T-Shirts and Fandom Demographics

Probably because I’m a nerd and a holder of a degree in sociology, I like looking for patterns.  And if I can do this in relation to things I geek out about, so much the better.  I have a small collection of t-shirts from the site TeeFury, and I really need to start making a record of the reactions I get from people when I wear them.  I’ve had this thought before, but I was reminded of the plan when I wore the above Doctor Who shirt today (I was wearing the shirt when I took the photo, which is why it looks a bit wavy).  A British guy came up to me in Wormhole Coffee (one of my new favorite places in the city) and gushed about how much he liked the shirt.

What I’m interested in finding out is: (1) what types of people comment on what shirts and (2) what level of appreciation do people show?  My Doctor Who shirt has gotten far and away the most comments, and most of the commenters are women who are roughly my age.  I think the guy today was the first man to comment on it.  Meanwhile, only middle-aged to elderly people have commented on the Calvin & Hobbes shirt, and almost no one has said anything about my Joss Whedon or Harry Potter shirts.  And when people do comment, the Doctor Who praise is extensive, while everything else gets about two words.  I wonder: is it because Doctor Who is currently airing/the new thing to geek out over in the U.S., while Firefly, Dr. Horrible, Harry Potter, and Calvin and Hobbes have been over long enough for people to not get as enthusiastic?