http://www.psychohistory.com/htm/06a1_incest.html
The most commonly reported incest occurs when the mother sees her son
masturbate as a teenager and tells him, "It's not good to do it alone.
Your IQ becomes lower. I will help you," or "You cannot study if you
cannot have sex. You may use my body," or "I don't want you to get
into trouble with a girl. Have sex with me instead."(166) The
researchers found that Japanese mothers and sons often sleep in the
same bed and have sex together, although the exact incidence in the
population was not investigated. According to the phone interviews,
Japanese mothers teach their sons how to masturbate, helping them to
achieve first ejaculation in much the same manner as they earlier
helped them with toilet training.(167) Most of the sons had no sexual
experience with another woman, and became jealous of the mothers'
having sex with their fathers, feeling they should have the right to
monopolize the mothers - perhaps helping explain why one informant
told a family planning expert: "We have no Oedipal problems in Japan -
there's no competition from the father.(168) Mother recent Japanese
book, based on one hundred incest reports, confirms these
observations, including the unusually high rate of mother - son
incest, although it, too, provides no way to determine true national
incidence rates.(169)