Essays of an Information Scientist, Vol:7, p.397-404, 1984

There are many anecdotes about identical twins who were separated early in life and later reunited. Although reared in different environments, their remarkable similarities include mannerisms, tastes in food, choices of hobbies, and vocations. Some people even believe that there is an almost extrasensory sympathy between identical twins—shared pain and moods and vague awareness about how the other is doing or what the other is thinking, While these anecdotes receive widespread attention in the media, the truly profound research on twins is not so well known outside the scientific community. In the first part of thk essay, 1 we reviewed the unique biology involved in the conception and birthing of twins. In this part, we'll review what researchers in the behavioral, cognitive, and medical sciences have learned about twins. Much of thk research has profound implications for the old philosophical " nature vs. nurture " debate. The extent to which intelligence, personality, and even health and life span are determined by genetics or environmental factors is still very controversial. As discussed in the first part of this essay, there are two types of twins— monozygotic (MZ) and dizygotic (DZ). DZ twins are like any other siblings— that is, on the average, only half of their genes are the same. They can be of the same or opposite sex. MZ twins are genetically identical, They have duplicate genes and they are always of the same sex. Thus, by comparing MZ and DZ twins, researchers are able to study the genetic and environmental factors in various human characteristics and diseases. Before we review the results of these twin studies, we'll briefly discuss some of the various ways twins are used in research. The classic twin study method is a tool for estimating the relative influence of genetic and environmental factors on a specific trait. If one assumes that environmental effects are the same in MZ and DZ twins, then a greater similarity between members of MZ pairs than between members of DZ pairs would indicate that the similarity is a result of their matching genes. Twins are said to be concordant when both members of the twin pair are alike in a trait or discordant when one twin is different from the other in that trait. The concordance rate, or the percent of MZ and D Z twin pairs sharing similarity for a given trait, is used as a measure of the …