What is a key behavior of babies at birth that demonstrates attachment?

Establishing eye contact is a key behavior of babies at birth that demonstrates attachment because it plays a critical role in forming a bond between the infant and their primary caregivers. When a baby makes eye contact, it indicates an inherent drive to connect emotionally and socially with others. This behavior helps initiate interactions that lead to further engagement, such as cooing and smiling, which reinforce the attachment relationship. Eye contact helps infants learn about their environment and builds a sense of security and recognition as they begin to respond to caregivers' faces, voices, and emotions.

The other options, while significant in their own right, do not demonstrate attachment in the same primary way. Crying can be a means to communicate needs but does not establish a bond in itself. Smiling typically develops a bit later as a form of social engagement after attachment has been initially formed. Reaching for objects reflects a developmental milestone in motor skills and object permanence rather than a direct sign of attachment to caregivers. Thus, eye contact is essential for the early attachment process, making it the most relevant behavior in this context.