Over the last four decades the transactional model has emerged as a central fixture of modern developmental science. Despite this, we are aware of no principled approach for determining (a) whether it is actually necessary to invoke transactional mechanisms to explain observed patterns of stability in a given domain of adaptive functioning and (b) the extent to which transactional processes, once identified in aggregate, are accounted for by measured domains with which an aspect of adaptive functioning is theoretically in transaction. Leveraging the fact that transactional mechanisms produce excess stability in an outcome domain above and beyond autoregressive processes, along with the basic logic of mediational analysis, we introduce two novel indexes for studying transactional processes strategically. We apply these metrics to data from the NICHD Study of Early Child Care and Youth Development cohort on mother- and teacher-reported externalizing problems and social competence along with teacher-reported and objective assessments of academic skills acquired in Grades 1, 3, and 5. During this developmental period we find that (a) transactional contributions to stability are strongest for teacher-reported outcomes, next strongest for mother-reported outcomes, and relatively weak for objective assessments of academic skills and (b) observed maternal sensitivity (but not child-reported friendship quality) accounts for a modest proportion of the total transactional effects operative in most of the domains of adaptive functioning we studied. Discussion focuses on extending the logic of our approach to additional waves of measurement.