To establish which social interactive behaviours predict later psychiatric diagnosis, we examined 180 videos of a parent-infant interaction when children were aged one year, from within the Avon Longitudinal Study of Parents and Children (ALSPAC) cohort. Sixty of the videos involved infants who were later diagnosed with a psychiatric disorder at seven years, and 120 were a randomly selected sex-matched control group. Interactive behaviours for both the caregiver and the one year old infant were coded from the videos according to eight holistic categories of interpersonal engagement: Well-being, Contingent Responsiveness, Cooperativeness, Involvement, Activity, Playfulness, Fussiness, and Speech. Lower levels of adult activity and speech in interaction at one year significantly predicted overall diagnosis of child psychiatric disorder. (Contains 6 tables and 1 figure.)