My Blemished Past (washingtonpost.com):
It is too late for me -- my psyche is already pitted -- but let me congratulate the scientific team at Birmingham University in England, whose researches have led them to the thesis that acne may be a kind of evolutionary contraceptive. In mimicking the symptoms of illness, the scientists speculate, pimples might form a sexual no-go zone that prevents humans from reproducing before they are physically and psychologically ready for it.
Zits as a Darwinian cold shower: It is a theory that many teenagers will find immediately convincing.
I don't know the origin of this, but it's an interesting evolutionary hypothesis, if nothing else.