Comment: Chemicals do have an effect

but other things contribute as well. For example, all female reproductive eggs (~400-700) are completed when a girl is still a fetus. Late marriages make eggs less wholesome, on average, due to environmental exposure to cosmic radiation and natural decay.

Regarding the fact that most teenage mass killers were on psycho drugs does not prove that drugs are the cause. If a person is mentally unstable, by the age of 12, school teachers and administrators would force the parents to have the child examined by a doctor. Doctor prescribes different drugs. Psycho drugs simply do not always work for all patients. Same with any drugs.