“Zygote to embryo to fetus” is the correct order. In development, a zygote is defined as a fertilized egg cell. An embryo forms at about 2 weeks post fertilization. Last, a fetus is the developing human from 9 weeks until birth.

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.

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.

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.

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.

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

Permissive parenting is the most "relaxed" of the parenting styles. It's defined by a lack of consistency in rules and consequences that are usually not enforced, or are enforced in an inconsistent manner. Permissive parenting is on the opposite end of the spectrum from Authoritarian parenting, which features extremely strict rules with strong enforcement of punishment, and little explanation. Most parenting styles fall somewhere between these two polarized points on the spectrum of parenting style.

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.

Authoritarian parents typically create strict rules that are not open to being updated based on the child's input. These parents have high expectations for their children, and are usually not nurturing.

Mary Ainsworth's study of attachment styles in parent-child relationships showed that babies with secure attachments to their parents would be distressed when their parents left the room and look for their comfort upon return. Ainsworth held that the baby had unconsciously recognized and accepted their dependence on their caregivers for their needs. The other attachment styles identified in the study were avoidant and anxious/ambivalent, where the babies may not have felt they could depend on their caregivers to fulfill their needs.