In Entwined Lives, researcher Nancy Segal draws upon hundreds of case studies to explore the physical development of twins, and to tackle the thorny nature-versus-nurture question. Segal is competent and often interesting when she sticks to the former; her forays into the latter, on the other hand, are long-winded and anecdotal, and clearly highlight her shaky grasp of statistical methods, not to mention the benefits of editing.
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.