"There's no third party," says the source, who adds that the pair made the decision to end their marriage about a month ago. "Ryan and Scarlett had been having some issues for a few months now. Distance and their careers played a big role. When you don't spend a lot of time with your partner you grow apart."
Reynolds, 34 and Johansson, 26, were married in 2008. Recently, they had "started to lead different lives, and it was harder and harder to come back together and pick up where they left off," the source says. "Scarlett was young when they married. She's a mature person but at points they were just on different pages."
Reynolds, who is filming a movie in Atlanta, and Johansson, who is currently in Jamaica with some girlfriends, remain firmly on good terms, the source says. "They had an intense and loving relationship, and they'll always love and respect each other."