I'm not sure that's correct. I've heard that high CO2 levels trigger the parasympathetic nervous system which calms us down. One could get the same effect from breathing into a paper bag, I think.
Hmm. In the Apollo 13 movie, high CO2 levels triggered a panic/anxiety attack in one of the astronauts. (Which may not have happened, or not as severely, in reality – but that’s adifferent issue. For one, astronauts are trained not to panic.) A quick google seems to confirm this. As for breathing in a paper bag, I believe that is one common way to deal with hyperventilation, which may also induce anxiety and thus lead to more of the same.