Danish historian Jørgen Nybo Rasmussen (Rasmussen 1974, 1986) argues that Jacob was the son, apparently extramarital, of King John of the Kalmar Union and a younger brother of King Christian II, both Danes.
A number of modern authors[REF][REF][REF] have counted Jacob - or James - as a Danish-Norwegian-Swedish prince and one of the legitimate children of King John and Queen Christina, but Rasmussen's thesis was also met with scepticism [REF].