Background: Social understanding is often thought to be relatively "protected" in children with Down's syndrome (DS) and to underlie the outgoing personality characteristically attributed to them. This paper draws together findings from our studies of behaviours during object concept testing, generally considered a theoretically "pure" measure of early cognitive ability, and from more recent work focusing on two key socio-cognitive skills: recognising facial expressions of emotion and collaborative learning. Age range of children studied was 4 months to 18 years. Methods: Using standardised hiding tasks, object concept ability was assessed cross-sectionally and longitudinally in children with DS and in age- or stage-matched typically developing (TD) children. Stability of cognitive advances was assessed and similarities/differences in developmental pathways explored. In the emotion recognition studies, the ability to distinguish six primary emotions was measured, with performance compared with that of developmentally-matched TD children and age-matched children with intellectual disabilities of similar severity but differing aetiology (non-specific intellectual disability (NSID) or fragile X syndrome (FXS)). In the collaborative learning study, the impact on sorting skills of working with a peer on a conceptually related task was measured, with outcomes compared in DS, NSID and TD child pairings. Results: Evidence of counterproductive, socially-based strategies being inappropriately applied by children with DS in learning contexts was found in both the object concept and collaborative learning studies, along with inefficient use of current cognitive and linguistic abilities. Emotion recognition weaknesses were also identified, although deficits were relative rather than absolute and individual variability was marked. Conclusions: In line with emerging evidence from other research teams, findings suggest possible difficulties in some core aspects of interpersonal functioning in DS, with both qualitative and quantitative differences in how social cognition develops and is applied in learning contexts. Implications for development and for behavioural phenotype theory are briefly discussed.