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troperville

Lots of people masturbate! It's nothing to be ashamed of. Or maybe it is. Either way, it's not something you'd bring up in polite conversation. However, it's given quite a different treatment in fiction. Unlike Nobody Poops or No Periods, Period, where the function is simply not mentioned because it would be Nausea Fuel, masturbation in fiction often fits into four different categories:

It's aimed at or about teenagers (usually Played for Laughs) and the character is either a horny teenage boy, or another character, usually the parents, think that he's masturbating when he is very obviously (or sometimes not so obviously) not.
The character masturbating is sexually frustrated, disturbed, perverted, or is a nerd/has no social life.
It's a military or a prison setting. The masturbation here would be explained by the shortage of women (or men), boredom and loneliness.
In porn or erotica, it can be used for sexual titillation for the viewers.

Of course, there's a pretty valid reason why "normal" masturbation isn't depicted very often — it doesn't accomplish much. There aren't many ways that masturbation could advance the plot of a story, and because it's generally a solitary activity, there usually won't be any character interaction involved.

Also note that male masturbation is used far more often than female (except in porn, for the above-mentioned reasons)... though that's not to say that female masturbation isn't given any attention in fiction (the "Rumpelstiltskin" tale supposedly had the original message, "women who masturbate become infertile"). In older media, expect references to hairy palms and/or blindness, two widespread myths used to keep boys from masturbating.

And because this page wouldn't be complete without her: In Ankh-Morpork on Discworld, Rosie Palm, the head of the Seamstresses' Guild, is named as a Shout Out to this, derived from an old British euphemism, "Rosie Palms and her five lovely daughters".

List of masturbation tropes:

Breaking In Old Habits - A character receives a replacement hand and immediately uses it to masturbate.
Caught with Your Pants Down - A character is caught masturbating.
I'll Be in My Bunk - A character who steps off-stage is humorously portrayed as leaving to masturbate.
Jizzed in My Pants - Yeah, self explanatory.
Mistaken for Masturbating - Characters think someone who isn't masturbating is.
Porn Stash - A character has a personal collection of pornography, and can usually be assumed to use it as an aid to masturbation.
Poor Man's Porn - A character gets off to something not strictly intended to be used for that purpose.
Self-Abuse - Masturbation is portrayed as bad for you.