This paper questions social interaction studies that use frequency or duration of behaviors in mother-infant relations as reliable measures of infant interactive competence. An infant's high score on frequency and duration tests may falsely indicate that the child is more communicative than other subjects. Such results may suggest that mother and child actually influence each other's behavior and consequently produce deceptively inflated scores. A methodological solution to this problem of mutual influence is proposed and the results of a study on the interactions of mothers and handicapped infants are used to illustrate the utility of this solution. In order to understand the relative influence of each actor in a social situation, it was deemed important to define and measure the stable, intrinsic characteristics each actor brings to the interaction. These "variables of independent influence" were measured by methods that prevented the immediate influence of the nontarget actor. Measures of independent influence were used to define how the degree of infant handicap influenced the frequency with which mothers of handicapped infants said their infants communicated. Researchers found that severely handicapped infants exhibited fewer communicative behaviors than their mothers supposed, indicating that mothers were predisposed to mistake pre-linguistic infant behaviors for communication, thus compensating for the scarcity of infant communication. (DR)