Sheldon from the Big Bang Theory wears a lot of superhero T-shirts, because that is a nerdy thing to do and Sheldon is an uber nerd. But I noticed that all the superheros are DC, Detective Comics, and none are from DC's competitor, Marvel Comics. This made me suspect that Warner Brothers, which owns the DC characters was producing The Big Bang Theory. Which turns out to be true, as you can see from the Wikipedia article for The Big Bang Theory
I found a discussion on Quora that noted this, and also noted that Cisco a character on The Flash televisions show, also produced by Warner Brothers, often wears shows referencing The Big Bang Theory television show. So Warner Brothers is doing exactly the type of cross promotion you would expect.
More recently I was watching season one of Once Upon a Time Henry a ten year old boy who is son, grand son, and adopted son of many of the shows main characters is reading Marvel Comics, which is owned by Disney. Once again cross promotion.
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