The attachment that is developed in the first two years of life between an infant and caregiver has been explained by John Bowlby's ethological theory of attachment. In 1969, Bowlby introduced his theory to world giving us insight to the important effects of a baby's feelings of security and capacity to develop trusting relationships (Berk, 2014). Bowlby described the development of attachment as progressing through four phases but there may be more than just these reasons for attachment. Genetics and the child's environment may also play vital roles in developing attachments.
Berk (2014) shows us that the first phase of Bowlby's theory is the pre-attachment phase which lasts from birth to around six weeks of age. This phase deals with the natural signals (e.g. smiling, crying, grasping, and gazing into the adult's eyes) that draw infants into close contact with the people that comfort them. Infants are able to distinguish their mother's face, voice and smell, but are not necessarily attached because the babies do not really mind being left in the care of an unfamiliar person. (p. 196).
The second phase explained by Berk (2014) is the attachment-in-the-making that last from about six weeks to six to eight months of age. Babies, in this phase, respond to a stranger differently than to a familiar caregiver. Trust is being built in this phase due to the baby recognizing that a response is caused by a signal given off. Babies in this stage still do not normally object much when left with an unfamiliar person. (p. 196).
Berk (2104) provides the third phase is the clear-cut attachment phase. This phase lasts from the age of six to eight months through 18 to 24 months. Attachment to the familiar caregiver is obvious in this stage. Babies may now display separation anxiety, which is when they become unhappy when left by their caregiver. Not all babies display separation anxiety or stranger anxiety depending on their situation and temperament.