Are you saying that outside influence have no impact on the choices that a person does?
How did we go to talking about from responsibility to influence? If your friends dare you to throw a rock through a window are you no longer responsible for your actions?
And what if you didn't get lower IQ, just high agression from the lead poisoning? Then what? Still fully responsible?
Of course, unless you have some form of mental defect that makes you compulsively act out on your aggression. I covered that already.