What Gives Identical Twins Different Personalities?

Does life experience shape our personalities, or are they already written in our genetic code? The nature-versus-nurture debate rages on as new findings show the brain can be sculpted by interactions with one’s environment.

In a study published this week in Science, German researchers examining 40 genetically identical twin mice found they could develop very distinct personalities. The researchers identified a link between exploratory behavior in the young mice and the birth of new neurons in their brains during adulthood.

The mice were both genetically identical and living in the same maze-like environment with twists, turns, and toys. Researchers equipped the mice with a special microchip emitting electromagnetic signals, which allowed the scientists to track the mice's movements and to rate their exploratory behavior. 

Despite a common environment and identical genes, the mice showed highly individual patterns of behavior. They reacted to their environment differently, and throughout the three-month experiment those differences increased. Most importantly, some mice traveled and explored a wider area than others did.

“Though the animals shared the same life space, they increasingly differed in their activity levels," said lead investigator Gerd Kempermann, professor at the Center for Regenerative Therapies Dresden and site speaker at the German Center for Neurodegenerative Diseases

The differences were associated with the generation of new neurons in the hippocampus, a region of the brain that supports learning and memory, according to Kempermann.

"This environment was so rich that each mouse gathered its own individual experiences in it," he said. "Over time, the animals therefore increasingly differed in their realm of experience and behavior.”

The birth of new neurons, also known as neurogenesis, depended on how thoroughly the mice explored their environment. Kempermann's study shows for the first time that personal experiences help shape how the brain reacts to new information and lead to the development of new behaviors going forward.

“Adult neurogenesis also occurs in the hippocampus of humans,” Kempermann said. “Hence, we assume that we have tracked down a neurobiological foundation for individuality that also applies to humans.”

The Drawbacks of Inbreeding

Thomas Bouchard, director of the Minnesota Center for Twin and Adoption Research, said that while he respects the work of Kempermann's team, he also has his reservations. Bouchard has spent his career studying identical twins who did not grow up together but who have developed many of the same traits. 

He points to the fact that this current study uses inbred animals, which are more sensitive to the environment than hybrids and less robust. Both wild animals and humans are considered hybrids.

“My criticisms should not detract from the excellence of the study,” Bouchard said. “I have no idea how one could explore this idea except with inbred animals. Every experimental design has its drawbacks.”

Still, the new research does suggest that experience influences how the human mind ages and matures.

“The finding that behavior and experience contribute to differences between individuals has implications for debates in psychology, education science, biology, and medicine," said study co-author Professor Ulman Lindenberger of the Max Planck Institute for Human Development in Berlin. "Our findings show that development itself contributes to differences in adult behavior.”

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