeats matter. More than 40 percent of the known copy number variations involve genes that affect human development, and there are strong indications they explain observed differences between monozygotic twins. For example, it's often the case that one identical twin will end up victimized by a genetically based disease like Parkinson's while the other does not. This is probably the result of variations in the number of copies of a certain piece of DNA. Copy number variations are also thought to play a role in autism spectrum disorder, schizophrenia, and ADHD, all of which can appear in only one member of a monozygotic twin pair (PDF). If copy number variations can affect discrete and diagnosable disorders, then why shouldn't they influence far more complex behaviors like your inclination to head to the polls on a Tuesday night in November? 

That's just the beginning of the genetic differences between monozygotic twins. As a result of mutations during development, about one in 10 human brain cells has more or less than the typical two copies of a chromosome. Identical twins also have different mitochondrial DNA, the genetic information stored in the cellar organelle responsible for processing glucose. Research suggests that mitochondrial DNA affects brain size among a host of other neurological traits.

Twin studies also rely on the false assumption that genetics are constant throughout one's lifetime. Mutations and environmental factors cause measurable changes to the genome as life progresses. Charney cites the example of exercise, which can accelerate the formation of new neurons and potentially increase genetic variation among individual brain cells. By the time a pair of twins reaches middle age, it's very difficult to make any assumptions whatsoever about the similarity of their genes.

To his credit, Matthew Yglesias smells a rat:

That doesn’t seem to me to follow. It’s still the case that identical twins are more genetically similar than other kinds of siblings. So if we have a study showing that identical twins are systematically more similar in some respect than non-identical twins, we’re still in possession of evidence about the influence of genetic similarity on behavioral similarity. 

One of his commenters gets to the heart of the problem.

I'd go further. The case where identical twins are truly identical is a floor, not a ceiling. If identical twins are so similar despite some "copy number" and "mtDNA" differences, well, think how much more they'd be similar if you took even those additional genetic differences away. Unless the guy thinks these residual differences are anti-correlated (instead of being uncorrelated) with any differences in environmental inputs - a fairly bizarre supposition - what he's really saying is that twin studies *understate* the impact of genes.

Palmer goes on:

The equal environments assumption is similarly questionable. As anyone who's ever seen a pair of toddler twins in matching sailor suits is surely aware, monozygotic twins do get special treatment. They are more likely than their dizygotic peers to be treated as "two of a kind" by family, friends, and teachers, which must increase their chances of developing similar behaviors. There have been numerous studies showing that dizygotic twins who look similar have more personality traits in common than those who are easily distinguishable.