It’s an age-old question: Which is more important in determining the course of our lives, nature or nurture? Actors Hayato Kakizawa and Eiji Wentz grapple with this absorbing issue in the musical “Blood Brothers,” which kicked off its eighth all-Japanese production on March 21 and will run until April 3 at Tokyo International Forum.

Written by English playwright Willy Russell, “Blood Brothers” traces the checkered lives of a pair of fraternal twins who are separated at birth, and it has long been a favorite among musical lovers. It first ran from 1983 to 2012 in London’s West End, winning an Olivier Award in its opening season and becoming the third longest-running musical there, after “Les Miserables” and “The Phantom of the Opera.” It premiered in Tokyo with an all-Japanese cast in 1991 and has had multiple reruns featuring fresh cast members.

Set in the hardscrabble city of Liverpool in the 1960s, the story — often described as "a Liverpudlian folk opera" — centers on Eddie (Wentz), who is adopted by a wealthy family, and Mickey (Kakizawa), who is raised in poverty by a single mother with numerous other children.