IF American television is a tinted window on the national psyche, someone had better call a doctor quick. The symptoms are troubling, to say the least: violent fantasies, inability to distinguish fact from fiction, an insatiable curiosity about pathological behaviour. With the daily body count on networks and cable channels running well into the hundreds, it doesn't take a team of PhDs to figure out that increasingly graphic violence has proliferated on the small screen: throw-away killings in action comedies, local newscasters rattling off the deaths du jour, grisly voyeurism of tabloid talk shows. And here's a new wrinkle: since violence sells advertising so well, now advertising is selling violence. One of those relentless late-night commercials, for example, uses the cloak of ''nature'' to peddle six hours of flesh-ripping, animal-kingdom carnage. There's also the plug for a series of Time-Life books with titles like Serial Killers (Volumes I & II), Mass Murderers, and Crimes of Passion. This expanding pool of blood understandably gives rise to a censorial urge. Convinced that seeing violence inspires it, an alarmed minority seeks to shield itself, and especially its children, by enforcing ''decency'' codes that more effectively curb profanity and the depiction of sex. But prohibition is not the answer. Indeed, it is not even the question. What we need to know is not how to suppress violent images but why people like to watch. Understanding why British television has less butchery than American is less important than figuring out why 2,000 people were murdered in New York City last year as opposed to 200 in London. We might also ask why the self-appointed arbiters of taste inthe US are prudes about sex, compared to Europe, and libertines when it comes to maiming, mutilation and murder. Is there a relationship between violence on the screen and on the street? Of course there is. Does the former cause the latter? Of course not. Most movies and television, like most politicians, are reactive - they reflect genuine realities and fears, even as they distort them for their purposes. They are followers, not trend-setters. The crescendo of blood-letting on the tube, already worrisome, is all the more disturbing to the extent it has now converged with another TV trend: the blending of truth and lies, The most successful new genre in television is the one that obliterates the once-solid (if permeable) barrier between fact and fiction, especially when the subject is violent criminal behaviour. A whole crop of series - America's Most Wanted, Top Cops, Street Stories, Prime Suspect, - play fast and loose with reality. Interviews with criminals and victims are mixed in with dramatic re-creations of the crimes in which they were involved; real police actions are planned and co-ordinated for maximum entertainment value and video access; facts are omitted, added or rearranged to make a better story. Even so-called ''news magazines'' such as Inside Edition, and A Current Affair resort to some of these techniques, which is why most of them fall within entertainment, rather than news. Court-based programmes offer a similar hodgepodge of the whole truth, half-truth, and everything but the truth. There is nothing new, of course, about television's obsession with ''true crime'' stories, but nor is their any doubt that the obsession has become magnificent, that it has reached unprecedented heights. It used to take years - long enough for someone to write a book - before a sordid murder found its way on to the tube. The average turnaround now is measured in months. If there is a danger of life imitating art, as some think, it is of violence motivated not by images but by fame. Criminals and victims are routinely interviewed on late-night news immediately following the broadcast of movies in which they were portrayed. John Lennon's murderer, the subject of a new biography, is a much sought-after guest. Celebrity psychopaths? Let's get real.