1. Robertson & Bowlwby observed young children aged 1-4 years old placed in a residential nursery and identified three progressive reactions - protest, despair and detachment - to developing the PDD model of responses to separation.
2. The research involved naturalistic observation of children's behavioral and emotional reactions to assess the effects of separation, with observations filmed and sampled over time.
3. While the research identified short-term distress from separation, the PDD model has been criticized for being too simplistic, as a child's attachment type affects their response.