Do as I say - or as I do?! How 18- and 24-month-olds integrate words and actions to infer intentions in situations of match or mismatch.

Infants use behavioral and verbal cues to infer another person's action intention. However, it is still unclear how infants integrate these often co-occurring cues depending on the cues' coherence (i.e., the degree to which the cues provide coherent information about another's intention). This study investigated how 18- and 24-month-olds' (N = 88 per age group) action selection was influenced by varying the coherence of a model's verbal and behavioral cues. Using a between-subjects design, infants received six trials with different stimulus objects. In the conditions Congruent, Incongruent, and Failed-attempt, the model uttered a telic verb particle that was followed by a matching or contradicting goal-directed action demonstration, or by a non goal-directed slipping motion, respectively. In the condition Pseudo-word, a nonsense word was combined with a goal-directed action demonstration. Infants' action selection indicated an adherence to the verbal cue in Congruent, Incongruent, and Failed-attempt, and this was stronger in 24- than 18-month-olds. Additionally, in Incongruent and Failed-attempt, patterns of cue integration across the six trials varied in the two age groups. Regarding the behavioral cue, infants in Congruent and Pseudo-word preferentially followed this cue in both age groups, which also suggested a rather unspecific effect of the verbal cue in Congruent. Relatively longer first action-latencies in Incongruent and Failed-attempt implied that these types of coherence elicited higher cognitive demands than in Congruent and Pseudo-word. Results are discussed in light of infants' flexibility in using social cues, depending on the cue's coherence and on age-related social-cognitive differences.