There seems to be a misunderstanding about feminine and masculine features for faces. I see people here saying that having big jaw, big chin is a masculine trait. There are plenty of low testosterone women with big square jaws: link
Big jaw is more representative of health (and growth hormone), not testosterone. Also chin is more about testosterone levels, not the jaw.
So why is a big square jawline considered manly?
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Girls do not look at a guy and think, "ohh he has more testosterone, he is manly ;) ". It is the features. For example, I love defined cheek bones, do not really care about if that is testosterone. It is just a trait that is considered attractive. I picture a cowboy when you said strong jaw, which is attractive/manly. Personally, I like a defined jawline, not neccesarily a big jaw.