So which is the greater influence on a person, nature or nurture? From the article (my emphasis):
Despite periods of controversy, twins studies proliferated. Over the last 50 years, some 17,000 traits have been studied, according to a meta-analysis led by Tinca Polderman, a Dutch researcher, and Beben Benyamin, an Australian, and published this year in the journal Nature Genetics. Researchers have claimed to divine a genetic influence in such varied traits as gun ownership, voting preferences, homosexuality, job satisfaction, coffee consumption, rule enforcement and insomnia. Virtually wherever researchers have looked, they have found that identical twins’ test results are more similar than those of fraternal twins. The studies point to the influence of genes on almost every aspect of our being (a conclusion so sweeping that it indicates, to some scientists, only that the methodology must be fatally flawed). ‘‘Everything is heritable,’’ says Eric Turkheimer, a behavioral geneticist at the University of Virginia. ‘‘The more genetically related a pair of people are, the more similar they are on any other outcome of interest’’ — whether it be personality, TV watching or political leaning. ‘‘But this can be true without there being some kind of specific mechanism that is driving it, some version of a Huntington’s-disease gene. It is based on the complex combined effects of an unaccountable number of genes.’’
Arguably the most intriguing branch of twins research involves a small and unusual class of research subjects: identical twins who were reared apart. Thomas Bouchard Jr., a psychologist at the University of Minnesota, began studying them in 1979, when he first learned of Jim and Jim, two Ohio men reunited that year at age 39. They not only looked remarkably similar, but had also vacationed on the same Florida beach, married women with the same first name, divorced those women and married second wives who also shared the same name, smoked the same brand of cigarette and built miniature furniture for fun. Similar in personality as well as in vocal intonation, they seemed to have been wholly formed from conception, impervious to the effects of parenting, siblings or geography. Bouchard went on to research more than 80 identical-twin pairs reared apart, comparing them with identical twins reared together, fraternal twins reared together and fraternal twins reared apart. He found that in almost every instance, the identical twins, whether reared together or reared apart, were more similar to each other than their fraternal counterparts were for traits like personality and, more controversial, intelligence. One unexpected finding in his research suggested that the effect of a pair’s shared environment — say, their parents — had little bearing on personality. Genes and unique experiences — a semester abroad, an important friend — were more influential.
So that's it then; it's all settled. Just as I suspected: nature trumps nurture. Right? Not so fast.
As pure science, the study of twins reared apart has troubled some researchers. Those twins either self-select and step forward or become known to researchers through media reports — which are less inclined to cover identical twins who do not look remarkably alike, who did not marry and divorce women of the same name or choose the same obscure hobby. Identical twins who do not look remarkably alike, of course, are also less likely to be spotted and reunited in the first place. And few studies of twins, whether reared apart or reared together, have included twins from extremely different backgrounds.
‘‘Every study will have its critics,’’ says Nancy Segal, a professor at California State University, Fullerton, who worked with Bouchard from 1982 to 1991. ‘‘But studying twins reared apart separates genetic and environmental effects on behavior better than any research design I know.’’
Segal has been studying Chinese twins (fraternal and identical pairs reared together and reared apart) since 2003. In several books about twins, Segal has merged science with human-interest tales, walking readers through statistical evidence but also highlighting anecdotal details: the identical twins reared apart who each showed up for research wearing seven rings, or the reared-apart sisters who rubbed their noses the same way and called it ‘‘squidging.’’
At the moment that a sperm penetrates an egg, that single-cell zygote is what is known as totipotent: It is pure potential. It has in it the makings of an eyebrow’s curve, a heart’s thick muscle, a neuron’s electrochemical power; it has in it the finicky instructional manual that will direct the building of the body’s every fiber and the regulation of those fibers. But that one cell splits into two, and instantly, lights begin to go out, potential dims. In order for that one cell to become a tiny bit of flesh in a heart, and not the hair of an eyebrow, one or more of its genetic signaling pathways must shut down. The result is differentiation, a steady process of elimination that allows complex biological universes to be built. Every time a group of cells divides, each one becomes more like one thing, less like another.
By the time that embryo is five or six days old, which is when a majority of fateful twin splits occur, some of those cells, by chance, go to one twin and some to the other. This means that the expression of some genes in one of those future twins is already, in subtle ways, likely to be different from the expression of genes in the other future twin, theorizes Harvey Kliman, the director of the reproductive and placental research unit at the Yale School of Medicine. From the moment that most identical twins separate, they may well have different epigenetics, a term that refers to the way genes are read and expressed, depending on environment. They are already different products of their environment, the environment being whatever uterine conditions rendered them separate beings in the first place.
The casual observer is fascinated by how similar identical twins are, but some geneticists are more interested in identifying all the reasons they might differ, sometimes in significant ways. Why might one identical twin be gay or transgender and not the other? Why do identical twins, born with the same DNA, sometimes die of different diseases at different times in their lives? Their environments must be different, but which aspect of their environment is the one that took their biology in a different direction? Smoking, stress, obesity — those are some of the factors that researchers have been able to link to specific changes in the expression of specific genes. They expect, in time, to find hundreds, possibly thousands, of others.
The meta-analysis published this spring in Nature Genetics, which examined 50 years of studies of twins, arrived at a conclusion about the impact of heredity and environment on human beings’ lives. On average, the researchers found, any particular trait or disease in an individual is about 50 percent influenced by environment and 50 percent influenced by genes. But that simple ratio does not capture our complicated systems of genetic circuitry, the way our genes steadily interact with the environment, switching on, switching off, depending on the stimulus, sometimes with lasting results that will continue on in our genome, passed to the next generation. How an individual’s genes respond to that environment — how they are expressed — creates what scientists call an epigenetic profile.
So, getting back to those two sets of twins in Colombia:
Before starting her research, Segal would not have been surprised if each young man tested similarly to his identical twin, despite their different environments. But her preliminary results, she said, show that on a number of traits, the identical twins were less alike than she initially anticipated. ‘‘I came away with a real respect for the effect of an extremely different environment,’’ Segal said.
The debate continues.