Here’s this week’s Media Mix about Japan’s age of consent, which has been 13 for more than 110 years. A good portion of the column is taken up by the story of a girl who was allegedly molested by her biological father when she was 13, which means she was over the age of consent and would therefore have to prove in court that she resisted her father’s advances in order for him to be convicted of sexual assault. What makes the situation doubly perplexing is that the father could be prosecuted for having sexually assaulted his own daughter, but when the alleged assault happened he was divorced from her mother, who had sole custody of the daughter, so that particular statute didn’t apply. For sure, many people will find this legal point puzzling—how can fatherhood be revoked?—but it follows the peculiar logic of family law in Japan, which gravitates around the primacy of the family register (koseki).

Determining motherhood is easy, but fatherhood is essentially a matter of taking somebody’s word for it (minus a DNA check, that is). The family register system allows the state to be the sole arbiter for paternity. That’s why the state considers the father of a baby born to a woman within 300 days after her divorce to be her ex-husband, even if the biological father is really someone different. The reason for this law is because it is possible that the ex-husband could be the father if he had sex with the mother up until the eve of their divorce. In any case, it makes it easier for the bureaucrats. The state basically wants the final say on who the father of a child is, so when a couple get divorced and the mother gains sole custody of their children, the ex-husband is, technically at least, no longer the children’s father in the eyes of the state. Japan also doesn’t really recognize joint custody, and in most divorce cases it’s the mother who gets sole custody, a circumstance that has led to a great deal of misery on the part of fathers, both Japanese and non-Japanese, who end up being denied any contact with their children. However, sometimes it is the father who gets sole custody (usually because he or his parents are more invested in having an heir) and it is the mother who is cut off. The main controversy about this legal nicety is whether it is fair to the children, who, as the low age of consent shows, don’t merit much consideration in Japanese family law.