Background and method: The aim of this study was to examine whether a mother's sensitivity towards her one-year-old infant is related to the infant's propensity to engage in "triadic" relations--that is, to orientate to an adult's engagement with objects and events in the world, for example in sharing experiences with an adult. In order to determine that any effects were specific to infants' behaviour in the interpersonal domain, we also tested their performance on tests of understanding means-ends relations and object permanence. Results: The results were that high maternal sensitivity and low intrusiveness correlated with high levels of infant triadic interpersonal engagement with a stranger "vis-a-vis" performance on the non-social tasks. There was also suggestive evidence that maternal sensitivity might be related to infants' propensity to share experiences with the mother. Exploratory analyses revealed that these findings held up when the effects of maternal socio-economic status and ethnic group were taken into account; and there was some indication that the effects of maternal intrusiveness on infant profiles of performance were more marked for mothers who did not have a partner. Conclusion: There is a specific relation between maternal sensitivity and one-year-old infants' propensity to engage with someone else in relation to the world.