John Bowlby developed the attachment theory after observing children separated from their parents during World War 2 developing emotional problems stemming from the separation. Bowlby himself experienced separation from his mother at a young age when sent to boarding school. His main theory was that strong attachments are formed through proximity maintenance, a secure base, and separation distress. He believed children need continuous care from their primary attachment figure, usually the mother, for the first two years to develop properly.