— 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.

François Brunelle, a photographer in Montreal, takes pictures of people who look alike but are not twins. Segal was sent to 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, then unrelated look-alikes should be as similar in behavior as identical twins reared apart,” she said. “Alternatively, if personality traits are influenced by genetic factors, then unrelated look-alikes should show negligible personality similarity.”

For 14 years, Brunelle, 64, has been working on a project he calls “I’m Not a Look-Alike!”— more than 200 black-and-white portraits of pairs who look startlingly alike.

Most come to him through social media links to his website. “It has taken on a life of its own,” he said. “I have heard from people in China — and even a man who has an uncle in Uzbekistan who is a dead ringer for former President George W. Bush.”

Two of his subjects, Roniel Tessler and Garrett Levenbrook, met three years ago at the University of Michigan, where Levenbrook was a student and Tessler was visiting with an a cappella group from the University of Maryland. Mutual friends steered them to Brunelle’s website.

When the two got together, at a pizza parlor in New York City, “we ordered the same toppings,” said Levenbrook, 25. “I found it difficult at first to look at Roniel because I felt I was looking into a mirror.”

But other than that, the two have little in common. Tessler, 27, describes himself as a free spirit; he called Levenbrook his “exact opposite” — “the most focused and organized person I know.”

For Segal’s initial study, she asked Brunelle to send questionnaires to some of his subjects, and she received completed forms from 23 pairs of unrelated look-alikes. The questionnaires measured stability, openness, extroversion, agreeableness and conscientiousness. The participants also took the Rosenberg Self-Esteem Scale, a widely used research measure.

As she expected, the unrelated look-alikes showed little similarity in personality or self-esteem. But twins — especially identical twins — score high on both scales, suggesting that the similarities were largely because of genetics. Her results were published in the journal Personality and Individual Differences.