Monozygotic (MZ) twins/ identical twins are two babies that develop from the same egg and are genetically identical.
Dizygotic (DZ) twins/ fraternal twins share the womb at the same time but develop from seperate eggs, no more genetically similar than ordinary siblings.
Twin studies are used to see if behaviours are shared by those who are genetically similar.
See what traits are shared by looking at concordance rates(likihood that, if one twin has a certain trait, the other twin will have the same.)
E.g if one twin develops schizophrenia, psychologists look at whether the other twin develops it. They can then work out the likelihood that if one tein has schizophrenia the other will develop…