He was pointing out that in the civilized West, you survived or not based on your immunity to diseases common in crowded places, and your intelligence had littl

He was pointing out that in the civilized West, you survived or not based on your immunity to diseases common in crowded places, and your intelligence had little to do with it, where if you lived in a hunter-gatherer society, a whole lot of knowledge was required to keep you alive. So if intelligence is selected for it would play a much larger role in New Guinea. He was also describing his personal experience with New Guineans, where he spent a lot of time. But I didn't take that part as a rigorous argument, more like a statement that you could make Murray's argument in reverse (that living in a "being stupid gets you killed" place might produce more brains than living in a "having a weak immune system gets you killed" place), and he didn't attempt to build on that point.