The special relationship between twins allows researchers to explore differences between genetic and environmental influences on physical and mental health, as well as traits and behaviors. By studying twins, we can learn a lot about diseases, disorders, and human nature in general. Twin studies help answer questions about many aspects of being human.

Approximately three or four out of every 1,000 newborns are identical twins. There are different types of twins, most often identical and fraternal. Identical, or monozygotic, twins develop from the same fertilized egg that divides into two. These twins are of the same sex and have the same genes.

Identical twins, sometimes referred to as paternal twins, show a high degree of genetic influence on human development. Fraternal or dizygotic twins develop from two different eggs fertilized by different spermatozoa; they usually share about half of their genes. Thus, fraternal twins are essentially like typical siblings.