Research has shown that loneliness is prevalent throughout society, including people in marriages, relationships, families, veterans, and those with successful careers. It has been a long explored theme in the literature of human beings since classical antiquity. Loneliness has also been described as social pain—a psychological mechanism meant to motivate an individual to seek social connections. Loneliness is often defined in terms of one's connectedness to others, or more specifically as "the unpleasant experience that occurs when a person's network of social relations is deficient in some important way".
People can experience loneliness for many reasons, and many life events may cause it, such as a lack of friendship relations during childhood and adolescence, or the physical absence of meaningful people around a person. At the same time, loneliness may be a symptom of another social or psychological problem, such as chronic depression.
Many people experience loneliness for the first time when they are left alone as infants. It is also a very common, though normally temporary, consequence of a breakup, divorce, or loss of any important long-term relationship. In these cases, it may stem both from the loss of a specific person and from the withdrawal from social circles caused by the event or the associated sadness.
The loss of a significant person in one's life will typically initiate a grief response; in this situation, one might feel lonely, even while in the company of others. Loneliness may also occur after the birth of a child (often expressed in postpartum depression), after marriage, or following any other socially disruptive event, such as moving from one's home town into an unfamiliar community, leading to homesickness. Loneliness can occur within unstable marriages or other close relationships of a similar nature, in which feelings present may include anger or resentment, or in which the feeling of love cannot be given or received. Loneliness may represent a dysfunction of communication, and can also result from places with low population densities in which there are comparatively few people to interact with. Loneliness can also be seen as a social phenomenon, capable of spreading like a disease. When one person in a group begins to feel lonely, this feeling can spread to others, increasing everybody's risk for feelings of loneliness. People can feel lonely even when they are surrounded by other people.
A twin study found evidence that genetics account for approximately half of the measurable differences in loneliness among adults, which was similar to the heritability estimates found previously in children. These genes operate in a similar manner in males and females. The study found no common environmental contributions to adult loneliness.