At least in Germany they admit it. In 2003, Harshad, a British citizen of Indian descent, had a baby daughter with his German girlfriend. Knowing nothing of Germany’s idiosyncratic custody laws, Harshad went along with his girlfriend’s suggestion that they skip the laborious process of registering joint custody. It wasn’t until the couple split that Harshad discovered the enormity of that choice. “I had no idea it would cause so many problems,” said Harshad, a 44-year-old IT professional. “My ex-girlfriend had said, ‘It’s nothing to worry about; from the paperwork point of view, it’s far easier not to do it,...