I'm not sure you can know, really, right now. If she has crushes on boys, then I think she probably doesn't really want to BE a boy, but maybe she likes some of the benefits of being a boy. Did you consider that? I mean.. if she is really athletic, is she allowed to play on the boys' teams at school? Maybe she thinks the girls teams are lame. Maybe she sees the drama that girls start dealing with around this age and doesn't want any part of it, and thinks boys don't have the drama to deal with.
OR, is there something else going on, where she might be saying this to get under your skin?
I just think perhaps you are getting worked up over something that isn't even an issue for her. She's a middle child. Does she have any brothers or only sisters? Do some of her male friends have more freedoms than she notices with her female friends? That can often be the case for some reason... that we allow boys to DO more things with less worry that something bad will happen to them. I have a 16 yr old son and I worry about him being alone elsewhere (he drives now!) obviously, but I am not paranoid that every time he stops to put gas in the car someone might try to grab him. My daughter is 13, and I already know that when she is 16, I will be very cautious and concerned about her stopping alone for gas. It's not equal. I don't expect it to be, and I don't think there is anything wrong with it. But, perhaps your daughter is becoming exposed to some of these differences and thinks it might be fun to have a little more freedom that perhaps she sees her male friends having.
Or.. if she's super competitive athletically, maybe she things bigger muscles would make sports more fun. Who knows.
But, honestly, from the little information here, it doesn't sound like any big thing to be concerned about.
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I agree with the most recent response.. I wouldn't drag a huge conversation on about this. It's an unnecessary highlighting of whatever is on her mind, that can make her feel like she is wrong for whatever it is she is feeling.
With that in mind, did you ask her what she meant when she said, she "feels like a boy"? That could mean all kinds of things as well. I mean, what exactly does she think it means to "feel like a boy"? In her terms, what does that mean? Does it mean she doesn't like dolls and would rather climb trees? Does it mean that instead of CatWoman or Storm, she'd rather pretend to be Batman or Iron Man? Does it mean she would rather have short hair that doesn't require "fixing" and she can shower and be done with it? Does is mean she'd rather wear sneakers and shorts all the time, and not dresses and dressy shoes with heels and jewelry?
Does it mean she would like to be the class clown (since these often tend to be boys)? What does that mean in her mind? I cannot imagine that in her mind it is a sexual idea.
When I was in 5th grade, my best friend and I weren't into doing our nails or hair or makeup or gossip or many of the things that were starting with our female classmates. We didn't like sitting at the end of PE and whispering in small groups. We'd rather swing, as high as we could, and pretend we were driving motorcycles while we were swinging.
I'm still not a "high maintenance" kind of gal. And I am always more comfortable in jeans. But I look nice in a dress if the occasion calls for it. I don't spend $$$ on my hair or nails. I don't match every outfit with the precise piece of costume jewelry accessories. (and if you do, that's great. More power to you!) I prefer easy understated things. Pearl studs. A simple chain with birth stones of my kids around my neck. A watch I never take off. I wear this every day, all day. On a "special" occasion I might change my earrings or add a bracelet.
But, when I was your daughter's age, I did everything my brothers did. Shot guns. Played basketball (with them). Football (with them in the yard). Baseball (with them and on the field after their games were over for the night). Climbed trees. Rode bikes. You name it. I even tried to compete with my middle brother bench pressing free weights.
I didn't feel like a girl back then, either, b/c I thought that girls were shy little violets that sat around and were waited on and expected to be dainty and not get their hands dirty. I liked to fish and gut my own catch.
I suspect your daughter just doesn't have the same idea in her head of what it means to "feel like a boy" that you are thinking.