Klein worked with young children using toys and games. This led her to theorize how humans develop a sense of who they are. She believed that we enter the world without being able to relate to anyone else, and that through a predictable series of inner change we come to realize that we need the love of others.
Some children negotiate these developmental stages with difficulty and she was able to determine their inner state using play as a means of determining specific difficulties. Her thought was controversial in its time but is now widely accepted.
