Final answer:
The assessment of the foster child's behavior when the caregiver leaves and returns is guided by Attachment Theory, suggesting a focus on the child-caregiver bond and possible signs of insecurity or other attachment-related issues.
Step-by-step explanation:
The social worker is using Attachment Theory to assess the 2-year-old foster child's behavior. This theory, developed by John Bowlby and extended by Mary Ainsworth, examines the importance of the bond formed between a child and their primary caregiver. Ainsworth's research, particularly the Strange Situation procedure, identified different patterns of attachment, such as secure, avoidant, resistant (or ambivalent), and disorganized. The described behavior in the question suggests difficulties with the caregiver-child attachment, but does not align precisely with the definition of avoidant attachment, where a child is usually not distressed by the caregiver's absence and is indifferent upon return. Instead, it could indicate forms of insecure attachment, where the child shows distress but cannot be soothed upon reunion, or potentially another issue related to social and emotional development.