they did a study a few years ago, it indicated red heads have a lower pain threshold than other people. Have you Googled "pain threshold research"? I was already on morphine and other opiates at the time so I did not care too much one way or another. (I have red hair)

Here it is, "Red heads suffer more pain" 15 Oct 2002 in New Scientist. That might explain my reaction to several professors I had in college.

Good Luck with your research ... this is a good forum, hopefully others will have a better answer than I did.