Most human babies arrive one at a time, but at the end of one out of 20 pregnancies, twins emerge. Why?
Looking at the human female, UCLA's physiologist/ecologist Jared Diamond answers the question indirectly by solving an ancient evolutionary puzzle. Why are women double-breasted? Aristotle, he reminds us in a recent issue of Nature, believed that the number of teats on an animal determined the size of its litter. Referring to recent research on the number of teats on 266 rodent species, Diamond suggests that human females have two breasts because although the mean litter size for our species is one, the maximum size whereby offspring can survive is close to two. Human females can give birth to two viable infants and the second breast provides a safety margin for the 5% of human babies who will need a nipple of their own. The production of twins, he explains, is an acceptable, if not optimal, strategy for human reproduction.
Twins may also be explained as a reproductive phenomenon necessary to provide data for scientists. Twin studies are especially revealing. They allow researchers to compare individuals with the same genes (identical twins) with those who are merely siblings (fraternal twins) but 2003332908household, develop in the same environment.
The most ambitious and rigorous twin studies to date have been under way at the University of Minnesota, where since 1979 more than 350 sets of twins have volunteered to be tested by psychologists, neurobiologists and physiologists.
The Minnesota team's psychologists have just reported on their study of the heritability of personality traits. They examined only twins of the same sex, and confirmed their status (identical vs. fraternal) by blood tests, including both twins who had grown up together, and others who had been adopted and raised in different homes, sometimes in different countries. The results, "Personality Similarity in Twins Reared Apart and Together," have startled a majority of American researchers, including some of those in the project. Their approach included identifying 11 different personality traits such as social potency (leadership aimed at getting attention), traditionalism (follows rules, authority and strict discipline), aggression and achievement. They concluded that there is more than a 50% chance of these traits being inherited. Although there are questions concerning the average age of the subjects in one category (45 years) and some of the trait descriptions, most social scientists seem satisfied that the data was collected objectively and analyzed accurately.
The investigators discovered that twins forced to dress the same did not develop very differently from those encouraged to dress differently. Their personalities seem to have been determined and unaffected by this kind of parental suasion. In fact, greater similarity in the twins' experiences accounted for only a small fraction of similarities in personality.
The nature-nuture controversy between those who believe that genes control human destiny and those who believe that environment shapes human lives is at least a century old. American psychologists have tended to believe that environment outweighs heredity in the formation of personality. This study challenges that view.
But it is a question of the glass being half empty, or half full. For if a genetic mixture is responsible for a person being outgoing or introverted, aggressive or timid, environment is still responsible for whether the extrovert becomes a terrorist or a philanthropist, or the aggressive individual a murderer or an athlete. The 50% that modifies personality is still environmental.
These results offer a reassuring pat on the back to parents. They can no longer be blamed for every aspect of a child's personality. This is part of a trend that includes removing blame from the parents of children afflicted with serious mental illness such as schizophrenia and depression, conditions we understand to result from chemical imbalances that may be inherited. Now parents of shy or domineering children can stop berating themselves. Children are apparently born with certain personality tendencies.
But parents are by no means off the hook. In fact, knowing that environment can moderate a child's personality makes it incumbent upon them to increase their efforts to help extremely aggressive or retiring youngsters. Effective parenting may be able to make the difference between a good citizen and a threat to the community.
As to the study of twins, the results from the rest of the Minnesota studies are just coming in. We should soon have a better idea of the heritability of allergies, sleep patterns and a host of other psychological and physiological mysteries.