A longstanding question for attachment theory and research is whether genetically based characteristics of the child influence the development of attachment security and its stability over time. This study attempted to replicate and extend recent findings indicating that the developmental stability of attachment security is moderated by oxytocin receptor (OXTR) genetic variants. Using longitudinal data from over 550 individuals, there was no evidence that OXTR rs53576 moderated the association between attachment security during early childhood and overall coherence of mind ("security") during the Adult Attachment Interview at age 18 years. Additional analyses involving a second commonly investigated OXTR variant (rs2254298) and indices of individuals' dismissing and preoccupied attachment states of mind also failed to provide robust evidence for oxytonergic moderation of the stability in attachment security across development. The discussion focuses on research strategies for investigating genetic contributions to attachment security across the life span.