The singular world of twins and twin studies

Personality Similarity in Unrelated Look-alikes

What they tell us about twin research

All of us know people who look alike, but are not related to one another. Perhaps we have even been confused for someone else. An interesting question is the extent to which people who look alike act alike—that is, how similar are their personalities? This question is important to answer because it tells us about a range of factors affecting personality. It is especially important for anyone interested in twins. Here is why.

A large number of twin studies have reported genetic influence on personality. This conclusion comes from the greater similarities between identical twins, compared to fraternal twins. However, twin research findings are still questioned by some people in the scientific community. A recurring misconception is that identical twin brothers and identical twin sisters resemble one another in personality because other people treat them alike. It is also argued that identical twins’ similar treatment is caused by their identical physical resemblance. I conducted a study to test this idea. I did this by comparing the personality similarities and self-esteem of 23 unrelated look-alike pairs. I call them U-LAs. They are rare, indeed, but I located them thorough the Canadian photographer François Brunelle, who has become famous for his pictures of people who physically resemble one another.

I reasoned that if treatment by others strongly affects personality, than ULAs should be as alike as identical twins. Alternatively, if genetic factors contribute substantially to personality then ULAs should be much less alike than both identical and fraternal twins.

The average age of the people I tested was about 46 years and the average age difference between them was about 65 years. I had each pair member complete items from the French Questionnaire de Personnalite au Travail, composed by the French psychologist Jean-Pierre Roland and his Belgian collaborator Filip Fruyt. This questionnaire yields score on five personality measures: stability, openness, extraversion, agreeableness and conscientiousness. I also had participants complete items from the Rosenberg Self-Esteem Scale, a widely used inventory that has been translated into many languages.

It turned out that the ULAs showed negligible similarity in both personality and in self-esteem—as I expected from my years of research with twins. I, therefore, concluded that identical twins’ personality similarity mostly reflects their shared genes. However, people do tend to treat identical twins more alike than they treat fraternal twins. This could be explained by what behavioral geneticists call reactive gene-environment correlation. That is to say, identical twins evoke similar responses form people based on their genetically influenced personality traits and other behaviors. I hope to report soon on the ULAs’ responses to their first meeting and the nature of their ongoing relationships with one another. More details about this work can be found in my publication of this study:


Segal, N.L. (2013). Personality similarity in unrelated look-alike pairs: Addressing a twin study challenge. Personality and Individual Differences, 54, 23-28.

NOTE: I have received a number of comments from twins who have list their twin brothers and sisters. Please contact me directly at or go to my web site to take part in research on this important subject.

Nancy L. Segal, Ph.D., is a Professor of Psychology and Director of the Twin Studies Center, at California State University, Fullerton.


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