1) Well, if a person chooses not to reveal their gender on the internet, there may be several reasons. When I first got subjected to the internet, I didn't want to reveal I was a female. I was in a "clan" on a game, and I was worried they wouldn't take me seriously or would ostracize me if they found out. So I let them believe I was a boy, and avoided revealing my gender. That was my reasoning for it, but I was a kid back then.
Nowadays, there can be a bunch of reasons. The main reason I see these days, is people not wanting to reveal their gender because they don't want their gender to be a focal point, or a deciding factor on how people interact with them. They don't necessarily see their gender as a key trait in their personality. And on the internet, gender isn't really too important as it is, depending on what you do. Being a girl or a boy makes no difference on these forums, though there are people who insist females get more positive attention. So as a result, females might not to reveal themselves for fear of unwanted attention, etc.
Another reason may be that people just are ashamed of the gender they are, and don't know if they should reveal the gender they are, or the gender they wish to be. In my opinion, only the gender you want to be matters, since gender plays very little role otherwise.
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2) Becoming mute made me a smarter, wiser, and overall much better person. However, it's made me bitter (at times) and more cynical too. It immediately made me value the act of listening, as opposed to talking. You learn more when you listen, than when you talk. At first, I refused to communicate with anyone, at all, for months. But then I began to understand the importance of choosing my words carefully. When you're forced to think before you speak, and listen before it's your turn to speak, it changes who you are and how you think about things.
I never learned sign language because nobody else understood it. So my way of communicating was writing everything down. Typical conversations tend to go "I speak, then I will read what you wrote." They could take away my power to communicate by just refusing to read, or by taking my notebook away. There is no way for me to yell or shout in these conversations, so in a way, it made me realize how utterly useless shouting and yelling is to effective conversation and discussion. That knowledge alone made me get much less angry and flustered in arguments, and overall made me a more mellow person. So in heated debates or conversations, I choose my words in such a way so I don't offend whoever I'm talking with, because they can take away my power to talk just by refusing to read.
I speak with people generally the same way I type on this site though. However, the issue is that I type super fast, but handwriting / texting is much, much slower. So if I'm talking to an impatient person, or if they don't want to read a lot, I don't say as much, and instead get right to the point. When I type, I do tend to say more and more, because it's stress relieving and nice to be able to communicate with people.
3) Hmm, I dunno if I believe how much or how little someone types on a website can be attributed or indicated by their personality so much. Even though the mood of my typing is overall the same as when I communicate in real life, I may type walls of text quite often, but I'm not interested in saying a whole lot with my pen and pad. So people might see me as overly talkative on websites, but in reality I'm a wallflower who prefers to listen and watch rather than talk. Because as I said, you learn more when you do so, than when you talk.
4) Hrm...OC's are an interesting source of thought for me. Some people make OC's to represent the person / creature they want to be, and some people make them to represent the type of person they are. I think it's a very nice, unique OC, but forgive me for being cryptic, but this question isn't really one I can answer too well. Perhaps it's a question best answered by you, if you'll forgive me for saying it. I don't know you quite well enough to answer it, but I think asking yourself that may lead to a better answer anyway.