Quick, gut reaction: which of these two patterns do you prefer? There is no right answer.
Pattern A
Pattern B
Step 2 of 2
A small bonus is being paid to two strangers you will never meet: one in your group, one in the other group. You decide how it is paid. Pick one.
What just happened
Arbitrary categorisation alone produces in-group favouritism (Tajfel et al., 1971). And favouring your own does not require harming the other: most group bias is in-group preference, not out-group hostility, so belonging need not have an enemy (Brewer, 1999).