Twin studies are pretty much useless.
One of the main messages of science over the last couple of decades is that genes are destiny. With every new issue of a psychology journal, it seems that the portion of your life governed purely by your own free will gets smaller and smaller. Genes determine 50 percent of the likelihood that you will vote. Half of your altruism. One-quarter of y our financial decisions.
How do we know? 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.
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. The movement eventually gave us forced sterilization and the most offensive passage in the history of the U.S. Supreme Court (and that's really saying something): "Three generations of imbeciles are enough."
Galton's seminal 1875 study of twins was designed to prove that England's "chief men of genius" were the product more of good breeding than of good rearing. Based on the incredible similarities he found between twins in 80 questionnaires, Galton trumpeted his conclusion to the world that nature soundly beats nurture. By modern standards, though, Galton's methodology was only good enough for cocktail conversation. His sample was too small, his subjects were all upper-class, and, since he didn't understand the difference between monozygotic and dizygotic twins, Galton had no control group. (To his credit, he acknowledged this limitation, even though it didn't get in the way of his sweeping claims.)
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.)
Consider a 2005 study by Rice University's John Alford and his colleagues claiming to show that 43 percent of the variation in political ideology in the U.S. could be attributed to genetics (PDF). With surprising precision, the authors listed several political issues, and the degree to which the public's views on each were shaped by genetics. They concluded that genetic predetermination could explain the "otherwise puzzling consistency in ideological divisions that is present across space and time."
Pause for a moment to examine that astonishing claim—that Americans' stubborn insistence on disagreeing over hot-button national issues is the result neither of two parties adjusting their views to appeal to a shifting political center nor of the fact that the issues on which we have learned to agree simply fall out of the political debate. (How many slavery proponents do you know? I hear there used to be a few of them in the U.S. Senate.) Nope. It has to be the series of base pairs in our cellular nuclei.
Fortunately for the future of our democracy, the study's conclusions far outpace its evidence. Three years after the Alford study came out, a Duke political scientist named Evan Charney (PDF) and Harvard geneticists Jon Beckwith and Corey Morris examined the flaws in the Alford study—and showed why all the other twin studies on heritability can't possibly show what they purport to show.
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.