Four parent-infant joint-action variables were derived by combining parents' vocal or object-stimulation behaviors with infants' vocal or object-directed behaviors observed in the home setting. Measures of simultaneous vocalization, joint-object play, parent-vocalize/infant-object play, and infant-vocalize/parent-object stimulation were constructed to represent social-construction experiences in which shared actions between infants and parents are thought to contribute to infants' cognitive development. The analyses (based on 66 infants and their parents) focused on the normative changes and stability of individual differences of these measures from 6 to 12 months and their relations with 30-month cognitive-development status. The durations of all measures increased with age. All joint-action measures showed consistency across time and the 12-month mother-infant (but not father-infant) joint-action measures were strongly related with 30-month McCarthy scores.