LONDON (AP) _ If you think they're out to get you, you're not alone. Paranoia, once assumed to afflict only schizophrenics, may be a lot more common than previously thought. According to British psychologist Daniel Freeman, nearly one in four Londoners regularly have paranoid thoughts. Freeman is a paranoia expert at the Institute of Psychiatry at King's College and the author of a book on the subject. Experts say there is a wide spectrum of paranoia, from the dangerous delusions that drive schizophrenics to violence, to the irrational fears many people have daily. "We are now starting to discover that madness is human and that we need to look at normal people to understand it," said Dr. Jim van Os, a professor of psychiatry at Maastricht University in the Netherlands. Van Os was not connected to Freeman's studies. Paranoia is defined as the exaggerated or unfounded fear that others are trying to hurt you. That includes thoughts that other people are trying to upset or annoy you, for example, by staring, laughing, or making unfriendly gestures. Surveys of several thousands of people in Britain, the United States and elsewhere have found that rates of paranoia are slowly rising, although researchers' estimates of how many of us have paranoid thoughts varies widely, from 5 percent to 50 percent. A British survey of more than 8,500 adults found that 21 percent of people thought there had been times when others were acting against them. Another survey of about 1,000 adults in New York found that nearly 11 percent thought other people were following or spying on them. Dennis Combs, an assistant professor of psychology at the University of Texas at Tyler, has been studying paranoia for about a decade. When he first started conducting paranoia studies, mostly in college students, he found that about 5 percent of them had paranoid thoughts. In recent years, that has tripled to about 15 percent, he said. In a small experiment in London, Freeman concluded that a quarter of people riding the subway in the capital probably have regular thoughts that qualify as paranoia. In the study, 200 randomly selected people (those with a history of mental problems were excluded) took a virtual reality train ride. They recorded their reactions to computerized passengers programmed to be neutral. More than 40 percent of study participants had at least some paranoid thoughts. Some felt intimidated by the computer passengers, claiming they were aggressive, had made obscene gestures, or tried to start a fight. Freeman said that in big cities, many ambiguous events can lead to paranoid thoughts. Because we constantly make snap judgments based on limited information, like which street to take or whether or not strangers are dangerous, the decision-making process is prone to error. Van Os said Freeman's virtual reality experiment was solid and confirmed previous research. Experts say not everyone with paranoid thoughts needs professional help. It all depends on how disturbing the thoughts are and if they disrupt your life. "People walk around with odd thoughts all the time," said David Penn, a professor of psychology at the University of North Carolina. "The question is if that translates into real behavior." Van Os recalled a delusional patient who was convinced that the French singer Charles Aznavour was in love with her, and had been whispering to her before she went to sleep every night for more than two decades. "You could call it a psychotic experience, but she was very happy about it," van Os said. "There isn't always a need for care when there's an instance of psychosis." He hoped that being able to identify milder delusional symptoms in people could help doctors intervene earlier to prevent more serious cases. The post-Sept. 11 atmosphere and the war on terror also have increased levels of paranoia in the West, some experts said. "We are bombarded with information about our alert status and we're told to report suspicious-looking characters," Penn said. "That primes people to be more paranoid." Traumatic events can make people more vulnerable to having paranoid thoughts. Since the attacks, Penn said Americans have been conditioned to be more vigilant of anything out of the ordinary. While heightened awareness may be good thing, Penn said it can also lead to false accusations and an atmosphere where strangers are negatively viewed. That can result in more social isolation, hostility, and possibly even crime. And it can take a toll on physical health. More paranoia means more stress, a known risk factor for heart disease and strokes. Still, some experts said that a little bit of paranoia could be helpful. "In a world full of threat, it may be kind of beneficial for people to be on guard. It's good to be looking around and see who's following you and what's happening," Combs said. "Not everybody is trying to get you, but some people may be."