The thing about alien psychology is in order for it to seem alien, a human with those traits would have to appear crazy to other humans. It may be full on bats in the attic crazy or slow burn crazy, but if it’s not then it’s just more of the Roman playwright Terence’s dictum: I am human, and I think nothing human is alien to me. (And maybe your SF premise is that aliens, no matter how weird, are really just humans inside, which is fine, but not what we’re going for here.) For game purposes every member of the alien race has the same trait, unless they are mad by the standards of their race. The trait won’t be their only feature, and may not be the dominant one (particularly if the trait would make it too hard to play the alien character or for other players to enjoy playing with the alien character if it was front and center), but it’s part of that race’s mental make-up, not an individual quirk. Also note that from the point of view of the alien species, the trait may not be what humans perceive it as at all.
So here’s some random alien psychology to play with. Players should roll or choose. More alien aliens should go directly for the Mental Illness Equivalent table, but a possibly easier alien would start with the milder Alien Psychological Traits, remembering though that as an alien trait it should probably be more extreme or oddly directed than it would be in a human.
Alien Psychological Traits (d30)
- Flirtatious
- Addict
- Primps constantly
- Absent-minded
- Fumble-fingered
- Collector
- Romantic
- Know-it-all
- Perfectionist
- Squeamish
- Sexy
- Bloodthirsty
- An attitude
- Dislike something
- High pain threshold
- Low pain threshold
- Insomniac
- Fanatic
- Picky eater
- Glutton
- Pacifist
- Gung ho
- Pet-lover
- Practical joker
- Neatnik
- Slob
- Pack-rat
- Figures of speech
- roll on Mental Illness Equivalent
- Quite eccentric; reroll 1d3 times
Mental Illness Equivalent (d20)
As explained above, this is meant to indicate not that the alien is mentally ill, but that it regards as standard behavior something that in a human would be a mental illness.
- Catatonia
- Multiple personalities
- Compulsive liar
- Paranoia
- Hallucinations
- Megalomania
- Severely phobic (roll on Phobias)
- Hypochondria
- Manic-depressive
- Quixotism
- Panzaism (seeing the most extraordinary things as normal and logical)
- Depression
- Hysterical injury
- Obsession
- Delusional identity
- Kleptomania
- Pyromania
- Hebephrenia
- Berserker rage
- reroll 1d3+1 times.
Phobias (d100)
- animals
- being alone
- beards
- bridges
- buried alive
- blood
- cancer
- cats
- children
- cold
- colors
- comets
- computers
- corpses
- crowds
- darkness
- depth
- disease
- dreams
- drink
- drugs
- electricity
- empty rooms
- enclosed spaces
- everything
- eyes
- failure
- females
- feces
- fire
- flowers
- flying
- fog
- foreigners
- germs
- glass
- god
- ghosts
- graves
- hair
- heights
- horses (or culturally appropriate domestic animal)
- ice
- idleness
- imperfection
- infinity
- insanity
- insects
- justice
- lakes
- males
- metal
- mirrors
- missiles
- monsters
- motion
- narrowness
- new things
- night
- nudity
- open spaces
- pain
- parasites
- philosophy
- physical love
- poison
- psychic powers
- punishment
- rain
- reptiles
- responsibility
- ridicule
- rivers
- see
- sex
- shadows
- sinning
- slime
- sleep
- small things
- snakes
- space
- spiders
- standing
- storms
- string
- symmetry
- trees
- travel
- touch
- tyrants
- vehicles
- voids
- water
- wind
- words
- work
- wounds
- writing
- reroll 1d3+1 times
Space Crawl 
I like the idea, but the specifics seem odd to me. These lists of alien traits are all familiar tics or disorders of humans. I’d expect alien traits to be less familiar, things that perhaps a human could do but that you’ve never heard a specific example of. Like being disgusted by radial symmetry, or obsessed with what every room smells like.
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Perhaps I made them too prosaic, but I’m a bit worried about playability. I’m afraid it might be too hard on both the player and GM to be constantly worrying about the presence of cues in the environment that wouldn’t normally be described as part of game-play, if that makes sense. I’ll see about adding a section of more exotic traits/concerns for people that want to try it.
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