Interesting question. Most of the girls I know are a bit more hesitant before they say something. In college, guys mostly just say what they're thinking and it's usually funny. Girls think first and end up saying something that is more intelligent and less funny.

In the realm of comedians, it's kind of the same. Guys aren't as hesitant to say something that might be misconstrued as controversial. Take half of Adam Sandler's jokes and imagine them coming out of... *thinking of a female comedian* ... Amy Poehler's mouth. It just doesn't fit.

That might be because girls kinda have it harder than us guys
You don't know women unless you've grown up in a house of 4 women and you were there day laborer
Girls weren't equal to boys until it was to late for that to matter
Plus there period knocks all the funny out em

Most of the funny people I know are girls to be honest, myself among them according to my friends.

In the professional comedian circuit, it's still catching up to the increasing gender equality perceptions that we have in society. It isn't that female comedians are not funny, it's that agents and venues are still hesitant to give them a chance because they don't think they will sell as much. But let us actually list some of those big names and you can see that there are far more well-know female comedians that you might think (this is just a small list of relatively well-known female comedians, wiki has pages for at least 403):

Amy Poehler
Tina Fey
Ellen DeGeneres
Kathy Griffin
Whoopi Goldberg
Rosanne Barr
Phyllis Diller
Lily Tomlin
Caroline Rhea
Joan Rivers
Kristin Schaal
Minnie Pearl
Margaret Cho
Wanda Sykes
Sarah Silverman
Lucille Ball
Maria Bamford
Joy Behar
Samantha Bee (the female reporter on the Daily Show)
Janeane Garofalo
Aubrey Plaza

because overwelmingly, guys say something stupid, girls dont.given stupidity or dumb statements are some of the funniest things, guys have a natural advantage there.

One of the funniest people I know is a girl but she is a tom boy so I am not sure if that counts...

I think it is because men say what comes to mind and that tends to be funny.

Probably because guys aren't afraid to crack jokes, especially perverted jokes. Most people(girls) find them funny but wont make jokes usually.
That's just a theory though.

I think it has to do with the fact that men have always (for the most part) been able to speak their mind with more freedom than women. For example, a man may make a crude sex joke, and typically his guy friends find it hilarious and perhaps respect him a bit more if it was based on an actual experience or whatever. But, if a woman were to joke about something crude like that, she'd be labeled a whore among other derogatory term. Get most women alone, free from people judging her, and she will be just as open as a man.

Also, let's not forget, what one person finds funny, another person may not. Some subjects are very polarizing. I for one hate dead baby jokes. I couldn't believe that it was a real category of jokes.

Point is, biologically, there is nothing preventing a woman from being funny.

Probably because people expect more out of girls than guys. Guys in our society have done everything and everyone has seen them do all these things and because of that, they have little expectations from them as a gender. We can clown around and act like idiots and people just look and say, ha ha that guy is such an idiot. Typical male behavior. But when It comes to girls, we expect them to be more refined than us guys. And if we see them degrading themselves to the humor that is dominated by males, we say, " Your a lady aren't you? Then you should act like one. Stop acting like an idiot." And everyone is used to seeing degrading male comedy. It's been used to for 100s or even 1000s of years. But since girls have just recently (In terms of how long they have been unaccepted as equals to the mid 1900s to now) become accepted as equals in every way, there still exists a cultural barrier between males and females. We simply don't feel as comfortable seeing women participate in degrading and/or gross humor. If we see a fat gross guy urinating his name in the snow, we might say something like "ha ha that's so stupid but funny. That guys looks funny too" But if a fat gross woman were to do the same thing, the response from the male audience might be something like "Ew, aren't girls supposed to be more clean and sophisticated than guys? This is just disgusting. I don't know how I feel about seeing a gross women degrade herself like this. Now excuse me while I go to watch some inappropriate videos about girls being exploited in degrading sexual escapades." There was an episode of South Park in which the guys were watching a show about guys farting when the show was compromised by a show about girls releasing gases from a different region. This absolutely disgusted the guys because due to the gender barrier they didn't think that girls should be doing something like that. They found it absolutely gross and disgusting even though it was on the same level as the show that they were watching about farting. The lack of understanding of girls led to their disgust with their comedy. We've been forcing our male comedy (I,e P*nis jokes and such) down their throats for the past 1000s of years so it's nothing new to girls to see a guy degrade himself but when we suddenly see girls, the ones who we expect to be civilized, sophisticated, and mature engaging in potty humor, we might think," Um, what is this? I don't think girls are supposed to be doing this. Ok um, eww." Because we don't understand enough about girls to be comfortable with them acting like that. It's not a good thing, but that's how it is. People tend not think that girls are capable of being nasty and it becomes awkward when there's a rude awakening to guys that girls are capable of being just as nasty as guys. Like I said before, it's not a good thing that it's like this, but sadly that's how it is. Not everyone is like this, but many people are. Since there are just as many guys as there are girls, if many guys don't like to watch comedy by girls, that cuts the audience in half. This is just IMO though. Don't assume that I think that what I said is absolutely true. And don't quote me! /