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Five Factor Model (FFM) personality traits are implicated in long-term health-risk behaviors and outcomes. Less research has addressed how early-life experiences are associated with individual differences in these traits in adulthood. We examine whether having been breastfed is associated with adult personality and well-being in the National Longitudinal Study of Adolescent to Adult Health. At Wave 1, caregivers reported whether the target child had been breastfed. At Wave 4, participants (=13,113; 53% female; =28.98) completed measures of psychological functioning. We tested for mean-level differences in the traits by breastfeeding status (yes/no) and by the duration of breastfeeding, controlling for basic demographic factors and early-life factors that could confound the breastfeeding-personality association (e.g., mother education). Participants who had been breastfed scored lower in neuroticism, anxiety, and hostility and higher in openness and optimism than those not breastfed. A curvilinear relation suggested that neuroticism was lowest for those breastfed for 9-12 months and highest for those either breastfed for >24 months or exclusively bottle-fed. Breastfeeding was unrelated to conscientiousness or state psychological functioning. This research suggests long-term psychological benefits to breastfeeding and indicates that early life experiences are associated with traits that are consequential for adult health.