Sentence:  Whether modern, industrial man is less or more warlike than his hunter-gatherer ancestors is impossible to determine....One thing that is true, though, is that murder rates have <WSD>fallen</WSD> over the centuries, as policing has spread and the routine carrying of weapons has diminished. Modern society may not have done anything about war. But peace is a lot more peaceful. 
Meaning of the word marked with <WSD> tags:  To become lower (in quantity, pitch, etc.).