"I've not done any great study on it," only partly for fear of what she might find out about herself, "but the general idea is that the Other magic genes and the human magic-handling genes are mutually incompatible, and make the child mentally unstable while magically very powerful. It usually manifests itself early, when the kid's a toddler, getting getting into the really gruesome kinds of mischief. But it can also be that the kid in question is completely normal until some sudden shock or stress kind of... snaps them."

"It's part of why magic-handling families are very careful about which families they marry into - Dad's family was dead set against him marrying my mother, for example, because she was an unknown. The general discrimination towards Others and Partblood Others, though, comes from the Wars."