While Sayumi Kusachi's raised each of her identical twin daughters the same way, each of the 21-year-olds has a very different personality, The Daily Yomiuri reported.

For example, Minami is shy and attending a school for voice actors, while Honami is outgoing and enrolled in a university, the news source stated. However, the twins' lives diverged when they were each sent to different high schools. This meant that the girls made their own friends and had unique experiences, which, in turn, may have influenced their personalities.

In Honami's case, the fact that her friends at the university are free to express their opinions helped her learn how to better assert herself.

"...There's no denying both genetic and environmental factors play a role in forming character," Eiji Shimizu, psychiatrist and head of Chiba University's Research Center for Child Mental Development, told the news outlet.

According to CBS News, individuals who wish to change their personalities should avoid spending time with people whose behavior is the opposite of what they are trying to achieve. To measure their progress, they may also want to take a personality test and see if there has been a significant change in their characteristics.