Holding a Mirror to Their Natures

When twins have similar personalities, is it mainly because they share so much genetic material or because their physical resemblance makes other people treat them alike?

Most researchers believe the former, but the proposition has been hard to prove. So Nancy L. Segal, a psychologist who directs the Twin Studies Center at California State University, Fullerton, decided to test it–and enlisted an unlikely ally.

He is François Brunelle, a photographer in Montreal who takes pictures of pairs of people who look alike but are not twins.

Dr. Segal was sent to Mr. Brunelle’s website by a graduate student who knew of her research with twins. When she saw the photographs, she realized that the unrelated look-alikes would be ideal study subjects: She could compare their similarities and differences to those of actual twins.

“I reasoned that if personality resides in the face,” she said, “then unrelated look-alikes should be as similar in behavior as identical twins reared apart. Alternatively, if personality traits are influenced by genetic factors, then unrelated look-alikes should show negligible personality similarity.”

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For Dr. Segal’s initial study, she asked Mr. Brunelle to send questionnaires to some of his subjects, and she received completed forms from 23 pairs of unrelated look-alikes. The questionnaires yield a score based on five personality measures: stability, openness, extroversion, agreeableness and conscientiousness. The participants also took the Rosenberg Self-Esteem Scale, a widely used measure in social science research.

As she expected, the unrelated look-alikes showed little similarity in either personality or self-esteem. By contrast, twins–especially identical twins–score similarly on both scales, suggesting that the likeness is largely because of genetics. Her results were published in the journal Personality and Individual Differences.

For a second study, she teamed with a skeptic, Ulrich Ettinger, a psychologist at the University of Bonn in Germany who had heard about the look-alike project during a postdoctorate at the University of Montreal.

“I thought that if two people looked alike, they would have similar personality traits because people would treat them the same,” he said. “For example, I thought men who looked alike and were tall and handsome would probably be extroverts.”

Their analysis was consistent with the findings of Dr. Segal’s first study: Personality traits do not appear to be influenced by the way people are treated because of appearance. Moreover, they found, there appears to be no special bond between look-alikes.

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