A measure that samples both the sophistication of infants' behaviors and infants' propensity to demonstrate their most sophisticated repertoire of skills was developed and tested for predictability of infant functioning across a 6-mo time span. This measure of spontaneous mastery, unlike a measure of executive capacity with which it is compared, was highly stable across 6 mo. Spontaneous mastery scores of 38 12-mo-olds predicted 18-mo-old performance on the Mental Development Index (MDI) of the Bayley Scales of Infant Development as well as the Bayley scale itself. Common to both the spontaneous mastery measure and the Bayley MDI is the construct of mastery leading toward developmental competence, suggesting that the more proficient children are at mastering tasks on their own, the more inclined they might be to maximize their potentials in testing and learning environments. The greater predictability of the spontaneous mastery measure suggests a means for elucidating individual differences in the motivation–competence relationship that remain stable across developmental epochs.