Cloning, Nature, and Nurture.
But what triggers the release of oxytocin in the rat brain when there's no researcher to inject the hormone? Warmth, touch, and friendly social interactions. So, does a hormone cause an action? Or does an action cause a hormone to be released? It seems to be a bit of both, especially in big-brained animals, like us, that have a lot of flexibility to their behavior. In primates, says Kim Wallen, a behaviorist at the Yerkes Regional Primate Center in Atlanta, hormones "act in concert with so many other factors. I think we might see these not as chemical signals that turn things on and off but as things that permit an organism to do certain things under certain circumstances."
The question of where characteristics come from also turns out to be too complicated to divide neatly into Nature or Nurture, according to Stephen Suomi, director of the Laboratory of Comparative Ethology at the National Institutes of Health. His research suggests that a rhesus monkey's sexual behavior, aggression, thinking, and responses to stress are affected by "prenatal stressing"--that the strains and pains to which the pregnant mother is subjected will have an immense impact on her infant long after it is born.
Not even DNA, that supposedly impregnable "digital" code, is viewed as platonically separate anymore. "DNA doesn't do anything," says James Shapiro, a cell biologist at the University of Chicago Medical School. A gene is an instruction for making a protein. Reading that instruction, following it, folding the protein into the shape it must have to do its job--all this is the job of the cell, and the cell is affected by its environment.
In twin studies, too, as Elizabeth Spitz, a researcher at the Université René Descartes in Paris, has noted, Nurture is nearly impossible to tease out. Identical twins, at the moment of birth, may have already had very different experiences during their nine months in the womb. This is because the number of membranes identical fetuses share depends on when exactly their single egg split into two embryos. If this divisional split occurred early (within four days after fertilization), then each twin grew to birth in its own little world. Each developed its own chorion, the outermost membrane, and its own amnion, the inner membrane that contains the amniotic fluid in which the fetus floats. If the cell division happened later (four to eight days after fertilization), each twin got its own amnion but shared a chorion. If the division happened later still (eight to 10 days after conception), the two fetuses shared both chorion and amnion.
How similar twins are depends in part on how many membranes they shared. That amazing story about how two separated twins both married red-headed engineers in the same year--the resemblance could be because they shared the same amnion, and not because of their genes. So it's a little premature to propose a genetic predisposition to divorce.
In fact, the standard laboratory procedure for sorting out one influence from another--controlling the genetics, the prenatal environment, and the rearing of the twins--turns out to be impossible. The point may seem abstruse now. But once all those millions of Dollies have been gamboling about for a few years, and it's plain that they don't all look and act alike, we'll have no choice but to get off Galton's seesaw.
David Berreby, author of Us and Them: The Science of Identity, has been a Slate contributor since 1996.