Twin research is an informative approach for understanding the genetic and environmental influences affecting behavioral, physical, and medical traits. The simple yet elegant logic of the twin method derives from the differences in genetic relatedness between the two types of twins. Identical (monozygotic or MZ) twins share 100 percent of their genes, while fraternal (dizygotic or DZ) share 50 percent of their genes, on average. MZ twins result when a fertilized egg (ovum) divides during the first two weeks following conception, while DZ twins result when a woman simultaneously releases two eggs that are fertilized by two separate sperm. MZ twins are always same sex, whereas DZ twins may be same-sex or opposite-sex. However, rare events occasionally produce unusual MZ and DZ twin variations. Twin researchers compare the resemblance between MZ and DZ twins with reference to a particular trait, such as height or weight. Greater resemblance between MZ twins than DZ twins demonstrates that the trait under study is under partial genetic control. There are also various ways that twins and their families can be used in research to increase the potential yield of a study. Sophisticated biometrical techniques can estimate the extent of difference among people associated with their genes, shared environments and nonshared environments. Twin research has proliferated in recent years. This is largely because the power of the twin method for understanding the origin and development of human traits has become increasingly appreciated by investigators representing diverse fields. Twinning rates have also increased dramatically since 1980, especially the rate of fraternal twinning as a consequence of fertility treatments. There have been stunning advances in quantitative mathematical methodology that continue to increase the value of twin studies. Lastly, there have been enormous developments in the molecular genetics and genomics fields with respect to associating genes posing increased risks for specific behaviors and disease. Twins will continue to play a prominent role in these endeavors. The sources presented in this article represent a wide range of areas and topics within twin research. General overviews of the field, both historical and current, are provided, as well as a listing of special collections in twin research, that is, books and journals focusing on a particular topic or theme and web addresses. The largest section includes topics reflecting the widening range of psychological, biological, and medical traits that have been examined via twin research methods. The section on twin-based perspectives provides sources treating unusual twin-related topics.
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