The Problem with Twin Studies

Posted by Osagie Obasogie August 26, 2011
Biopolitical Times
Since the August 10 New York Times Magazine story on couples carrying twins or triplets choosing to reduce their pregnancies to a single fetus, Slate has run an unusual number of articles on twins – fourteen in ten days, to be exact. Hidden in this bunch is one by Brian Palmer that critiques the proliferation of scientific findings based on twin studies that claim to isolate the genetic underpinnings of human behavior. Palmer notes that in twin studies,
Researchers compare some behavior or trait in a set of pairs of monozygotic (identical) twins and a set of pairs of dizygotic (fraternal) twins. In theory, the siblings in each pair have been raised in the same way—i.e., they have "nurture" in common. But their "natures" might be different: Identical twins come from the same sperm and egg and are assumed to share their entire genomes; fraternal twins match up at only about half their genes. So if the pairs of monozygotic twins tend to share a trait more often than the pairs of dizygotic twins—be it the likelihood they will vote, a tendency toward altruism, or a strategy for managing their financial portfolios—the difference can be chalked up to genetics. Some call this approach beautiful in its simplicity, but critics say it's crude, potentially misleading, and based on an antiquated view of genetics. The implications of the studies are also just a little bit dangerous, because they suggest, for example, that some people just aren't cut out for being nice to one another.
Palmer correctly identifies Francis Galton as the first person to use twin studies as a methodology to seek the heritable components of human behavior. Galton also coined the term “eugenics,” giving twin studies an intellectual and political origin that few modern scientists care to acknowledge. While Galton’s initial approach to twin studies was surely rudimentary compared to today’s standards, Palmer draws attention to the assumptions embedded in modern twin studies that continue to raise questions about this approach:
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. 
Palmer’s critique has (predictably) caused a firestorm in the article’s comments section. Regardless of which side you are on, Palmer ends the piece with a comparison of twin studies to phrenology that is worth thinking about:
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.
h/t  to Jay Kaufman