Mary Ainsworth conducted the Strange Situation Experiment to study infant attachment. Babies were observed interacting with their mother and a stranger in an unfamiliar room. Researchers discovered that infants could be securely or insecurely attached based on their reactions. Secure infants cried when their mother left but were happy when she returned, while insecure infants showed avoidant or anxious behaviors. Insecure attachment has been linked to later emotional and behavioral problems in children.