The Tele­graph looks at Why We Scream:

We all have a core set of five facial mus­cles that con­trol our abil­ity to pro­duce stan­dard expres­sions which con­vey anger, hap­pi­ness, sur­prise, fear, sad­ness and dis­gust. But there are up to 19 mus­cles present in the face, and many peo­ple do not pos­sess all of them.

The riso­rius mus­cle, which con­trols expres­sions of extreme fear, is found in only two thirds of people.

via Mind Hacks