The special relationship between twins allows researchers to examine the differences between genetic and environmental influences on both physical and mental health, as well as traits and behaviors, in the general population. Ultimately, twin research helps answer questions not only about how humans differ, but why.
Since identical (monozygotic) twins develop from the same fertilized egg, they are the same sex and share 100 percent of their genes. Some of their similarities provide evidence of a high degree of genetic influence. But since fraternal (dizygotic) twins develop from two different eggs fertilized by separate sperm cells, they generally share only about 50 percent of their genes and, in that sense, are more like typical brothers and sisters. Similarities found between sets of identical and fraternal twins suggest more environmental influences than genetic.
The information gathered from twin studies lays the groundwork for future genetic research, including the degree to which any and all aspects of life are determined by genetics, the location of specific genes, and the prevention and treatment of genetic disorders.