Previous research has suggested that when a social partner, such as a parent, pays attention to an object, this increases the attention that infants pay to that object during spontaneous, naturalistic play. There are two contrasting reasons why this might be: first, social context may influence increases in infants' endogenous (voluntary) attention control; second, social settings may offer increased opportunities for exogenous attentional capture. To differentiate these possibilities, we compared 12-month-old infants' naturalistic attention patterns in two settings: Solo Play and Joint Play with a social partner (the parent). Consistent with previous research, we found that infants' look durations toward play objects were longer during Joint Play, and that moments of inattentiveness were fewer, and shorter. Follow-up analyses, conducted to differentiate the two above-proposed hypotheses, were more consistent with the latter hypothesis. We found that infants' rate of change of attentiveness was faster during Joint Play than Solo Play, suggesting that internal attention factors, such as attentional inertia, may influence looking behaviour less during Joint Play. We also found that adults' attention forwards-predicted infants' subsequent attention more than vice versa, suggesting that adults' behaviour may drive infants' behaviour. Finally, we found that mutual gaze did not directly facilitate infant attentiveness. Overall, our results suggest that infants spend more time attending to objects during Joint Play than Solo Play, but that these differences are more likely attributable to increased exogenous attentional scaffolding from the parent during social play, rather than to increased endogenous attention control from the infant.