@DuDuDuLangaLangaBingBong
I've been doing some thinking about this, and it's such a complex issue, with so many differing opinions, that I can only throw out a few of my own, which are by no means "authoritative" on the issue in any training or academic sense.
And, actually, the word "complex" is key here, because I believe this phenomenon is exactly that: the Jungian definition of a complex, which Jung writes about as follows:
‘Everyone knows nowadays that people ‘have complexes’. What is not so well known, though far more important theoretically, is that complexes can have us. The existence of complexes throws serious doubt on the naïve assumption of the unity of consciousness, which is equated with ‘psyche’, and on the supremacy of the will. Every constellation of a complex postulates a disturbed state of consciousness …. The complex must therefore be a psychic factor which, in terms of energy, possesses a value that sometimes exceeds that of our conscious intentions … And in fact, an active complex puts us momentarily under a state of duress, of compulsive thinking and acting, for which under certain conditions the only appropriate term would be the judicial concept of diminished responsibility’ (CW 8, para 200).
And this particular complex is collective, and contagious in nature. If there is any personal susceptibility to defer to group thinking, then it's more likely to occur. So, to focus on girls and young women in this context - yes, I think the complex is different. And here I get really speculative, because I'm not an academic who has spent years researching this, nor am I an analyst who works with children. But maybe one or some (or none!) of these factors come into play:
A girl's body changes more perceptibly than a boy's - particularly with breast development. Breasts are sexualised in our society, and many girls have a deep-seated ambivalence towards them when they appear. It's not just the breasts themselves, but what they symbolise: sexual maturity. Add in a child's personal psycho-emotional experiences, and their family dynamics, and maybe there is a deep need to stop this process, or to stop identifying with "woman/mother", or to identify more closely with "man/father". Parent-parent, and parent-child, dynamics are key here - many of which will be unconscious and therefore sometimes heavily defended against or denied.
Add societal changes: the advent of porn, the easy access to porn by young people and their changing sexual attitudes and habits as a result, and growing up a girl must be downright confusing for some at best, and terrifying for some at worst.
But the thing about complexes - and I do think that this is complex-based or -related - is that they will evade easy explanation because they reside in and are driven by the unconscious (which is why asking a child if they "really want to transition" is pointless and irresponsible); and when it is collective, we are often too close into our current social material to fully get a grip on it - which is why they're potentially so dangerous and destructive.
As for how to help girls now - I don't know. That's my truthful answer. The best I can suggest is to try and deal with the root cause, which isn't about the girls at all - that is where the complex is manifesting. But that root cause is deep, entrenched, and, by its nature, will be defended to the hilt by its (primarily unconsciously driven, even "possessed") proponents/victims. Working with a personal complex in a single individual is challenging enough. Put group dynamics into play, and you are taking on a leviathan with a butter knife. Though I was thinking of one possibility yesterday ...
In therapy, there is the concept of "confrontation". It's tricky to do, and frankly my own experience of it as a "therapee" has been that it was too much, too quickly: it risks traumatising, because it is a direct shot of insight into a key element of the unconscious.
However, applying this approach, one of the ways perhaps to dismantle a collective complex is at the level of the unconscious: to find that one true shot that gets to the heart of the Death Star. That shot will be collective in nature; it will pertain to a collective element of the unconscious. It is designed to compromise the integrity of the structure, not of the individuals in it. It is that moment where this "possession" disintigrates - as it has done with many an ideology when those in power have been toppled. What that is specifically? Who knows? Maybe we don't find it by looking for it directly. Maybe we do what we can right now, which can feel so inadequate but which is perhaps far more powerful than we can know: working on our own unconscious material. Honestly, if I could get everyone who was even halfway willing or curious into therapy with a decent therapist, I would. This is personal work first, and that then affects the collective. Currently, I have no better suggestion.