GMAT Verbal Reasoning Practice Test 3

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Question : 56 of 100
 
Marks: +1, -0
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?
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