Kohlberg found that the first two stages are reached by most children, that stages 3 and 4 are reached by older children and most adults, but that the stage 6 is reached by only 20% of the population.
Carol Gilligan examined certain differences between the moral development of males and that of females. In younger children, she found that girls are more concerned with a morality based on caring and boys with a morality based on justice. Gilligan proposed that this gender difference is in part due to children's relationship with their mother.
Social development. Social development begins at birth as a child forms an attachment (a strong emotional bond) with the primary caregiver(s), usually the mother. Harry Harlow studied attachment deprivation with baby monkeys raised in isolation. Although their physical needs were met and they were given surrogate mothers made of cloth, these monkeys suffered severe behavior pathologies. They recovered if the isolation was limited to three months, but longer periods produced abnormal adults. Ethically, this type of study could not be conducted with humans, but parallels have been found with children reared in cold, isolated, emotionally deprived environments. Emotional attachments to caregivers are thought to be essential for social development.
Konrad Lorenz studied imprinting, a rapid and relatively permanent type of learning that occurs for a limited time (called a critical period) early in life, particularly in birds. Baby ducks learn to follow their mother if they see her moving during the first 30‐hour period after their birth. If, however, they don't see their mother, they can imprint on and follow a human or even a moving object instead. Imprinting demonstrates that attachments by the young to a parent can occur early and can have lifelong consequences.
The term gender stereotyping refers to patterns of behavior expected of people according to their gender. The development of gender‐related differences is complex. Gender stereotyping occurs not only because of parental differences in rearing children of each gender but also because of socialization experiences. Eleanor Maccoby has observed that children with widely different personalities play together simply because they are of the same gender.
Personality development. Developmental psychologists also study personality development in children.