Atheists, being a moderate proportion of the USA population (about 8-16%) are disproportionately less numerous in the prison population (0.21%)
Japan (the most atheistic nation in the G-8) has the lowest murder rate while the United States (the most Christian nation in the G-8) has the highest. Japan used to have much stronger religious faith, and a state religion, and guess what: Japan was remarkably aggressive and militaristic when "Shinto" was at its peak, and during WW2, when its Emperor was regarded as a God.
Louisiana, with America's highest church attendance rate, has twice the national average murder rate.
If atheism causes violence, why are right-wing fundamentalists unable to find a shred of statistical evidence to back that claim up?
These are all very good points. Some discrepancy between the number of atheists in the population and in prison might not mean much, but the discrepancy we have is quite large. If there were something about atheism that were incompatible with morality, wouldn't we see the discrepancy going in the other direction? Why are less religious nations (not just Japan, but also countries like Britain) also nations that have less violent crime? Why are the more "liberal, secular" portions of America those with less divorce and murder?