Attachment Theory, conceived by John Bowlby and furthered by Mary Ainsworth, explores the centrality of attachment bonds in human development and emotional life. Particular attention is paid to the degree of security infants and children feel in relationship to their caregivers and the consequences when a felt sense of security is lacking (as in cases of even mild emotional neglect). Mary Ainsworth's experimental "strange situation" examined the responses of children to different caregiver behaviors and identified a set of attachment patterns (e.g., anxious-resistant, avoidant...) which followed the children into their adult relationships. Radical when originated, attachment theory has since been thoroughly integrated into much clinical practice, especially that of social workers.
For futher review: Attachment theory at Wikipedia, at Simply Psychology, attachment theory books at Amazon.