Twin Studies

Twin studies can tell us about how genes and environments affect behavioral and physical development. There are two kinds of twins: identical and fraternal. Identical twins result when one fertilized egg splits during the first two weeks of pregnancy. These twins share all their genes and are always of the same sex. They occur in about one third of natural twin conceptions. Fraternal or nonidentical twins result when two eggs released by the mother are fertilized by two sperm from the father. These twins share half their genes, on average, just like ordinary siblings. Fraternal twins can be same-sex or opposite-sex.

The classic twin design involves comparing the similarity of identical and fraternal twins. If identical twins are more alike in intelligence, personality, or physical skills ...