To ensure survival, infants need to be physically close to the mother and form an enduring emotional bond. In the first book of his trilogy Attachment and Loss (1969), Bowlby postulates that attachment behavior serves an evolutionary function in protection of the infant. Attachment bonds explain separation distress and the child wanting to be physically close to the attachment figure, especially when sensing danger.
Mary Ainsworth, a member of Bowlby’s research team and the person who brought his theory to the United States, proposes that infants use the attachment figure as a secure base for exploration of the environment and as a safe haven to which to return for reassurance (see her book Infancy in Uganda: Infant Care and the Growth of Love published in 1967). Her development of an assessment tool called “The Strange Situation”, which examines the balance of attachment and exploratory behaviors under conditions of low and high stress, prompted her to study patterns of infant reunion behaviors. Building on her observations, Ainsworth developed a classification system of patterns of attachment in which, to briefly summarize, secure attachment (as opposed to less adaptive, avoidant, and resistant attachments) manifests when an infant who cries at the mother’s departure is readily comforted when she returns.
Sensitive caregiving and consistent responsiveness to the infant’s signals is fundamental for the infant’s development of secure attachment. In contrast, if caregivers are not responsive to children’s needs for reassurance and provide insensitive and inconsistent care, babies develop alternative strategies for interaction under conditions of stress, such as turning away from caregivers (avoidant attachment), simultaneously seeking and resisting contact (resistant attachment), or displaying a breakdown in strategy (disorganized attachment), which are broadly characterized as insecure attachments.
Research since that time has supported Ainsworth’s classification attachment styles. Studies that followed children over time (i.e. one study done by Erickson, Sroufe and Egeland in 1985) posit that secure attachment is associated with better developmental outcomes and has a positive impact on behaviors later in life.
Sensitive and consistent caregiving plays a crucial role in normative development of a person. Children develop an internal working model of relationships based on the character of early attachments. This model serves as a template for future relationships. The lack of a caregiver that could be trusted may be a reason why children with insecure attachments have a difficult time in interactions with other people. Read more to find out different attachment patterns, including attachment “disturbances” and “atypical” attachment behaviors (What are the Effects of Early Severe Deprivation on Attachment?)
Researchers of attachment have shaped policies and international laws. International conventions postulate that being raised in a permanent and optimal caregiving environment is in the best interest of the child in the way that it corresponds to the physical, emotional, and developmental needs of children.

Suggested online resource: Download Attachment Relationships: Quality of care for young children 

References

Ainsworth, M. D. S. (1967), Infancy in Uganda: Infant care and the growth of love, Baltimore: Johns Hopkins University Press.

Ainsworth, M. D. S., Blehar, M. C., Waters, E., & Wall, S. (1978). Patterns of attachment: A psychological study of the strange situation. Hillsdale, N.J.: Erlbaum.

Bowlby, J. (1982). Attachment and loss: Vol. 1. Attachment. New York: Basic Books. (Originally published 1969).

Bretherton, I. (1992). The origins of attachment theory: John Bowlby and Mary Ainsworth. Developmental Psychology, 28, 759-775.

Cassidy, J. (1999). The nature of the child’s ties. In J. Cassidy & P. R. Shaver (Eds.), Handbook of attachment: Theory, research, and clinical applications. New York: Guilford Press. Erickson, M.F., Sroufe, L.A., & Egeland, B. (1985). The relation between quality of attachment and behavior problems in preschool in a high-risk sample. In I. Bretherton & E. Waters (Eds.), Child Development Monographs, 50 (1-2), 147-166.