Phase 1 (Birth to 8–12 weeks) Indiscriminate Responsiveness to Humans

During this phase, infants orient to persons in their environment, visually tracking them, grasping and reaching for them, and smiling and babbling. The infant often ceases to cry upon seeing a face or hearing a voice. These behaviors sustain the attentions of others and thus their proximity to the infant, which is the infant's goal.

Phase 2 (3 to 6 months) Focusing on Familiar People

The infant's behaviors toward others remain virtually the same except that they are more marked in relation to the mother or perhaps the father. Social responses begin to become more selective, however, with the social smile reserved for familiar people. Strangers receive a long, intent stare. Cooing, babbling, and gurgling occur with familiar people. A principal attachment figure begins to emerge, usually the mother.

Phase 3 (6 months to 3 years) Active Proximity Seeking

Infants show greater discrimination in their interactions with people. They become deeply concerned for the attachment figure's presence and cry when that person starts to leave. Infants will monitor the attachment figure's movements, calling out to them or using whatever means of locomotion they have to maintain proximity to them. The attachment figure serves as a base from which to explore and is followed when departing and greeted warmly upon return. Certain other people may become subsidiary attachment figures; however, strangers are now treated with caution and will soon evoke alarm and withdrawal.

During phase 3, two very predictable fears emerge. Separation anxiety occurs as the relationship between the infant and the attachment figure becomes more intense and exclusive. The infant cries, sometimes quite vociferously, upon the departure of the attachment figure and exhibits intense joy upon their reunion.

Stranger anxiety is another characteristic fear of phase 3. Occurring around 7 to 8 months, the infant's stranger anxiety is characterized by lengthy stares and subsequent crying at the sight of an unfamiliar person. Alarmed, the infant will cling tightly to the attachment figure and resist letting go.

Phase 4 (3 years to the end of childhood) Partnership Behavior

Prior to this phase, the child is unable to consider the attachment figure's intentions. For instance, the suggestion that "I'll be right back" is meaningless to the child, who will insist on going along anyway. By age 3, the child has developed a greater understanding of parental intent and plans and can envision the parent's behavior while separated. The child is now more willing and able to let go and can be more flexible.

Source: The Young Child: Development from Prebirth through Age Eight by J. Black and M. Puckett, © 2000. Reprinted by permission of Prentice-Hall, Inc., Upper Saddle River, NJ.