In shorthand, 'it's genetic', though my understanding of it is slightly more complex than that, going down past genes to... particles and quarks, maybe. It is not my field. Magic is a dominant trait, anyway, while 'non-magic' is recessive; we are as vastly outnumbered as we are due to many instances of mankind deciding that we're evil, or Satanic, or simply not to be trusted.
private.
It's a lovely film.