Twin Studies

I note that Cal State Fullerton Psychology Professor Nancy L. Segal has a new book out on twins separated at birth or in early childhood, entitled Born Together, Reared Apart. I have not yet been able to get this book through my university library or local public library, but I’ve put in requests for this latest contribution by an important researcher on the genetic influences on human character and behavior.

 The historical origins of twin studies lie in the work of Sir Francis Galton, a cousin of Charles Darwin,  an early pioneer in the study of genetics, and a founder of the controversial theory of eugenics. Galton conducted some of the earliest systematic studies of human twins in the 1870's. He recognized the difficulty of identifying the extent to which human traits are biologically inherited and the extent to which traits are produced by diet, upbringing, education, and other environmental influences. Borrowing a phrase from William Shakespeare, Galton called this the "nature versus nurture" problem, a phrase that has become standard in the social sciences. Galton reasoned that he could attempt to find an answer to this problem by comparing similarities among people who obviously shared a great deal of biological inheritance with each other with similarities among people sharing less biological inheritance. Twins offered the clearest example of people who shared common biological backgrounds.

Galton contacted all of the twins he knew and he asked them to supply him with names of other twins. He obtained 94 sets of twins. Of these, 35 were sets of twins who were very similar, people that we would today call identical twins. These 35 pairs reported that people often had difficulty telling them apart and 9 pairs were so close to one another in appearance that the 18 individuals reported having mistaken their own images in mirrors with their twins.

Using questionnaires and interviews, Galton compared the 35 identical pairs with the other twins. He found that the identical twins were much more similar to one another in habits, interests, and personalities, as well as in appearance. They were even much more alike in physical health and in susceptibility to illness. The one area in which all individuals seemed to differ markedly was in handwriting.

Since Galton's time, researchers have learned how biological inheritance occurs and this has made possible an understanding of why twins are similar. It has also enabled researchers to make more sophisticated use of twins in studies that address various aspects of the "nature versus nurture" problem. Parents pass their physical traits to their children by means of genes in chromosomes. Each chromosome carries two genes for every hereditary trait. One gene comes from the father and one comes from the mother.

Any set of full brothers and sisters will have a good deal of heredity in common, since all of their genes come from the same parents. However, brothers and sisters usually also differ substantially. The cell that develops into a living creature, the zygote, is created when the ovum, or egg, is fertilized by the sperm. Each zygote will combine genes from the father and from the mother in a unique combination, so different zygotes will develop into quite different people. Even when two fertilized eggs are present at the same time, as in the case of dizygotic or fraternal twins, the two will be different combinations of genes from the mother and the father.

Identical twins are an exception to the rule of unique combinations of genes. Identical twins develop from a single zygote, a cell created by one union of egg and sperm. Therefore, monozygotic twins, twins from one zygote, will normally have the same genetic make-up. Differences between genetic twins, researchers argue, must therefore be produced by environmental factors following birth.

The ideal way to conduct twin studies is to take the approach of Segal in this new book and compare monozygotic twins who have been reared apart from one another in vastly different types of families. This is rarely possible because the numbers of twins separated at birth and adopted are quite limited. For this reason, researchers in most twin studies use fraternal twins as a comparison group, since the major difference between monozygotic and dizygotic twins is that the former are genetically identical. Statistical similarities among monozygotic twins that are not found among dizygotic twins are therefore believed to be due to genetic inheritance.

One of the main sources for twin studies is the Minnesota Twin Registry. A second major source of twin studies is the Virginia Twin Registry. This is a register of twins constructed from a systematic review of public birth records in the Commonwealth of Virginia. A few other states also maintain records of twins. Some other organizations, such as the American Association of Retired People (AARP) keep records of twins who volunteer to participate and make these records available to researchers.

Although twin studies are one of the best available means for studying genetic influences on human beings, questions about this approach do exist. Twin studies assume that monozygotic twins are biologically identical, but some critics have claimed that there are reasons to question this assumption. Even though these twins tend to show greater uniformity than other people, developmental differences may emerge even in the womb after the splitting of the zygote.

Twins who show a great physical similarity may also be subject to environmental similarities, so that traits believed to be due to genetics may in fact be a result of upbringing. Some parents, for example, even dress twins in matching clothing. Even when twins grow up in separate homes without being in contact with one another, their appearances and mannerisms may evoke the same kinds of responses from others. Physical attractiveness, height, and other characteristics often affect how individuals are treated by others, so that the biologically based resemblances of twins can lead to common experiences. Critics of twin studies point out that twins constitute a special group of people and that it may be difficult to generalize findings from twin studies to the population at large.

Despite the concerns about twin studies, these have provided evidence that a substantial amount of human character and behavior may be genetically determined. In 1976, psychologists John C. Loehlin and Robert C. Nichols published their analyses of the backgrounds and performances of 850 sets of twins who took the 1962 National Merit Scholarship test. Results showed that identical twins showed greater similarities than fraternal twins in abilities, personalities, opinions, and ambitions. A careful examination of backgrounds indicated that these similarities could not be explained by the close treatment received by identical twins during upbringing.

Later twin studies continued to provide evidence that genes shape many areas of human life. Monozygotic twins tend to resemble each other in probabilities of developing mental illnesses, such as schizophrenia and depression, suggesting that these psychological problems are partly genetic in origin. A 1996 study published in the Journal of Personality and Social Psychology used a sample from the Minnesota Twin Registry to establish that identical twins are similar in probabilities of divorce. A 1997 study in the American Journal of Psychiatry indicated that there is even a great resemblence between twins in intensity of religious faith. Twin studies have offered evidence that homosexual or heterosexual orientation may be partly a genetic matter.

For my own part, I’ve concluded that much of the variation among individual human beings can be attributed to genetic predispositions, and I see the study of separated twins as the best evidence for this conclusion, which is why I’m eager to obtain Dr. Segal’s new book. Beyond this, though, I’m curious about several other “nature versus nurture” (or “nature interacting with nurture”) questions. One of those concerns just what genetic identity means among highly adaptable human beings. The same individual will react in dramatically different ways to different circumstances, so one of the meanings of “socialization” could be how genetically shaped personalities respond to varying social environments.

One of the biggest issues, opening great possibilities for controversy, is whether significant genetic variation exists at the aggregate or categorical levels, as well as at the individual level. Even twins separated early in life generally grow up within the same cultural and national settings. Even if we could find individual sets of twins reared in different countries or within different racial or ethnic groups, we would not have a large enough number to draw conclusions about the extent to which variations at group levels result from genetic predispositions, shared historical legacies, or environmental influences.