The California Twin Program
is a non-profit effort conducted at the University of Southern California to collect the basic characteristics of all twins born in California regardless of where they are living currently. Click on the left to learn about the nature and goals of the program and who is eligible to participate.
Fraternal twins may seem to share the same placenta and identical twins may seem not to.
About one in every 50 Americans is a twin, and about one in every three twins is an identical twin. Identical twins are fairly evenly dispersed in the population, no matter when, where, or to whom they were born.
It is possible to identify twins who are similar with respect to genetic inheritance but not the environment, or those who are genetically different but similar with respect to environmental experience.
Our first questionnaire was sent to twins who in 1992 were over the age of 37.
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About Twins

About one in every 50 Americans is a twin, and about one in every three twins is an identical twin. Identical twins are fairly evenly dispersed in the population, no matter when, where, or to whom they were born. Fraternal twins, in contrast, vary substantially in frequency over the world, being common among the pregnancies of older women, especially common among families with roots in Africa, and relatively rare among those of Asian heritage. Half of the fraternal twin pairs are composed of individuals of different sex. As the use of fertility drugs to assist women with conception have become more common, the frequency of fraternal twinning has also. Most twins know if they are fraternal, because they simply don’t look at all alike. Pairs who are confused within their own families or in their own schoolroom are almost always identical, even when the mother has been told that they are fraternal on the basis of the look of the placenta. Fraternal twins may seem to share the same placenta and identical twins may seem not to. Sometimes it is necessary to do laboratory analysis, especially when the twins disagree.