Why do we scream in pain and fear? Why do humans (other living things too, but not sure) scream and make high pitched sounds when in pain*  or when in fear that something painful* will imminently be inflicted upon them?

*  both physical and emotional.
This question came up in our tri-monthly site evaluation queue (Let's get critical: Feb 2014 Site Self-Evaluation), and I marked it as "needs improvement," so here's an indication why, and my attempt to do so.

The next-highest Google hit for this question's title, "Science: why we scream" (Highfield, 2008) seems pretty useful. Telegraph's terms and conditions are a little scary though, so I'll paraphrase some ideas rather than quoting directly.

Darwin himself argued that emotional expressions may improve selective fitness. This could explain the relatively universal uniformity of fear expressions, which generally include widened eyes and nostrils, lifted eyebrows, and maybe some inclination to scream (Susskind et al., 2008). This may improve olfactory and visual sensory intake and motion tracking. Pragmatic benefits aside, William James, another prominent historical figure in psychology, was well ahead of his time in pointing out that expressions affect emotions; they also affect neural circulation!

However, there are limits to the universality of fear expressions, and even to the uniformity of facial muscular anatomy (Waller, Cray, &amp; Burrows, 2008)! Basic expressions like "happiness, sadness, fear, anger, surprise, and disgust" are handled pretty well by five essentially ubiquitous facial muscles, but people differ in the number of additional, facially expressive muscles they have (19 may be the maximum). I've edited this summary of Waller and colleagues' results into the Wikipedia page on the risorius muscle just now:


  A study of 18 Caucasian cadavers found individual differences in the presence and bilateral symmetry of risorius muscles. Seven of these individuals (both men and women) clearly lacked the muscle; two were inconclusive; only four were conclusively symmetric.


Highfield (2008) associates the risorius muscle with "expressions of extreme fear," but Wikipedia only associates it with an "insincere-looking" smile; Waller and colleagues (2008) associate no expressions explicitly with the risorius muscle, which they don't really discuss in specific depth. However, Highfield quotes Waller directly (suggesting further unpublished discussion that may have supported his interpretation of risorius' function) as follows:


  Everyone communicates using a set of common signals...so we would expect to find that the muscles do not vary. The results are surprising. Some less common facial expressions may be unique to certain people.


Other quick Google Scholar results for "scream fear" ("scream pain" didn't produce anything promising at first glance) seem useful to mention here too. A study of rats and 160 undergrads found (in the human research) that a "yell, scream, or call for help...was frequent for females, rare for males" (Blanchard, Hynd, Minke, Minemoto, &amp; Blanchard, 2001). A more general discussion of the function of fear screams in humans and other animals has also been published (Högstedt, 1983), so you may want to check that out too. It includes some literature review, so its references could probably lead much deeper into the question than you may wish to pursue it... ;)

References

Caroline Blanchard, D., Hynd, A. L., Minke, K. A., Minemoto, T., &amp; Blanchard, R. J. (2001). Human defensive behaviors to threat scenarios show parallels to fear-and anxiety-related defense patterns of non-human mammals. Neuroscience &amp; Biobehavioral Reviews, 25(7), 761–770. Available online, URL: http://www.researchgate.net/publication/11558273_Human_defensive_behaviors_to_threat_scenarios_show_parallels_to_fear-_and_anxiety-related_defense_patterns_of_non-human_mammals/file/9fcfd50c755e22230b.pdf. Retrieved February 8, 2014.

Highfield, R. (2008, June 17). Science: Why we scream. The Telegraph: Science News. Retrieved from http://www.telegraph.co.uk/science/science-news/3344701/Science-why-we-scream.html.

Högstedt, G. (1983). Adaptation unto death: Function of fear screams. American Naturalist, 121(4), 562–570. Available online, URL: http://www.jstor.org/stable/10.2307/2460982.

Susskind, J. M., Lee, D. H., Cusi, A., Feiman, R., Grabski, W., &amp; Anderson, A. K. (2008). Expressing fear enhances sensory acquisition. Nature Neuroscience, 11(7), 843–850. Available online, URL: http://www.yorku.ca/khoffman/Psyc6253/Susskind%2708_Meotions4perception.pdf. Retrieved February 8, 2014.

Waller, B. M., Cray Jr, J. J., &amp; Burrows, A. M. (2008). Selection for universal facial emotion. Emotion, 8(3), 435–439. Available online, URL: http://eprints.port.ac.uk/1629/1/WALLERetal2008b.pdf. Retrieved February 8, 2014.