Some people can eat raw jalapeños like they're apples. Why do some people have a high tolerance for spicy food while others can't stand it?

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Sources:
http://www.sciencedirect.com/science/article/pii/S0950329312001917
http://www.popsci.com/science/article/2013-06/fyi-are-people-born-tolerance-spicy-food
http://www.pbs.org/wgbh/nova/body/reed-taste.html
https://books.google.com/books?hl=en&lr=&id=vvalCgAAQBAJ&oi=fnd&pg=PA77 book
https://academic.oup.com/chemse/article-abstract/6/1/23/277878/The-role-of-desensitization-to-capsaicin-in-chili
https://www.ncbi.nlm.nih.gov/pubmed/9035246

Image Sources:
https://ja.wikipedia.org/wiki/%E3%82%AB%E3%83%97%E3%82%B5%E3%82%A4%E3%82%B7%E3%83%B3#/media/File:Capsaicin-3D-vdW.png