While the reading passage discusses twin studies, the professor is doubtful of the effeciency of twin studies.
According to the reading passage, researchers conduct twin studies to determine how much nature and nurture contribute to the personality and characteristics of a person. In traditional case studies, researchers compared identical twins and fraternal twins. As a result, it was found that fraternal twins have 50 % of same genetic traits. Therefore, if identical twins are more similar than fraternal twins, researchers can conclude that genes are more significant factor of a person's traits.
However, the professor casts doubts on the twin studies. She points out a flaw in premise of equal environment. That is, an environment similarity of fraternal twins and identical twins is different. Thus, it is not clear that they really share same experiences in the study. She says that parents and family actually treat the identical twins as one person. For example, identical twins usually wear same clothes and eat same ice cream. Therefore, it is natural that identical twins resemble each other more than fraternal twins. This makes unclear to conclude that heredity is a larger factor than environment. That is why the professor thinks she can not accept the twin studies.