Excerpt: 'Entwined Lives'
Feb. 21, 2005 — -- Ann and Claire Recht, 17, won the title of "World's Tallest Twins" from the Guinness Book of World Records in July 2004. The Rechts appeared on "Good Morning America" along with psychologist, Dr. Nancy Segal, an expert on twins, on what twins can tell us about nature vs. nurture.
Segal is the author of "Entwined Lives: Twins and What They Tell Us about Human Behavior." You can read an excerpt from the book below.
Chapter One
Identical and Fraternal Twins: Living Laboratories
I feel a rush of excitement waiting in my laboratory for nine-year-old twins, Rocky and Tony Novak, and their mother, Linda, to arrive for their research visit. For the next two hours the four of us will be players in a great discovery game, a search for how and why people grow and develop as they do. I wonder how alike Rocky and Tony will look to one another, how similarly they will respond to problems and questions, and how much they enjoy being twins. It will be fascinating to learn how their answers and test scores, and those of other twins, illuminate areas of human development that remain in the dark.
Occasionally, I am asked to recall a "favorite" pair, or a pair that was especially memorable in some way. I always answer that it is the pair I last studied in my laboratory whose particular personalities and preferences still linger in my mind. Each individual twin pair is a fresh and fascinating take on what it means to be human. It is, however, the collective story, repeated countless times and in different ways by each set, that tells us who we are and how we got there. It was extraordinary to discover that the "Jim twins," the first pair of reunited identical twins studied at the University of Minnesota, had sons with the same name, enjoyed woodworking and bit their fingernails to the nub. It was also striking to find other examples of highly matched idiosyncratic behaviors in many other reared apart identical pairs, and to find them so infrequently in reared apart fraternal pairs. The parallel paths of identical twins, and the diverging paths of fraternal twins, strongly suggest that human development proceeds according to a plan that is largely guided by our genes. Twins are truly living laboratories.
Twins offer a natural experiment for studying how behavioral and physical traits are shaped by nature and by nurture. In fact, when twins are not included in human developmental research, the findings may actually be suspect. The classic twin method was first described in 1875 by Sir Francis Galton, a gentleman-scholar from London. According to Galton, "It is, that their history affords means of distinguishing between the effects of tendencies received at birth, and of those that were imposed by the circumstances of their after lives; in other words, between the effects of nature and nurture." Greater resemblance between identical than fraternal twins, in height or IQ, shows that genetic factors affect the development of that trait. This conclusion is reasonable because the members of identical and fraternal twin pairs are the same age and were raised together in the same home, so the only critical difference is their biological relatedness. Nature has provided science with a convenient assembly of human subjects that allows control over factors that may affect the way we mature and develop.
If the entire twin chronicle were so tidy we could stop here, but twin research has endured a mixed reception in behavioral science research. Throughout history, the twin method has been treated as both honored guest and social outcast. Supporters have reveled in the rich body of knowledge that only twin studies provide concerning the development of intelligence, special abilities, personality, psychopathology, diseases and virtually any human trait that can be measured and studied. However, detractors have pointed to "flaws" in the research design, and misuse of findings. For example, some people have argued that identical twins are so alike because they are treated more alike than fraternal twins, violating the equal environments assumption of the twin design, and that their more similar genetic makeup has little to do with it. Other people have worried that gene-behavior relationships will be used to discriminate between different groups of people or to deny the usefulness of programs designed to improve child care or educational opportunities. I will return to these important issues in the last chapter. Actually, it is only in the last two decades that the twin method has achieved its stunning comeback. This is partly due to greater appreciation for what twin studies can accomplish and to improved methods for interpreting results.
What produces twins, who has twins and how twins differ from singletons, or nontwins, are among the vital questions that have fueled debates over the role of twins in scientific investigation. The biological events responsible for twinning are a good place to begin to sort through these issues. Identical (monozygotic or one-egg) twins result when a single fertilized egg, or zygote, divides between the first and fourteenth day after conception. Further splitting can lead to identical triplets, quadruplets, quintuplets or sometimes more. The identical Dionne quintuplets, born in Canada in 1934, resulted from a single fertilized egg that split four times. Based on the quints' relative likenesses in palm prints, hair whorl, handedness, facial form and ear shape, the University of Toronto research team constructed a "continuous chain of similarities," suggesting the position of each sister in the succession of divisions: Annette, Yvonne, C´cile, Marie and Emily.
What causes the fertilized egg to divide remains a mystery. One early theory linked identical twinning to delayed implantation of the fertilized egg in the uterus. More recently, Dr. Judith G. Hall from the University of British Columbia, Canada, suggested that the early developing mass "senses" an error, attempts to "rid itself of" the abnormality, and may produce identical twins in a minority of cases. Other possible explanations include time of fertilization and different inactivation patterns in female twins' X chromosomes. Some of these theories continue to be explored.
There is little evidence that the tendency to produce identical twins is tied to the genes. Identical twinning seems to occur randomly so such twins might show up in virtually any family at any time. The fairly constant rate at which identical twins occur worldwide (approximately 1/250 births) is also evidence against a genetic influence on this type of twinning. Nevertheless, the birth of more than one identical pair in some families has not gone unnoticed, and a closer look at the distinguishing features of these unusual families is worth doing. Identical twinning and left-handedness appear to occur together among the relatives of some identical twins, although increases in birth abnormalities have not been observed in twins from such families. A few investigators have proposed that a single gene whose expression varies may be responsible for identical twinning in some families. These observations, while intriguing, are tentative at best.
Fraternal (dizygotic or two-egg) twins result when two different eggs are fertilized by two different sperm cells so that, genetically speaking, their relationship is the same as ordinary brothers and sisters. These twins come in two varieties: same-sex and opposite-sex. Fertilization of three, four or even five eggs at the same time is possible, although these "higher order multiples" occur less frequently than twins. Many combinations of identical and fraternal "supertwin" partners are possible.
A great deal more is understood about what causes fraternal than identical twinning, and some of the more surprising factors might well affect family planning decisions once they are more widely known. Fraternal twinning happens most often to women who bear children between the ages of 35 to 39 years. The catalogue of other characteristics linked to fraternal twinning includes:
