Why are fangirls often obsessive over male celebrities, while fanboys aren't that obsessive over female celebrities?

Fangirls will often go crazy over male celebrities and act like they are married to them or something. I've seen fangirls who will literally fight each other over Justin Bieber or Zac Efron, saying "He's mine!" and acting like they own the celebrity who doesn't even know who they are. I know of a fangirl who photoshops pictures of herself next to her actor celebrity crush so it looks like they're a couple and uploads it to instagram, then sends him the photoshopped pictures by dm every day until the day he eventually notices her.

Fanboys on the other hand seem to often retain their sanity. Maybe they might have posters of a women on their walls, but they're generally less obsessive. Why are a lot of fangirls like this while fanboys keep it straight?

Why are fangirls often obsessive over male celebrities, while fanboys aren't that obsessive over female celebrities?
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