Identical, or monozygotic, twins are of the same sex and are genetically identical and physically similar, because they both come from one ovum (egg), which, after fertilization, divides in two and develops into two separate fetuses.
Identical twins are also called monozygotic, and non-identical, dizygotic or fraternal.
It is reported that some monozygotic twins have problems in establishing their identity and experience delayed language development and problems forming other relationships.
Identical twins, also referred to as monozygotic, are certainly less common than fraternal.
Researchers utilized monozygotic (identical) twins for this research as well as conducting some research with animal subjects.