Prenatal development begins at fertilization, when the male sperm unites with the female ovum (egg) to form a single-celled egg called a zygote. During the first 8 weeks of prenatal development, the organism is called an embryo; after that, it is known as a fetus. Harmful influences that can adversely affect the fetus's development include German measles, toxic substances, some sexually transmitted diseases, cigarettes, alcohol, illegal drugs, over-the-counter medications, and chronic maternal stress. Fathers affect prenatal development too; the sperm of teenage boys and men over age 50 may have mutations that increase the risk of miscarriage, birth defects, and certain diseases in their offspring.

Babies are born with motor reflexes, perceptual abilities, and rudimentary cognitive skills. Cultural practices affect the timing of physical milestones.

Babies' innate need for contact comfort gives rise to emotional attachment to their caregivers, and by the age of 6 to 8 months, infants begin to feel separation anxiety. Studies of the Strange Situation have distinguished secure from insecure attachment; insecurity can take one of two forms, avoidant or anxious-ambivalent attachment. Styles of attachment are relatively unaffected by the normal range of childrearing practices, and also by whether or not babies spend time in daycare. Insecure attachment is promoted by parents' rejection, mistreatment, or abandonment of their infants; by a mother's postpartum depression, which can affect her ability to care for her baby; by the child's own fearful, insecure temperament; or by stressful family situations.

In support of the view that the human brain contains a mental module that is sensitive to a universal grammar, children from different cultures go through similar stages of language development; adults do not consistently correct their children's syntax; and groups of children who have never been exposed to adult language often invent their own. However, languages also vary around the world, suggesting that language is a cultural tool. It's also possible that children learn the statistical probability that any given word or syllable will follow another. At 4 to 6 months of age, babies begin to recognize the sounds of their own language. They go through a babbling phase from age 6 months to 1 year, and at about 1 year, they start saying single words and using symbolic gestures. At age 2, children speak in two- or three-word telegraphic sentences that convey a variety of messages.

Jean Piaget argued that children's thinking changes and adapts through assimilation and accommodation. Piaget proposed four stages of cognitive development: sensorimotor (birth to age 2), during which the child learns object permanence; preoperational (ages 2 to 7), during which language and symbolic thought develop, although the child remains egocentric in reasoning; concrete operations (ages 7 to 12), during which the child comes to understand conservation; and formal operations (age 12 to adulthood), during which abstract reasoning develops. Today, we know that the changes from one stage to another are not as clear-cut as Piaget implied; development is more continuous and overlapping. Babies and young children have greater cognitive abilities, at earlier ages, than Piaget thought, and young children are not always egocentric.

Lawrence Kohlberg proposed that as children mature cognitively, they go through three levels of moral reasoning. But people can reason morally without behaving morally. Developmental psychologists study how children learn to internalize standards of right and wrong and to behave accordingly. This ability depends on the emergence of conscience and the moral emotions of guilt, shame, and empathy.

As a strategy for teaching children to behave, a parent's use of power assertion is associated with a child's aggressiveness and lack of empathy. Induction is associated with children who develop empathy, internalize moral standards, and can resist temptation. The capacity of very young children to delay gratification and control their feelings is associated with the development of internalized moral standards and conscience.

Gender development includes the emerging awareness of gender identity, the understanding that a person is biologically male or female regardless of what he or she does or wears, and gender typing, the process by which boys and girls learn what it means to be masculine or feminine in their culture. Some individuals are born with intersex physical conditions, living with the physical attributes of both sexes, and consider themselves to be transgender. Transsexuals feel that they are male in a female body or vice versa; their gender identity is at odds with their anatomical sex.

Universally, young children tend to prefer same-sex toys and playing with other children of their own sex. Biological psychologists account for this phenomenon in terms of genes and prenatal androgens. Cognitive psychologists study how children develop gender schemas for the categories “male” and “female,” which in turn shape their gender-typed behavior. Learning theorists study the direct and subtle reinforcers and social messages that foster gender typing.

During adrenarche, the adrenal glands begin releasing hormones that affect brain development. Adolescence begins with the physical changes of puberty. In girls, puberty is signaled by menarche and the development of breasts; in boys, it begins with the onset of nocturnal emissions and the development of the testes, scrotum, and penis. Hormones produce secondary sex characteristics, such as pubic hair in both sexes and a deeper voice in males.

Most American adolescents do not go through extreme emotional turmoil, anger, or rebellion. However, conflict with parents, mood swings and depression, and reckless or rule-breaking behavior increase.

Erik Erikson proposed that life consists of eight stages, each with a unique psychological challenge, or crisis, that must be resolved, such as an identity crisis in adolescence. Erikson identified many of the essential concerns of adulthood and showed that development is a lifelong process. However, psychological issues or crises are not confined to particular chronological periods or stages.

Many people between the ages of 18 and 25, especially if they are not financially independent, find themselves in a life phase called emerging adulthood. The middle years are generally not a time of turmoil or crisis but the prime of most people's lives. In women, menopause begins in the late 40s or early 50s. In middle-aged men, hormone production slows down and sperm counts decline; fertility continues, but with increased risk of fetal abnormalities.

The speed of cognitive processing slows down in old age, and fluid intelligence parallels other biological capacities in its eventual decline. Crystallized intelligence, in contrast, depends heavily on culture, education, and experience, and it tends to remain stable over the lifespan.

Many supposedly inevitable results of aging, such as senility, depression, and physical frailty, are often avoidable. Exercise and mental stimulation promote the growth of synapses in the human brain, even well into old age, although some mental losses are inevitable.