Depends on people having a realistic view on their relationships. Also is gathered retrospectively. This limits the validity of the research.
Correlational
This is because we cannot conclude that the infant's early attachment type causes differences in later relationships.
Contradictory evidence
Zimmermanetal (2000)- found that childhood attachment type did not predict adult attachment type. Life events had more of an impact.
Contradictory evidence
Koluchova (1976)- found that Czech twins were isolated from 18 months to 7 years later recovered after being looked after by a loving family.
Free will V Determinism
Research is deterministic. This is because of the negative implications of assuming that insecure attachment in infancy will cause an impact on later relationships.