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 fallen over the centuries, as policing has spread and the routine carrying of weapons has <WSD>diminished</WSD>. 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 smaller.