For example, the rise of video games which may be a stronger explanation. One way to test this is if their are dips in violence around the release of GTA or other ultra popular games. With the method of action simply being fewer human interactions.
That said, the indirect evidence is even low levels of environmental lead contamination are significant over a population. Which suggests other things may cause similar issues without people noticing. EX: mad as a hatter
The data is just not as clear as your suggesting. After controlling for various things there is a percentage of the variation that links back to lead levels, but that does not mean it's the only or even the strongest impact in the raw data. Further, lead usage is linked to other things and as such is not completely independent.
PS: Remember, people move so lifetime exposure get's really hard to track accurately without regular blood tests.
