Data from early mother-infant face-to-face interactions are used to demonstrate that a statistical association, widely accepted as the single indicator of sequential dependency between the behavior of two interactants, in fact reflects a combination of responsiveness and dependency comprising two independent aspects of the contingency environment. The derivation of these measures from behavior coded in real-time is illustrated in an analysis of the reciprocal relations between the smiling behavior of mothers and their 2- to 5-month-old infants
