Bowlby proposed that a child forms a mental representation of their relationship with their primary attachment figure called an internalworking model
The internal working model serves as a model for what relationships are like
A child whose first experience is of a loving relationship with a reliable caregiver will tend to form an expectation that all relationships are as loving and reliable, and they will bring these qualities to future relationships
A child whose first relationship involves poor treatment will tend to form further poor relationships in which they expect such treatment from others and/or treat others in that way
Most importantly the internal working model affects the child's later ability to be a parent themselves
People tend to base their parenting behaviour on their own experiences of being parented
This explains why children from functional families tend to have similar families themselves