The Dorians are one of the lost tribes of humanity who fled from Earth following the nuclear conflict of 1987.
Many years after the Dorians left Earth and founded their own society, they experienced a devastating war of their own, one which unleashed massive mutations in their genetic pool. However, these mutations took an unusual form, causing every male member of their species - and, it's implied, females as well - to look exactly alike. This 'generic' Dorian form appeared to be a normal Human, however it caused the Dorians such distress and embarrassment (as one Dorian man sadly pointed out, "Imagine a world...where every face you see is the mirror image of your own") that all Dorians wear masks in public to hide their true nature. ("The Dorian Secret")