Attachment theory is a theory that how we interact with a romantic partner is related to how we interacted with our mother as an infant.
This was initially tested in a test known as the Strange Situation. A child aged 12-18 months and their mother were put into a room and the child was given time to get comfortable. The mother then left the room and the child’s behaviour was observed.
The original theory separated people into three categories: securely attached, anxiously attached, and avoidantly attached.