Actually, to be fair and in practice my personal observation has been that:

1) The more skilled cyclist (real or perceived) is the one who usually drives the tandem, or
2) If the skills are equal, the larger of the two riders will drive the tandem, or
3) If the skills and size are equal, the one who bought the tandem will decide who drives the tandem, unless there are extenuating circumstances:
a) The tandem is a second tandem in a household with two adults and two children where both parents drive their respective tandems with children stoking, or
b) The stoker is sight impaired and enlists the assistance of a pilot to drive their tandem and/or is invited to stoke, or
c) It's a track tandem, in which case the stoker with the strongest legs stokes.

There you have it and that's why you'll find that when you have a mixed adult team on a tandem (that would be a man and woman), 99 times out of a hundred you'll find the man driving.