A study examined whether attachment and cognition at the age of 12 months are predicted by vocal rhythm coordination at the age of 4 months. The mother-infant, stranger-infant, and mother-stranger face-to-face interactions of 88 4-month-old infants were recorded. Results confirmed that adult-infant coordination at age 4 months predicted attachment and cognition at 12 months. Midrange degree of mother-infant and stranger-infant coordination was found to be optimal for attachment, while high stranger-infant coordination in the lab was optimal for cognition. It was concluded that either more or less optimal outcomes can be indexed by high coordination as a function of outcome measure, partner, and site. A fundamental dyadic timing matrix, a system that guides the trajectory of relatedness and that informs all relational theories of development, is further defined by this research.