Summary The dissertation provides a comprehensive picture of early-life factors that are associated with intelligence and personality in adulthood. Specifically, it aims at addressing the relationship between various measures of intelligence and personality assessed at different ages in adulthood, and the following early-life factors: timing of 1-year milestones, timing of 3-year milestones, birth weight, size in the first years of life, and parental socio-economic status. It contributes to the existing literature by underlining stability in development from early to late life. It thus demonstrates lifelong connections between factors in the first years of life and individual differences in intelligence and personality. The dissertation consists of 12 original papers and a scientific summary.