Identical twins tend to have similar personalities; if environment outweighs heredity in personality development, twins raised together should presumably have more similar personalities than those raised apart. A recent study of identical twins in both situations measured 11 key traits through a questionnaire, and concluded that 7 of the 11 are primarily products of heredity.
Which of the following, if established, would cast the most doubt on the study’s results?

Question

Examsnet · Accepted Answer

Correct Answer is :  Some of the traits that the study attributed to heredity developed in the separately raised twins because those pairs all grew up in similar families.

GMAT Verbal Reasoning Practice Test 3