Attachment as Selection: A Sensitive Period Framework for Exclusive Bond Formation

What makes one individual irreplaceable to another? Across vertebrate species that form exclusive social bonds, from precocial birds to monogamous mammals, including humans, bond formation begins with a common problem of how one individual becomes selected from among many. In animal research, this process is directly observable as a time-limited shift from broad social openness to exclusive preference within a bounded window of heightened plasticity. Early attachment theory described a comparable transition, but the field’s focus shifted toward the functions of established bonds, including safe haven use and felt security, rather than the process that produces specificity. As a result, the mechanisms of attachment formation remain poorly specified in humans. This paper returns selection to the center of attachment theory, advancing a four-phase biobehavioral framework grounded in sensitive period logic that separates bond formation from the regulatory functions that follow. Central to this framework is a gated mechanism in which heightened arousal disproportionately weights experience during a bounded window, driving competitive narrowing toward one individual. This reconceptualization resolves inconsistencies in the human literature and challenges the view that attachment is confined to protection-based relationships. Separating formation from quality reveals fundamental questions about whether selection-phase experience shapes internal representations, which relationships genuinely qualify as attachments, and whether parental bonding belongs within the attachment system. The framework generates testable predictions regarding gate opening, competitive dynamics, and closure, and provides a basis for developing formation-specific measures capable of identifying when attachment occurs and when it fails.