Nancy L. Segal, professor of Developmental Pyschology and director of the Twin Studies Center at Cal-State Fullerton University, looks at the twin bond from both viewpoints in a new book now available online and in bookstores,
'Entwined Lives: Twins and What They Tell Us About Human Behavior' (Dutton, March 1999).

Dr. Segal has studied twins her entire adult life and is one of the foremost experts on twin behavior in the world. In fact, one chapter in her book ('Double Indemnity: Twins in the Courtroom') talks about the many lawsuits involving twins in which she has served as an expert witness.

So what can a twin learn by reading Entwined Lives? One theme which returns over and over while reading the book is the differences between identical vs. fraternal twins and how they may impact behavior and relationships. My twin brother and myself don't know if we're identical or fraternal. Our mom says we are fraternal because we had two placentae, but Dr. Segal has done studies that show parents correctly identify their twins as either identical or fraternal just 74% of the time (she claims a personal 94% success rate in determining twin types). There are several ways to accurately determine the zygosity of twins with the two most effective systems being a DNA test or examining blood group systems of the twins. All fraternal twins have two placentae, two amnions and two chorions but so do one-third of all identical twins.

Why is important to know the difference? The author (herself a fraternal twin) claims that in addition to the scientific differences in identical twins (sharing 100% of their genes) vs. fraternal twins (sharing 50% of their genes), there are sociological differences between the two main types of twins.

An entire chapter of the book is devoted to the special relationship between twins. Indeed, several studies have shown that identical twins show more altruistic behavior towards each other than fraternal twins, displaying a greater propensity for cooperating with each other. Of course, all twins fare better in these type of studies than same-sex or opposite-sex siblings or cousins, so there are plenty of examples of extremely close fraternal twins (or identicals who drift apart).

The book has chapters on twinless twins, "pseudo-twins" (raising two same-age children who aren't twins), twins separated at birth ("nature vs. nurture"), the rise in multiple births due to fertility advances (Dr. Segal theorizes that twinning rates will subside soon as improved techniques will bring successful conceptions with fewer embryos) as well as non-human twins in the animal kingdom. If you're looking for a chart of the seven pairs of gorilla twins born in captivity, it's right there on pages 236.

Entwined Lives has a little of everything relating to multiples. The many questions surrounding twins and other multiples aren't all answered in this book, but it does an excellent job of summarizing recent trends and new knowledge. We highly recommend it for all twins and parents of multiples.