When reared apart, they do not share the same heredity or environment.
When reared together, they share the same heredity and same environment.
When reared together, they share the same environment, including pre-natal, but have a different heredity.
When reared apart, they share the same heredity but in a different environment.
The small number of identical twins makes it impossible to obtain a decent size sample for study.
Parents often resist being involved in such studies for fear the children will be negatively affected.
It is difficult to biologically distinguish between monozygotic and dizygotic twins.
Twins are often unpredictable, leading to random and chaotic results.
Twins often share very similar circumstances, skewing results.