Objectives: To compare the quality of mother-infant interaction between young A-sib infants, with and without phenotypic autism risk, and TD-sib controls.

Methods: Mother-infant play interactions involving 30 6-9-month-old A-sibs and 30 TD-sib controls were rated, blind to dyad characteristics. A global rating scale was developed for this purpose by combining appropriate elements of two validated mother-child interaction measures.

Results: A-sibs at high phenotypic risk were less attentive to mother, had fewer positive vocalisations and showed more neutral affect than A-sibs at low phenotypic risk. Their mothers exhibited less sensitive responsiveness and less acceptance, and their overall interactions were rated as slightly less dyadic. However, low-risk A-sibs tended to show similar interaction scores as TD-sibs. A-sibs at high phenotypic risk were also more likely to show behavioural atypicalities during the interaction session than the other infants.

Conclusions: This study is the first to demonstrate that A-sib infants at high phenotypic risk and their mothers tend to differ in their interactions from others dyads. Based on a gene-environment transactional model, and on developmental research in non-ASD contexts, the findings suggest that such mother-infant relations in high-risk A-sibs may exacerbate their social and communicative difficulties. We also report on preliminary 12-15 month follow-up data. Our findings have implications for early preventative intervention.