> That's why school shootings pretty much never happened before the 1970s

School shootings didn't happen for multiple reasons that are not SSRIs:

  - Semi-automatic and automatic weapons weren't available to the public
  - There were no video games and few movies glorifying a lone gunman "getting revenge" on a society that spurned them (there movies about gangsters, or war movies)
  - There was no anti-American/facist "militia/tactical" cultural meme
  - There was not yet any widely known stories of suicide-by-cop and fame via mass-murder
  - The American cultural ethos had not yet turned cynical; once Vietnam and Nixon's betrayal happened, it was all downhill
  - We stopped locking up crazy people in insane asylums
  - Social isolation and urbanism increased population density and animosity

> - Semi-automatic and automatic weapons weren't available to the public

They were fully available to the public. Automatic weapons were tax-gated, but still were (and are!) available, after 1934. Semi-automatic weapons have been freely available to any citizen pretty much since they were invented (circa 1893).