For most species, bared teeth are a threat, even on earth. So it shouldn’t be very surprising that most alien species tend to respond poorly to a human smiling at them. Humans who spend a lot of time around aliens do their best to train themselves out of the habit, adopting (as much as they are physically capable) the expression of enjoyment used by whatever species they socialise with most. But it’s really hard not to smile when you see another human… Harder still not to smile back when one smile at you. This leads to the common misapprehension that humans generally don’t get along with strangers.
When, by whatever series of events, a crew or team with a human member acquires an additional human or two, the atmosphere gets tense for a few cycles while the nonhumans wait for some kind of establishment of hierarchy to take place. Some humans humor the assumption and perform a mock battle in some public area - these are generally those who have encountered the scenario before and became tired of trying to explain.
Rap battles, trivia contests, simple sports matches and other activities that a human would recognise as popular recreational activities often feature in these dominance rituals. The participants find that the performance serves as a great ice breaker and so the practice is becoming increasingly common. It is likely, therefore, that the misinformation about human social strata will persist.
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