BEVERLY, Mass.

WHAT DO THE ASSASSINATION of John F. Kennedy and the terrorist attacks of Sept. 11, 2001, have in common? For many Americans, the answer is that they are both the result of a conspiracy.

That is not surprising. Since the president’s murder, which took place on Nov. 22, 1963, 45 years ago tomorrow, Americans have increasingly turned to conspiracy theories as the explanations for a wide range of events. Today, the words “conspiracy theory” are said so frequently that we usually don’t think too much about what they really mean. Yet, this eagerness to accept conspiracy theories tells us something important about our society. At their core, such ideas are expressions of deep cynicism. This pessimistic and jaded perspective is prevalent today.

Throughout most of American history, exactly the opposite was true. Americans were anything but a cynical people. Instead, hope and optimism fueled two centuries of nation-building. Even in the dark climate of the Cold War, Americans pushed aside much of the doom and gloom.

After President Kennedy’s death in 1963, things changed. The assassination came not long after the disastrous Bay of Pigs incident, the Cuban missile crisis and other international scares. The sudden loss of America’s youthful leader prompted new trepidation. These anxieties created a foundation upon which conspiracy theories grew in the American imagination.

Soon after Kennedy’s death, the Warren Commission concluded that assassin Lee Harvey Oswald had acted alone. There was no evidence of a conspiracy. For many Americans, this meant the case was closed. But other people suspected a cover-up.

At first, the skeptics were in the minority. Before long, however, suspicion started growing. In the years that followed, more and more Americans came to disbelieve the Warren Commission and wondered what the commission was hiding. Before long, this way of thinking spread to other topics.

Other discords of the 1960s fueled such thoughts. Soon after the president’s death, widespread civil unrest erupted. The president’s brother, Sen. Robert F. Kennedy, was assassinated in 1968, as was Martin Luther King Jr. Riots and protests about a host of political and social issues intruded into American life. A widespread sense of unease and alienation emerged. The nation appeared on the brink of chaos.

The era of suspicion reached its height with the Watergate scandal in the 1970s. This capped a period in which many citizens became disillusioned with government, business and other pillars of society. It was a profound change in the way Americans thought, signaling that trust had largely evaporated.

After the 1970s, America began to look at itself as the potential enemy. In earlier years, Americans sometimes feared plots and conspiracies. But the source of anxiety was outside the United States. By the mid-1970s, however, Americans increasingly feared that danger emanated from within American society itself. The thought that conspiracies were to be found under every stone percolated for the next quarter century.

During these years, conspiracy theory became a pop-culture staple. Hollywood aggressively pushed the idea. Conspiracies make good stories, and the entertainment industry frequently turned to it. The idea can be found in movies as varied as Chinatown, Silkwood, Capricorn One, Rambo and The X-Files, and even in family entertainments such as E.T. and Men in Black.

The cumulative effect was corrosive. Audiences got the message that elaborate plots surround them. Americans came to think of conspiracies as a normal part of everyday life.

Public reactions to the terrorist attacks of 9/11 show how often Americans turn to conspiracies to explain current events. The attacks briefly refocused attentions at external enemies. But soon, some people concluded the tragic events resulted from an internal conspiracy. As shocking as it may seem, surveys show that many Americans believe their own government had a role in the attacks. These ideas seem disturbing and absurd to many people, but they have become increasingly mainstream in recent years.

Sometimes there really are evil plots, of course. But this rare occurrence is seldom what we mean when we talk about conspiracy theories. Instead, the words more often signify people who feel alienated from their own society. The widespread use of conspiracy theory to explain the world’s ills, even when we don’t completely mean it, is a striking symptom of self-doubt.

Conspiracy theories do make entertaining movies. But they should not form the basis of our world view. The bleak outlook they represent is unlikely to help solve our problems or add to the quality of our lives. Better that we should leave that way of looking at the world, and the cynicism it entails, on the screen.

Gordon Arnold is a professor of liberal arts at the Montserrat College of Art in Beverly, Mass., and is the author of the new book Conspiracy Theory in Film, Television, and Politics (Praeger Publishers, 2008).