Answer to Question 1
Answer: Twin studies cannot rule out confounding variables such as the fact that identical twins look alike. People may treat the twins a certain way based on their identical appearance, which is an environmental influence and not a genetic one.
Answer to Question 2
Answer: The restriction of range problem in twin studies refers to the fact that the family environments for the twins being evaluated probably did not vary very much. Most twins in the twin studies grew up in stable, middle-class homes with married parents. Twin studies also leave out the potential influences of culture because they are usually done within one nation and are always done within one generation because twins are born simultaneously.