In a sample of 7,695 families in the prospective, nationally representative British Millennium Cohort Study, this study examined whether characteristics of the 1st-born child predicted parents' timing and probability of having another child within 5 years after the 1st child's birth. Infant temperament was assessed with the Carey Infant Temperament Scale (Carey, 1972; Carey & McDevitt, 1978) at age 9 months, childhood socioemotional and behavioral characteristics with the Strengths and Difficulties Questionnaire (Goodman, 2001), and childhood cognitive ability with the Bracken School Readiness Assessment (Bracken, 2002) test at age 3 years. Survival analysis modeling indicated that the 1st child's low reactivity to novelty in infancy, high prosociality, low conduct problems, and high cognitive ability in childhood were associated with increased probability of parents having another child. Except for reactivity to novelty, these associations became stronger with time. High emotional symptoms were also positively associated with childbearing, but this was likely to reflect reverse causality--that is, the effect of sibling birth on the 1st child's adjustment. The results suggest that child effects, particularly those related to the child's cognitive ability, adaptability to novelty, and prosocial behavior, may be relevant to parents' future childbearing. (Contains 2 figures and 5 tables.)