We observed 1,000 3‐year‐old children who exhibited five temperament types: Undercontrolled, Inhibited, Confident, Reserved, and Well‐adjusted. Twenty‐three years later, we reexamined 96% of the children as adults, using multiple methods of comprehensive personality assessment, including both self‐ and informant‐reports. These longitudinal data provide the longest and strongest evidence to date that children's early‐emerging behavioral styles can foretell their characteristic behaviors, thoughts, and feelings as adults, pointing to the foundations of the human personality in the early years of life.