Developmental psychologists study humans of all ages to observe how different stages of life (e.g., infants, adolescents) react to various stimuli. A significant proportion of research in developmental psychology focuses on children, seeing as childhood sees the greatest amount of change.

Erik Erikson was a famous psychologist who proposed eight psychosocial stages that every human being experiences during his or her lifetime. Erikson believed that adolescents experience confusion over which roles they wish to fulfill (e.g., rebel, good student, class clown). To successfully conquer this stage, adolescents must merge their various roles of choice into a complete identity. This conflict is known as identity vs. role confusion.

A general overview of Erikson's stages is given below:

Trust vs. mistrust, age 0-2 years

Autonomy vs. shame and doubt, age 2-4 years

Initiative vs. guilt, age 4-5 years

Industry vs. inferiority, age 5-12 years

Identity vs. role confusion, age 12-19 years

Intimacy vs. isolation, young adulthood

Generativity vs. stagnation, middle adulthood

Ego integrity vs. despair, late adulthood

Authoritarian parenting is typically depicted as the parent being controlling and power-assertive over their child. The authoritarian parent expects a lot of their child, and may reject him/her if he/she does not meet their high expectations.

Harry Harlow was an American psychologist who is best known for his maternal separation, dependency needs, and social isolation experiments with primates. His experiments would not have been allowed today because they would be considered animal cruelty by the IRB research approval panel.

If one is not developing in a healthy and positive manner throughout life, it is possible to become completely self-possessed during middle age. Instead of expressing concern about the people and society around them and their relationship with others, the middle-aged person is completely selfish and out to satisfy their own desires without much consideration for anyone else.

Jean Piaget set the intellectual foundations for Kohlberg's six stages of moral development. The major difference is that Piaget elucidates the process of moral reasoning in children, rather than breaking down moral development into evolutionary stages. Piaget said that children approach problems in two different ways: assimilation and accomodation. In assimilation, children easily internalize the rules of the outside world, including the ethical ones. In accomodation, children may have to change these internalized ideas or other pre-existing notions to fit what they see in the world. Kohlberg expanded on this and categorized ideas based on the stages of a person's life which, according to him, define the moral dilemnas a person faces.

In the presence of a Y chromosome, testosterone prevents the default female organs from developing in the embryo. However, to prevent female organs from developing and to initiate development of male reproductive organs, testosterone must be received by 5-alpha reductase receptors. If testosterone is present, but there are no receptors, female organs will continue to develop in spite of the Y chromosome. This is called 5-alpha reductase deficiency. Although genetically male, the baby has the appearance of a female. In many cases however, a surge of testosterone during puberty leads to development of male reproductive organs.

The Sensorimotor stage ranges from zero to two years of age. The phrase “out of sight, out of mind” describes a child before developing true object permanence. True object permanence is the ability of a child to know that even though an object is not visible to them, it still exists (imagine a game of peek-a-boo). In this stage, Piaget also proposed instinctual reflexes, intentional-ism, and coordination of speaking, hearing, and acting.  

Secondary sex characteristics are traits that develop during puberty, and include facial hair, enlarged breasts, pubic hair, and deeper voices. Primary sex characteristics are innate and include sex organs (the ovaries in women and testes in men) and genitalia. 

Longitudinal research is a popular method of conducting developmental psychology studies. This method takes one group of people and tracks the effect of the developmental factor being studied on them over time. This is a time-consuming and often tricky method of research and many researchers may turn to cross-sectional research instead.