Why is that in almost every science fiction work I’ve encountered, alien species are depicted as being monocultural?

Think of any popular sci-fi movie, book or series. If it has aliens in it, then chances are any particular alien species has only one language, has only one worldview/religion, one set of cultural practices, and probably only one style of fashion. Most of the time, individual members of this species all look alike and talk alike as well.

Even worse, sometimes certain alien species are associated with a particular emotion or quality; i.e. Star Trek‘s Klingons are “angry” and “warlike” while Vulcans are “wise” and “logical”. We would never put up with this kind of stereotyping if it were applied to human characters, but for other species, we are willing to suspend our disbelief.

When you consider the breadth of diversity in our own species when it comes to language, culture, worldview and fashion, I think it’s unlikely that other sentient life would tend toward monoculture. Within a single human culture there is more diversity and difference than there is in the entire species of Klingons.

Come to think of it, these aliens usually from planets with only kind of terrain. Think of Star Wars and the “ice world” of Hoth, the “desert planet” of Tatooine, the “forest moon” of Endor, etc. From the (admittedly very little) that we have seen of the cosmos, most planets with only one type of terrain are incapable of supporting life (Venus, Mercury, Mars, etc).

I think this shows a lack of imagination on the part of speculative fiction writers.