The methodological confusion of twin studies.

The methodological confusion of twin studies.

The methodological confusion of twin studies.

A couple of things.
Aug. 24 2011 3:17 AM

Double Inanity

Twin studies are pretty much useless.

Twins. Click image to expand.
How reliable is studying twins?














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.

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.

In addition, genetics and environment aren't separable elements, so any attempt to discern their independent contributions to an observed trait is foolish. The epigenome, the part of our genetic makeup that determines when genes are transcribed—that is, when they actually become relevant to our existence—is deeply affected by conditions in the womb and the unquantifiable complexity of the external environment.

Our knowledge of the human genome is far too superficial for anyone to be making percentage estimates of the extent to which our biology sets our destiny. Consider the unfortunate story of the Texas rancher who cloned his beloved, sweet-natured bull only to find that the new model was the polar opposite in temperament. The clone gored his master repeatedly, dislocated his shoulder, ripped open his scrotum, and fractured his spine.

Duke's Evan Charney likes to compare off-the-shelf twin studies of human behavior to phrenology, the 19th-century pseudoscience that attempted to link intellectual capacity to skull size and shape. He may have something. There's a strong temptation to believe that the same genes that make identical twins look so similar also make them think and act identically. That assumption isn't just insulting to a twins' individuality; it displays a reductive attitude toward the incredible complexity of our genetic structure, which scientists are just beginning to understand.

None of this is to say that genetics don't play some role in our behavior—they absolutely must. But twin studies are a fundamentally flawed method for quantifying that influence. Don't believe for a second that anyone can put a percentage estimate on your genetic inclination toward altruism by asking a few questions to a bunch of twins.