Developmental psychologists study physical, cognitive, and social change throughout the life span with a focus on three major issues:

At conception, one sperm cell fuses with one egg cell.

Genes are the basic units of heredity that make up chromosomes, the threadlike coils of DNA. The human genome is the shared genetic profile that distinguishes humans from other species.

The interaction between heredity and environment influences development. Epigenetics is the study of environmental influences on gene expression (making genes active or inactive) that occur without a DNA change.

From conception to 2 weeks, the zygote is in a period of rapid cell development.

By 6 weeks, the embryo’s body organs begin to form and function.

By 9 weeks, the fetus is recognizably human.

Teratogens are potentially harmful agents that can pass through the placenta and interfere with normal development, as happens with fetal alcohol syndrome.

Newborns’ sensory systems and reflexes aid their survival and social interactions with adults.

Newborns smell and hear well, see what they need to see, and begin using their sensory equipment to learn.

Inborn temperament—emotional reactivity and intensity—heavily influences our developing personality.

Identical (monozygotic) twins develop from a single fertilized egg that splits into two; fraternal (dizygotic) twins develop from separate fertilized eggs.

Studies of separated identical twins allow researchers to maintain the same genes while testing the effects of different home environments. Studies of adoptive families let researchers maintain the same home environment while studying the effects of genetic differences.

Most brain cells form before birth. With maturation and experience, their interconnections multiply rapidly and become more complex. A pruning process strengthens heavily used links and weakens unused ones, and we seem to have a critical period for some skills, such as language.

Complex motor skills—sitting, standing, walking—develop in a predictable sequence. Timing may vary with individual maturation and with culture.

We have few conscious memories of events occurring before age 4, a blank space in our conscious memory that psychologists call infantile amnesia.

In his theory of cognitive development, Jean Piaget proposed that children actively construct and modify an understanding of the world through the processes of assimilation and accommodation. They form schemas that help them organize their experiences.

Piaget believed children construct an understanding of the world by interacting with it while moving through four cognitive stages:

Current research supports the sequence Piaget proposed, but finds young children more capable and their development more continuous.

Lev Vygotsky’s studies of child development focused on the ways a child’s mind grows by interacting with the social environment. Parents and other caregivers provide temporary scaffolds from which children can step to higher levels of thinking.

Infants develop stranger anxiety soon after object permanence.

Infants form attachments with caregivers who satisfy nutritional needs but, more importantly, who are comfortable, familiar, and responsive.

Attachment styles differ (secure or insecure) due to the child’s individual temperament and the responsiveness of the child’s caregivers.

Securely attached children develop basic trust and tend to have healthier adult relationships.

Neglect or abuse can disrupt the attachment process and put children at risk for physical, psychological, and social problems.

Parenting styles—authoritarian, permissive, negligent, and authoritative—reflect how responsive and how demanding parents are.

Child-raising practices reflect both individual and cultural values.

Children with the highest self-esteem, self-reliance, self-regulation, and social competence tend to have authoritative parents. Less positive outcomes are associated with authoritarian, permissive, and negligent parents.

However, correlation does not equal causation (it’s possible that children with positive characteristics are more likely to bring out positive parenting methods).

Adolescence, the transition period from childhood to adulthood, begins with puberty, a time of sexual maturation.

The brain’s frontal lobes mature during adolescence and the early twenties, enabling improved judgment, impulse control, and long-term planning.

In Jean Piaget’s view, formal operations (abstract reasoning) develop in adolescence, and this development is the basis for moral judgment. Research indicates that these abilities begin to emerge earlier than Piaget believed.

Lawrence Kohlberg proposed a stage theory of moral thinking: preconventional morality (self-interest), conventional morality (gaining others’ approval or doing one’s duty), and postconventional morality (basic rights and self-defined ethical principles). Kohlberg’s critics note that the postconventional level is culturally limited, representing morality only from the perspective of an individualist society.

Other researchers believe that morality lies in moral intuition and moral action as well as thinking.

Erik Erikson proposed eight stages of psychosocial development across the life span. He believed we need to achieve the following challenges: trust, autonomy, initiative, competency, identity (in adolescence), intimacy (in young adulthood), generativity, and integrity.

Each life stage has its own psychosocial task, with the chief task of adolescence being solidifying one’s sense of self, one’s identity. This often means trying out a number of different roles. Social identity is the part of the self-concept that comes from a person’s group memberships.

During adolescence, parental influence diminishes and peer influence increases, in part because of the selection effect—the tendency to choose similar others as friends.

Nature and nurture—genes and experiences—interact to guide our development.

Parents influence our manners, attitudes, values, faith, and politics. Language and other behaviors are shaped by peer groups, as children adjust to fit in.

Emerging adulthood is the period from age 18 to the mid-twenties, when many young people in Western cultures are no longer adolescents but have not yet achieved full independence as adults.

Muscular strength, reaction time, sensory abilities, and cardiac output begin to decline in the late twenties and continue to decline through middle adulthood (to age 65) and late adulthood (after 65).

Around age 50, menopause ends women’s period of fertility. Men experience a more gradual decline in sperm count, testosterone level, and speed of erection and ejaculation.

In late adulthood, the immune system also weakens, but good health habits help to enable better health in later life.

Recall begins to decline, especially for meaningless information. Recognition memory remains strong.

Researchers use cross-sectional and longitudinal studies to identify mental abilities that do and do not change as people age.

Adulthood’s two major commitments are love (Erikson’s intimacy—forming close relationships) and work (productive activity, or what Erikson called generativity).

Chance encounters affect many of our important decisions, such as our choice of romantic partners.

The social clock is a culture’s expected timing for social events such as marriage, parenthood, and retirement.

Most older people retain a sense of well-being, partly due to the tendency to focus more on positive emotions and memories.

People over 65 report as much happiness and satisfaction with life as younger people do.

Normal grief reactions vary widely. People do not grieve in predictable stages.

Death of a loved one is much harder to accept when it comes before its expected time.

Life can be affirmed even at death, especially for those who experience what Erikson called a sense of integrity—a feeling that one’s life has been meaningful.