The subjects consisted of 20 pairs of adults between the ages of 18 and 23; each pair contained a second- and fourthborn child in a family containing at least five children. The families in the sample were all white and middle-class with parents' marriages intact. The Relationship Communication Coding System was used to analyze the interaction between subjects in each pair. The system categories used here included question, nonsupport, instruction, assertion, support, and extension. The findings indicated a significant relationship between sibling position and interactional behavior, with the secondborns making more responses coded as assertions and the fourthborns making more responses coded as questions. These findings could be used to design experiments to study nonshared environments within families. 35 references