Personally I look at the lead poisoning argument like the alcohol argument.

People who drink alcohol (unless they only drink a tiny cup of wine a day, which helps, but then I doubt they drank so little) becomes more stupid because they're drunk and their minds aren't clear and lives shorter lives.

You don't see me arguing that alcohol caused the fall of empires. I mean the nomads all drink a lot, and so did Uesugi Kenshin and many, many other conquerors, Alexander for one. They still conquered lots of territory.

Of course since you're in a University. Maybe you can go dig up so books or papers showing the higher-than-normal content of lead in the bones dug up from Roman times.

If I was to say what reason contributed to the fall of the Roman Empire the most, I would say civil strife "tribes fighting tribes instead of banding together in one arm of force. As long as we fight, in this room, seeking petty victories, and acting on personal revenges Rome will remain SMALL." - Julius Caesar {the movie}