Loneliness is an unpleasant emotional response to perceived isolation. Loneliness is also described as social pain—a psychological mechanism which motivates individuals to seek social connections. It is often associated with an unwanted lack of connection and intimacy. Loneliness overlaps and yet is distinct from solitude. Solitude is simply the state of being apart from others, not everyone who experiences solitude feels lonely. As a subjective emotion, loneliness can be felt even when surrounded by other people; one who feels lonely, is lonely. The causes of loneliness are varied. They include social, mental, emotional, and environmental factors.
Research has shown that loneliness is found throughout society, including among people in marriages along with other strong relationships, and those with successful careers. Most people experience loneliness at some points in their lives, and some feel it very often. As a short term emotion, loneliness can be beneficial; it encourages the strengthening of relationships. Chronic loneliness on the other hand is widely considered harmful, with numerous reviews and meta-studies concluding it is a significant risk factor for poor mental and physical health outcomes.
Loneliness has long been a theme in literature, going back to the Epic of Gilgamesh. Yet academic study of loneliness was sparse until the late twentieth century. In the 21st century, loneliness has been increasingly recognised as a social problem, with both NGOs and governmental actors seeking to tackle it.