Book Review

Entwined Lives: Twins and what they tell us about human behavior

N Engl J Med 2000; 342:902March 23, 2000

Article

Entwined Lives: Twins and what they tell us about human behavior
By Nancy L. Segal. 416 pp., illustrated. New York, Dutton Books, 1999. $24.95. ISBN: 0-525-94465-6

The study of twins is a fundamental tool of genetic epidemiology and thus is important to all medical and behavioral sciences. Even in this era of study of the human genome, the definition of phenotypic boundaries in many fields still requires refinement, and genetic and environmental influences require further exploration. Despite their importance, I have heard the results of twin studies described as “opaque” by medical colleagues. They need look no further than Entwined Lives, by Nancy Segal, for clarification. The book covers the various facets of twin research, particularly in the study of behavior, in an accessible and sensible manner. It is ably illustrated with case descriptions and anecdotes of encounters with twins during the author's own work and describes aspects of life as a twin that are not usually included in textbooks, such as grief over the loss of a twin, the special relationship between twins, and legal issues involving twins. Although it focuses on behavior rather than on clinical disorders, the book will be of interest to clinicians because the principles of twin research it explains are widely applicable to an understanding of the human condition.

Segal discusses the biologic basis of twinning along with unusual types of “twins,” such as unrelated siblings of the same age who are reared together through adoption, and complicated sibling relationships, such as those that can result from modern fertility treatments involving sperm or egg donation or surrogate motherhood. For example, there are twins who had different intrauterine environments because they were carried by different surrogate mothers and, in rare cases, fraternal twins with different fathers. This book deals imaginatively with the ways in which such sibling relationships challenge our notion of twinning and offer new angles for the study of genetic and environmental influences on behavior. Identical twins reared apart offer a rare opportunity to examine similarities between persons who are genetically identical but do not share a common environment; such work is eloquently described and illustrated with fascinating cases. The importance of differences between the members of separated pairs is also highlighted, since these differences tell us as much about environmental influences on behavior as they do about genetic influences.

There are interesting chapters on what twin studies can tell us about athleticism, intelligence, personality, and mental disorders, but because the emphasis here is on behavior (as the title of the book suggests), a detailed review of studies of mental disorders in twins is outside its scope. For anyone interested in twin research, this book will provide a wealth of information illustrated with real-life examples and written in an understandable, readable style. Its wide-ranging and up-to-date content makes it a valuable review of all aspects of twin research.

Jane Scourfield, M.R.C.Psych.
University of Wales College of Medicine, Cardiff CF4 4XN, United Kingdom