What Personality and intelligence are today considered core factors in human development over the life course. Thus, research has consistently documented core personality traits and high intelligence to be strongly associated with a longer and happier life, fewer diseases, higher income, higher education, and better social adjustment. It is, however, still a mystery how these traits arise, how they develop, and to what degree they can be traced back to the early life psychological development. This doctoral dissertation we will lead to pioneering knowledge on the very early predictors of personality and intelligence. Using a unique birth cohort, it will thus be possible to identify early life factors that are of particular importance in shaping the development of intelligence and personality Why The first years of life are considered crucial to development of individual differences related to personality and intelligence. This is in line with the Nobel Prize winner James Heckman's ground-breaking findings that the highest rate of return comes from investing in early childhood, from birth until age three. Thus, differences in children's readiness for school as well as a range of different outcomes in adulthood may very well be related to differences in personality and cognitive skills at an early age. This study will not only lead to better understanding of these psychological phenomena - but also increase knowledge about potential interventions to create optimal conditions for development during the first years of life. How The studies are all based on one of the oldest birth cohorts in the world, Rigshospitalets mor-barn kohorte, which contains data on 8949 mothers and their 9125 children born between 1959 and 1961 at Rigshospitalet. The children were repeatedly followed up in several follow-up studies, with the most recent conducted in 2009-11 when the cohort was in midlife. Unique data on this cohort is thus available with thorough information on various early predictors (demographic information, prenatal factors, birth weight, milestones, growth, parental characteristics etc.) together with data on adult intelligence and personality. SSR By focusing on specific factors during the first years of life, it will be possible to identify early life factors that influence the development of personality and intelligence, providing a unique understanding of the origins of these individual differences. This will increase knowledge on optimal conditions for development during the first years of life and on potential interventions.