Until recently experts believed that environment, not genetics, largely determines human personality. A new study, however, has shown that there is more similarity in personality between identical twins raised together than between nonidentical twins raised together. The study concluded that genetics, therefore, does play an important role in determining personality.

Which of the following, if found to be true, would cast the most doubt on the study's conclusion?

(A) Identical twins raised separately in different adoptive families are usually more similar in personality than are nonidentical twins raised separately in different adoptive families.
(B) No matter how twins behave, parents treat identical twins in ways that tend to elicit similar personality traits but do not treat 'non-identical twins in such ways.
(C) Parents of both identical and non-identical twins have long claimed that their children, from early infanthood, had definite and well-established personality traits.
(D) Birth parents and their identical twin children tend to become more similar to each other in personality over time, but adoptive parents and their identical twin children do not.
(E) Neither identical nor nonidentical twins are likely to display drastic changes in their individual personalities as they grow up.