Answer:
This is actually something I learned about when I watched a debate about parenting!
Disclaimer before I get into my answer: I am not an expert in anything, I am a simple highschool student who watched a debate for a school paper. Please correct me if I am wrong in anything!
Anyways...
It was debated that identical twins who were separated at birth, one with the birth parents, one with their adopted parents had a lot of similarities (beside their looks lol) in IQ if I remember correctly, and even some behaviors. Such as they both refused to eat anything unless it has cinnamon on it. However, if one twin has a loving, accepting environment generally blossoms into a healthy, happy human while the other twin has a anger filled, abusive environment they generally will blossom into a damaged person.
Now this isn't just shown in twins who were separated at birth. If one twin gets more love and attention than the other twin, Twin B might grow to resent Twin A because they get the love and attention they are missing out on.
Another way environment can affect twins is that if they, at an early age, learn that they have to compete for attention, commonly one twin will come to bring that competition into adult life (an Example of this was actually on a Dr. Phil episode, where one twin was constantly tearing down the other because she had to be on top)
I hope this helps!
Step-by-step explanation: