Most people feel lonely sometimes, but it usually lasts only between a few minutes and a few hours. This kind of loneliness is not serious. In fact, it is quite normal. For some people, though, loneliness can last years. Psychologists are studying this complex phenomenon in an attempt to better understand long- term loneliness. These researchers have already identified three different types of loneliness. The first kind of loneliness is temporary. This is the most common type. It usually disappears quickly and does not require any special attention. The second kind, situational loneliness, is a natural result of a particular situation, for example, a divorce, the death of a loved one, or moving to a new place. Although this kind of loneliness can cause physical problems, such as headaches and sleeplessness, it usually does not last for more than a year. Situational loneliness is easy to understand and to predict. The third kind is the most severe. Unlike the second type, chronic loneliness usually lasts more than two years and has no specific cause. People who experience habitual loneliness have problems socializing and becoming close to others. Unfortunately, many chronically lonely people think there is little or nothing they can do to improve their condition. Psychologists agree that one important factor in loneliness is a person’s social contacts, i.e. friends, family members, coworkers, etc. We depend on various people for different reasons. However, psychologists have found that the number of social contacts we have is not the only reason for loneliness. It is more important how many social contacts we think or expect we should have. In other words, though lonely people may have many social contacts, they sometimes feel they should have more. Most researchers agree that the loneliest people are between the ages of 18 and 25. They found that more than 50 percent of the college freshmen were situation ally lonely at the beginning of the semester, but had adjusted after a few months. Thirteen percent were still lonely after seven months due to shyness and fear. They felt very uncomfortable meeting new people, even though they understood that their fear was not rational. The situation ally lonely freshmen overcame their loneliness by making new friends, but the chronically lonely people remained unhappy because they were afraid to do so. Psychologists are trying to find ways to help habitually lonely people for two reasons. First of all, they are unhappy and unable to socialize. Secondly, researchers have found a connection between chronic loneliness and serious illnesses such as heart disease. While temporary and situational loneliness can be a normal, healthy part of man, chronic loneliness can be a very sad, and sometimes dangerous condition. What is the main idea of this passage?