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This study examined contributions of maternal personality and infant temperament to infant vocabulary and cognitive development both directly and indirectly through parental stress. Participants were recruited at birth and included 63 infant twin pairs and their mothers. Assessments were completed at 6, 9, 12, and 18 months of age and included Dimensions of Temperament–Revised (maternal personality), Parenting Stress Index (parental stress), Infant Behavior Questionnaire–Revised (infant temperament), Bayley Scales of Infant Development II: Mental Development Index, and MacArthur-Bates Total Vocabulary. Structural equation modeling with a jackknife approach was used to analyze data separately for each twin in the pair. At 12 months, maternal personality and infant temperament contributed indirectly to MacArthur-Bates Total Vocabulary and Bayley Mental Development Index scores through parental stress. In addition, infant temperament directly contributed to 12-month MacArthur-Bates Total Vocabulary. At 18 months, these relationships were no longer significant. The different findings at 12 months compared to 18 months may reflect important developmental and environmental shifts, as well as possible differences in the method and measurements used at each age.