Masaki Kubota's delight at reuniting with his two sons gave way to heartbreak when the younger, two years old, stared at him uncomprehendingly as if to say: "Who are you?"

It was their first encounter since his wife left with the boys a year earlier — the kind of painful separation that Japan's new parental custody rules aim to prevent.

The country enacted laws last month allowing joint custody after divorce, replacing a decades-old system where it was granted to only one side, and almost always to the mother.