Aside from death and taxes, one thing is certain for many of the 122,000 pairs of twins born in the United States each year.
Twins will be courted and cajoled, from early childhood, by scientists eager to enroll them in “nature-or-nurture” research. Those studies try to determine whether diseases and traits are due to genes that people inherit from their parents, the environment in which they live, or both.
Twins have had that status since the 1800s, when a British scientist named Francis Galton pioneered research on twins. Galton conveniently concluded that the British upper classes dominated society because they inherited intelligence rather than wealth.
Despite the Human Genome Project and other advances in genetics, twin studies remain the classic way of getting that information. Indeed, twin studies are booming.
The National Institutes of Health, for instance, recently issued a nationwide plea for 400 pairs of twins to participate in studies on rheumatoid arthritis and other autoimmune diseases. Business also is brisk at the world's two dozen major “twin registries,” databases of information about twins that are available to scientists for research projects.
Among them are studies on the causes of heart disease, high blood pressure, cancer, schizophrenia, bone loss, eye disease, type A behavior, headache, personality traits, obesity, arthritis, homosexuality, suicide, senility, prostate disease, alcoholism, diabetes, violence and antisocial behavior, Alzheimer's disease, and differences in fingerprint patterns, body weight, intelligence, and financial success.
There are different kinds of twins.
“Identical” twins form from one fertilized egg that splits, into two identical halves. Each child is the same sex, and shares the same genes. Scientists think that some of those genes, although identical, may work a little differently.
Fraternal twins form from two fertilized eggs and inherit sets of genes as different as brothers and sisters born separately. Non-identical twins can be the same or opposite sexes.
Some scientists add a third type, “polar-body twining,” which results in twins that are part identical and part fraternal.
Twins who are raised together share the same environment. Some are separated at birth, adopted by different parents, and raised in different environments.
So, what can scientists find out by comparing twins?
Suppose, for instance, that scientists want to know how big a role genes vs. environment plays in causing asthma, clinical depression, or Alzheimer's disease. So they study 100 pairs of twins (at least one of whom has the condition) and check the “concordance rate” for identical and fraternal twins. That's the presence of disease in both twins.
If the disease is strictly genetic, the concordance rate in identical twins should be 100 per cent. Every time one has the disease, the other has it. If the disease were strictly environmental, genes would make no difference.
If they find a 40 percent rate for identical twins, it's evidence that environment plays a big role in causing the disease. And other scientists can begin searching for the environmental cause.