A fictional alien race could be as numerous as human beings or feature even greater numbers, and that's a lot of individual characters to get to know. Luckily, around three out of every four alien species throughout the endless expanse of space have a single concept that they and everyone else can define them by.
Some tropes emerge because a lot of authors came up with the same theoretical scientific advancement at once. Others come up as writers build on each other's work to constantly expand outward. Still, others are just time savers that become shorthand in an audience's mind. Unfortunately, some can subtract interesting detail and stumble into some troublesome ideas.
The Advanced Ancient Civilization Trope In Sci-fi, Explained
The planet of hats is a general term for an entire race, species, population, or culture of beings that theoretically have individual identities, but who all share a single central characteristic. Maybe every person on the planet is in tune with nature, or maybe they're all involved with organized crime, or maybe they all strictly follow the same code of ethics. Or indeed, maybe they all wear the same hat. This is extremely common in episodic fiction in which the main thrust of the narrative comes from exploring countless new planets. In real life, the one intelligent species we're aware of has almost infinite complexity between its individual members, so it stands to reason that any alien race would do the same. That would be very difficult to depict in a single novel or episode of TV. It's easier to just slap a label that says «hat planet» or whatever on each new culture and call it a day.
The go-to example of this trope that also happens to be the most prolific is unquestionably the proud
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