Researchers sought to pin down why identical twins are not perfect replicas of each other, even when they have been raised in the same environment, and studied the matter using 40 genetically identical mice.
The mice were kept in an elaborate, five-level cage connected by glass chutes and filled with toys, scaffolds, wooden flower pots, nesting places and more. The space available to explore spanned about five square meters (yards).
"This environment was so rich that each mouse gathered its own individual experiences in it," said principal investigator Gerd Kempermann of the German Center for Neurodegenerative Diseases.
Even though the mice were genetically the same, and the environment they were kept in was also the same, they showed individually different levels of activity. Some explored a lot, some did not.
And by fitting them with a special micro-chip that emitted electromagnetic signals, scientists could track how much the mice moved around and quantify their exploratory behavior.
"Over time, the animals therefore increasingly differed in their realm of experience and behavior," said Kempermann. Over the course of three months, they developed very different personalities.
Researchers found that the brains of the most explorative mice were building more new neurons -- a process known as neurogenesis -- in the hippocampus, the center for learning and memory, than the animals that were more passive.
Control mice kept in a less enriching environment showed less brain growth.
Kempermann and colleagues said they have shown for the first time how personal experiences and ensuing behavior contribute to individualization, and that neither genetics nor environment alone could cause this personal growth.
"Adult neurogenesis also occurs in the hippocampus of humans," according to Kempermann. "Hence we assume that we have tracked down a neurobiological foundation for individuality that also applies to humans."
The findings offer new understanding of how the brain works, and could shed light on the processes of learning and aging, said Ulman Lindenberger, director of the Center for Lifespan Psychology at the Max Planck Institute for Human Development in Berlin.
"When viewed from educational and psychological perspectives, the results of our experiment suggest that an enriched environment fosters the development of individuality," said Lindenberger.
An accompanying commentary in Science by Olaf Bergmann and Jonas Frisen of the Karolinska Institute in Stockholm, Sweden, said the research has two main uses.
"Molecular understanding of neurogenesis will hopefully aid in the rational development of new classes of drugs for psychiatric disease," they wrote.
Furthermore, it "may teach us... how living our lives makes us who we are."