Do politicians’ words, the president’s especially, matter?

Since he has been in office, President Trump has relentlessly demonized his political opponents as evil and belittled them as stupid. He has called undocumented immigrants animals. His rhetoric has been a powerful contributor to our climate of hate, which is amplified by the right-wing media and virulent online culture.

Of course, it’s difficult to prove that incendiary speech is a direct cause of violent acts. But humans are social creatures — including and perhaps especially the unhinged and misfits among us — who are easily influenced by the rage that is everywhere these days. Could that explain why just in the past two weeks we have seen the horrifying slaughter of 11 Jews in a synagogue in Pittsburgh, with the man arrested described as a rabid anti-Semite, as well as what the authorities say was the attempted bombing of prominent Trump critics by an ardent Trump supporter?

You don’t need to be a psychiatrist to understand that the kind of hate and fear-mongering that is the stock-in-trade of Mr. Trump and his enablers can goad deranged people to action. But psychology and neuroscience can give us some important insights into the power of powerful people’s words.

We know that repeated exposure to hate speech can increase prejudice, as a series of Polish studies confirmed last year. It can also desensitize individuals to verbal aggression, in part because it normalizes what is usually socially condemned behavior.