March 2 1997 3:30 AM


Cloning, Nature, and Nurture.


Cloning, Nature, and Nurture


Dolly, the world's first cloned mammal and thus probably history's most famous sheep, is likely to leave a somewhat ironic legacy. This triumph of genetic engineering, achieved last week by Scottish embryologist Ian Wilmut, might well mark the defeat of the idea that genes determine who and what we are.

A clone is an identical twin, a second creature with a genetic code identical to the first's. Dolly, in other words, is the twin sister her original never had. Identical twins are rare in nature, and because they're created when a single fertilized egg divides into two embryos, they're born at the same time. If Wilmut's techniques for cloning mammals prove to be practical, that time constraint will no longer exist. It will be possible to make twins whenever we want, to replace your old dog Pete with Puppy Pete, to replicate a deceased human infant, or to copy yourself.

But twins usually grow up in the same family and community (studies of their rearing suggest that people treat identical twins much more alike than they treat siblings or even fraternal twins). A clone's environment will be different. It will grow from a different egg, develop in a different womb, possibly grow up in a different place, and have different triumphs and disasters.

Cloning, in other words, offers the possibility of the mother of all "twin studies"--comparisons of how often a trait appears among identical twins vs. how often it appears among others. Twin studies these days are invoked to feed the popular notion that genes determine behavior, since they suggest that genes play a role in such things as schizophrenia, alcoholism, performance on intelligence tests--even proneness to divorce (a 1992 study of 1,500 sets of twins found that if one twin had been divorced, the likelihood that the co-twin had also been divorced was 45 percent in identical twins, but only 30 percent in fraternal twins).

It was Sir Francis Galton, cousin of Charles Darwin and founder of the eugenics movement, who first suggested twin studies as a means of teasing out the role of heredity in shaping human traits. Galton wanted to use the studies to establish the primacy of Nature over Nurture. In fact, he was the one who coined that cliché, seizing a snatch of verse from Shakespeare's The Tempest in which Prospero calls Caliban "[a] devil, a born devil, on whose nature/ Nurture can never stick; on whom my pains,/ Humanely taken, all, all lost, quite lost!"

Since Galton's time, the Nature-Nurture dichotomy has hardened into competing ideologies. A vigorous intellectual movement in favor of Nature in the 1920s gave rise to a countertradition emphasizing the importance of Nurture, which spawned the field of cultural anthropology. Meanwhile, the school of Nature, following the discovery of DNA in 1953, evolved into sociobiology and evolutionary psychology, whose adherents believe that the common heritage of human nature, transmitted through genes, has a greater effect on behavior than culture and history do.

The political and cultural arguments, however, lag behind the lab work. At conferences and seminars, biologists and psychologists have been overheard remarking to one another that the Nature-Nurture dichotomy can't accommodate what they're finding.

Consider, for example, oxytocin, a hormone found in the bodies of many mammals, including human beings, where it is associated with uterine contractions and breast-feeding. Add a little oxytocin to female rats' brains, and they get friendlier. Block receptors for oxytocin, and the creatures threaten each other more. Oxytocin also seems to raise tolerance for pain, and it decreases blood pressure. Sounds like the rule of Nature.