While still in utero, the brain is growing. Thus the longer the gestation period, the larger the relative size of the brain will be in the infant. It is assumed that the more time spent in infancy and childhood, the more an animal learns. There is a direct correlation to length of the gestation period, infancy and childhood development, and lifespan in primates.
What primate do you think has the longest gestation period, infancy, and childhood?