We receive genetic messages from each of our parents. Sometimes a zygote divides, separates into two zygotes, and each grows separately. Twins are the result. Sometimes two ova are released in the same month. If each is fertalized twins result, but this time not from the same genetic combination.
Monozygotic twins are individuals with the same genetics.
Studies of these individuals should allow estimates of what is NOT genetic since differences between individuals with the same genetics must be non-genetic. If the monozygotic twins are adopted and raised apart we have the potential for more interesting studies. Twins are not common, monozygotic twins are less common, adopted monozygotic twins are even less common, and adopted monozygotic twins raised separately are exceedingly rare. Therefore we must treat all twin studies with the same suspicion we would give any study with a very tiny non-representative sample. The branch of biology that studies heredity is called genetics .