Entwined Lives [Paperback]

Among the more interesting ideas raised in the book is a detailed description of the different ways in which twins develop in utero from conception onward. I also found intriguing a description of a third type of twin, one in which both siblings share their mother's, though not their father's genes. This second topic, though, is barely developed, and there is little mention of how twins of this type are identified. This is typical for the book - interesting ideas are raised and then promptly abandoned, leaving the reader with little understanding or context.

At the same time, Segal does not hesitate to make broad generalizations about genetics and socialization from small collections of anecdotes. Much of the book is devoted to demonstrating the influence of genetics upon intelligence, behaviour, and athletic ability. A chapter on twins separated at birth is well-written and its methods well-described, but it's the exception. On the whole, this section is filled with dozens of case studies and stories from which Segal draws a multitude of conclusions, some better founded than others. She has ample data to support her uncontroversial claim that genetics play a large role in determining intelligence and similarities in abilities, and belabours this obvious point for several chapters during much of the book. (Do we really need several paragraphs explaining that height and weight play a role in determining athletic ability?)

But she is just as prepared to base her theories on a story here, a fact there: one identical pair's preference for very rare meat, for instance, prompts Segal to hypothesize that not only do identical twins share preferences, but that the things they enjoy are likely to lie outside the mainstream. She also ponders at length the significance of one athletic twin claiming an Olympic gold while his cotwin ended up with the silver. After a paragraph of grade-school-level speculation, she raises the (most likely) possibility that the medal discrepancy doesn't mean anything. This book is filled with these sorts of useless, irrelevant tangents (the most bizarre involving a mention of the wholly theoretical "twins paradox" from special relativity in a section on a pair of identical astronauts) that go absolutely nowhere, and that should have been edited out of the first draft of the book. She devotes virtually no space whatsoever to explaining which of her data are statistically meaningful, but rambles at length about very minor aspects of her methodology, such as choosing the most suitable acronym to denote biologically unrelated same-age siblings.

The stories in Entwined Lives make for entertaining, if light, reading, and would have been more enjoyable if Segal didn't keep interrupting them with unsupported generalizations, or with the dozens of "see? Genetics ARE important" assertions that appear over and over again. And the scientific data would have been more useful had it been better, or more rigorously developed. As a book about human behaviour, which it claims to be, it is too long, and explains too little.

The book gives us significant insight far beyond the surface interest most have in twins. I found the book also very entertaining with its numerous ancedotes and real-life stories about twins. Being a twin myself, the book helped me gain new perspective about my twinship and eliminate many eroneous perceptions/myths about twins.

Segal, a twin herself, has been researching twins for over 20 years, since her undergraduate studies. This book sums up much of that research in a format that is approachable by general audiences (and includes extensive endnotes to help interested readers find the studies that are cited). The many topics in her book include: identical twins, fraternal twins, twins reared apart, children adopted together, conjoined twins, non-human twins, friendship between twins, loss of a twin, famous twins, mental skills, athletic skills, and behavioral traits.

One of the most interesting results of Segal's and others' twin research is the strong influence that genes have on intelligence, behavioral traits, and athletic ability. Segal reports that identical twins, reared together or apart, are remarkably similar in these areas, and become more so as they get older. The similarity is weaker with fraternal twins, siblings, and cousins, and hardly found at all between unrelated same-aged children raised together. Certain health factors on the other hand, seem to be more dependent on environmental factors and life choices than on genetics, such as aging of the skin and heart disease. Many of Segal's results come from research on identical twins reared apart, in whom strong similarities point to genetic programming since the environments in which they were raised were different. The logical opposite of identical twins reared apart is unrelated same-aged children reared together, in whom any similarities would suggest environmental influences. Segal has a short chapter on such siblings, but this area could really use more research to complement what has been done with twins reared apart.

This book is absolutely fascinating for what it says, not only about twins and other multiples, about the siblings and parents of twins, about adopted children, but also about humans in general.