The trial of John W. Hinckley Jr. is giving urgency once again to that peculiarly American question: What makes a human being violent?
One can hardly imagine how this long-standing stumper could have been answered before the motion picture was invented and Jimmy Cagney triggered his first Warner Brothers machine gun. For almost every hypothesis seems to begin with a provoking image on a screen. And so a parade of psychiatrists has testified to the probable baleful effect on Hinckley of the film ''Taxi Driver.''
Meanwhile, a new study from the Department of Health and Human Services affirms the disturbing behavioral consequences of viewing frantically flickering shadows for four hours or so a day. ''Violence on television,'' the report concludes, ''does lead to aggressive behavior by children and teen-agers.''
If Popeye had never eaten his spinach and swung wildly at that other big nasty sailor in the interests of Olive Oyl, would everything still be as tranquil as Eden in the sandbox?
If Edward G. Robinson (or whoever) had not first yelled, ''Come and get me, coppers!'' would it have been quite unnecessary to invent the ghastly term ''juvenile delinquent''?
If there were no war movies, would there be no more wars?
The temptation of simple cause-and-effect explanations is, of course, that they promise equally simple remedies.
Nobody but the studio cost accountants would wish to defend the excesses of horror films. Decapitation has become routine. Cannibalism is the least of it. Hollywood's special effects departments appear to be working around the clock to escalate the bestial so that the audience's scream does not suddenly turn in mid-gasp into a yawn.
Still, it is too easy to correlate violence in daily life with the number of maimed corpses shown per hour on a screen, and leave the matter there.
Children who witness racist incidents in their schools are experiencing a violence more excruciating than any ''Night Stalker'' script could devise.
A broken home, where the divorce turns ugly, can prove more devastating to the children caught in the middle than all the second-hand mayhem in a dozen sequels to ''Conan the Barbarian.''
We are just beginning to understand what the idea of war - particularly nuclear war - means to children. The New York Times recently sampled the feelings of a number of Manhattan schoolchildren on this Cubject:
''When I walk in the street, I look at things and imagine what would happen if they just disappeared.''
''If there was a nuclear war in my lifetime . . . I just wouldn't want to have a family.''
''I just want to grow up.''
The notion of a holocaust, it seems, produces unequaled fear and anger in children - the two emotions any parent would give anything for them not to feel too obten or too deeply.
The prospect of nuclear war finally threatens children with ''an intangible loss of faith in the general structure of human existence,'' in the words of the psychologist Robert Jay Lifton.
This is violence indeed.
By all means, we should remain concerned about the images children feed upon - the visions they so uncritically ingest for periods of time that generally exceed what they spend in the classroom. But we ought to understand that film-fantasized violence is as much a symptom as it is a cause.
It will do little good to fill our screens with sweetness and light until we can bring a measure of peace to history and our daily lives. Our children - silently as well as verbally - tell us this every day.