From Mind Hacks:
A curious term from anthropology describing the tendency for someone to come up with a counter-example from some usually obscure and remote tribe when anyone makes a general claim about human culture.
Bongo-bongoism: the venerable but ultimately sterile anthropological practice of countering every generalization with an exception located somewhere at some time.
Apparently, it was first used by anthropologist Mary Douglas in her book Natural Symbols.
Link to the culture evolves! blog (where I found the definition).