Diamond says that there were four factors that protected New Guineans in this way.
- Most importantly, there was malaria and some other diseases. These diseases made it very difficult for Europeans to live in the lowlands or to pass through them on the way to the more hospitable highlands.
- New Guineans were not decimated by European diseases. This was partly because they had been somewhat exposed to them through Indonesia and partly because New Guinea was not widely settled until a time when public health practices reduced the spread of disease.
- Europeans didn't reach the highlands (because of the diseases) where there lots of natives until the 1930s. By that time, the idea of killing native people to get their land was unacceptable.
- European crops and animals don't do well in New Guinea's climate. This meant that it was hard for Europeans to establish any sort of major settlements.