Yeah. If all you want is mutations which decrease IQ, well, studies of the profoundly retarded have turned up dozens, perhaps hundreds or thousands by now, confirming what everyone guessed in the first place (that the retarded had genetic problems which broke all sorts of biological systems, which intelligence is downstream of). But these findings have yet to lead to any breakthroughs in understanding the biology of intelligence that I've heard of.

(This reminds me a little of the paper "Can a Biologist Fix a Radio? — or, What I Learned while Studying Apoptosis".)