The study of human behavioral genetics

The first scientist to study human behavioral genetics systematically was Francis Galton, an English cousin of Charles Darwin. In 1869, Galton published Hereditary Genius, which summarized his study of nearly 1,000 eminent judges, politicans, military commanders, scientists, artists, religious leaders, and scholars. Because there were at that time no objective measures of intelligence or achievement, Galton developed his own scale of reputation as "a leader of opinion, of an originator, or a man to whom the world deliberately acknowledges itself largely indebted." A careful statistical analysis of the relatives of eminent men found that they were much more likely than the average person to achieve eminence themselves. Indeed, the closer the genetic relationship to an eminent man, the more one was likely to achieve.

Galton was aware that people in the same family not only share similar genes but are also ordinarily raised in similar environments. The resemblances he found could have been caused by heredity, by environment, or by both. Galton cited several arguments to support his idea that eminence resulted from "hereditary genius." For example, he noted that many men had risen to achievement from humble families. Further, he found that the biological sons of eminent men were more successful than the adopted relatives of Roman Catholic popes, despite the fact that these adopted children were given many advantages.

Since Galton, the measurement of human abilities has advanced considerably, and mathematical tools of analysis have also become far more sophisticated. By today's standards, Galton's studies prove little or nothing about the inheritance of genius. But his pioneer research did help to establish two of the most important techniques in human behavioral genetics-the study of twins and the study of adopted children. The rationale of adoptive studies is quite straightforward. If a characteristic is primarily based on heredity, children should resemble their biological parents even if they never knew them. However, if environment is the key to a particular trait, children should come to resemble the adoptive parents who raised them.

More common than adoptive studies are comparisons of the behavioral characteristics of twins. About lout of every 87 white births in North America is a twin birth. Thus, 2 out of every 88 babies, or about 2.3% of the white population, are twins. Interestingly, North American blacks have a somewhat higher proportion of twin births-1 in every 73. Indentical twins are called monozygotic because they develop from the division of a single fertilized egg (zygote) and thus have precisely the same genes. Fraternal twins are called dizygotic because they develop from two different eggs that happen to be fertilized by two different sperm at the same time. They are often male and female, whereas monozygotic twins, being identical, must always be of the same sex. Dizygotic twins are no more similar genetically than any other two siblings; that is, they share about half their genes.

To geneticists, the most interesting cases are those rare individuals who were separated from an identical twin at birth. Although few in number, these people are great in significance; they represent a kind of natural experiment in which heredity is held constant while the environment is varied.

A review of all the published cases of monozygotic twins raised apart found only 95 pairs that were adequately reported. The results showed striking "similarities in many dimensions, including physical characteristics as disparate as height and the pattern of tooth decay; temperament and personality; mannerisms, such as a firm or limp handshake; smoking and drinking habits; tastes in food; and special aptitudes and interests, especially in the arts or in athletics" Other similarities included weight, blood pressure, EEG, voice characteristics, and even symptoms of anxiety, such as nail biting and headaches.

Fascinating as these results are, they must be considered provocative rather than conclusive. Many of the twins had met before they were studied, and some were even raised in different branches of the same family. Thus, in some cases they shared the same environment, and their resemblances may not depend on heredity alone. A much larger body of research compares the degree of resemblance between identical and fraternal twins. Typically, these studies assume that pairs of twins who grow up in the same family are exposed to similar environments whether they are monozygotic or dizygotic. Thus, if identical twins generally resemble each other more in terms of some characteristic, it seems reasonable to conclude that heredity played a part.