Why do genetics studies compare identical twins behavior to fraternal twins behavior?

Identical twins come from one egg and one sperm resulting in them sharing 100% of their DNA. Alternatively, fraternal twins come from two eggs and two sperm, which means that they share roughly 50% of their DNA. Therefore, if we assume that twins who are raised together will share the same upbringing (i.e.) nurturing experience, we can deduct that if identical twins have more similar decisions to fraternal twins, DNA must explain the difference. Furthermore, this relationship should be stronger for fraternal twins of the same sex, as they were more likely to have more shared childhood experiences as it is possible that opposite sex fraternal twins may participate in different activities due to gender social norms such as boys participating more in sports and girls playing with dolls.