I used the least-squares method to make the trend lines in the second and third graphic, which was an interesting intellectual exercise.


Note that about 95% of homicide incidents are single-victim. I put the trend line on this one.




You can see how, after the peak in about 1990, the number of single-victim homicides dropped sharply while the levels of multi-victim homicides remained generally steady. This is almost certainly due to the removal of lead from gasoline, the widespread availability of hormonal and barrier birth control, and the legalization of abortion, all of which occurred about 20 years before the drop.