So there are a few interpretations there: it's a heritable mental illness; it's environmental and whatever's causing the mothers' mental illness is causing the sons'; the mentally ill mothers are inflicting this on their sons.
I feel like there's some support for all three explanations.
I wouldn't be surprised if all of them contribute in at least some cases, but I'd also guess (educated guess) that the mothers inflicting this on their sons is the biggest and most common contributor. I don't believe hardly any child believes they were born into the wrong body unless someone, in a position of authority or influence, suggests it to the child.
