Not all twins look identical. According to Dr. Nancy L. Segal in her book Entwined Lives, "Identical twins are not identical in every way, and in fact, show differences in virtually every trait that has ever been studied." Segal says that differences in the quantity of nutrition reaching each twin might explain a size difference. She also reports that timing of the splitting of the fertilized egg has been associated with difference in physical features i.e. why one twin may have a cleft lip or heart defects while the other twin remains unaffected.

Famous child TV starts Mary-Kate and Ashley Olsen are not identical but are actually fraternal twins who look very similar.

Not all twins play tricks on people. Many claim they are "victims of mistaken identity." In Twin Stories, many twins told me that sometimes people just get confused or may not know the twins as well as they think they do. They may be overconfident in their ability to identify identical twins. They say the end result is that people get fooled through no fault of their own. The most common experience is simply a case of mistaken identity and not playful tricks.