I'm trying to remember where I first read about the Trobrianders, and failing. The Ethical Slut, maybe? Malinowski seems to be responsible for most of the scholarship on them, which I agree is a little suspicious.

IIRC, the yams in question have an unusually high level of progesterone, which does (or at least should) have observable effects on fertility rates. But I think there was more to it than that--if I remember my original source, I'll definitely pass it along.