President Trump, conservatives and the National Rifle Association have once again tried to steer the national conversation after a mass shooting to the mental health of the people who pull the triggers, rather than the weapons they use. But how can the mental health system stop gun violence when mental illness is so rarely the cause of it?  Guest: Dr. Amy Barnhorst, the vice chairwoman of community psychiatry at the University of California, Davis. For more information on today’s episode, visit nytimes.com/thedaily.