empirical evidence: Evidence that is based on observation, experience, or data; not theoretical.

culture: A system of shared beliefs, norms, behaviors, and expectations that persist over time and prescribe social behavior and assumptions.

nature: In development, nature refers to the traits, capacities, and limitations that each individual inherits genetically from his or her parents at the moment of conception.

scientific method: A way to answer questions using empirical research and data-based conclusions.

hypothesis: A specific prediction that can be tested.

race: A group of people who are regarded by themselves or by others as distinct from other groups on the basis of physical appearance, typically skin color.

sensitive period: A time when a certain type of development is most likely, although it may still happen later with more difficulty. For example, early childhood is considered a sensitive period for language learning.

ethnic group: People whose ancestors were born in the same region and who often share a language, culture, and religion.

cohort: People born within the same historical period. They experience historical events (such as wars), technologies (such as the smartphone), and cultural shifts (such as women’s liberation) at the same ages.

difference-equals-deficit error: The mistaken belief that a deviation from some norm is necessarily inferior to behavior or characteristics that are more typical.

life-span perspective: An approach to the study of human development that takes into account all phases of life, not just childhood or adulthood.

ecological-systems approach: A perspective on human development that considers all the influences from the various contexts of development. (Later renamed bioecological theory.)

science of human development: The science that seeks to understand how and why people of all ages and circumstances change or remain the same over time.

differential susceptibility: The idea that people vary in how sensitive they are to particular experiences. Often such differences are genetic, which makes some people affected “for better or for worse” by life events. (Also called differential sensitivity.)

replication: Repeating a study, usually using different participants, perhaps of another age, location, socioeconomic status (SES), or culture.

critical period: A time when a particular type of developmental growth (in body or behavior) must happen for normal development to occur.

socioeconomic status (SES): A person’s position in society as determined by income, occupation, education, and place of residence. (Sometimes called social class.)

nurture: In development, nurture includes all the environmental influences that affect the individual after conception. This includes everything from the mother’s nutrition while pregnant to the cultural influences in the nation.

social construction: An idea that is built on shared perceptions, not on objective reality. Many age-related terms (such as childhood, adolescence, yuppie, and senior citizen) are social constructions, strongly influenced by social assumptions.

epigenetics: The study of how environmental factors affect genes and genetic expression—enhancing, halting, shaping, or altering the expression of genes.

cross-sectional research: A research design that compares groups of people who differ in age but are similar in other important characteristics.

dependent variable: In an experiment, the variable that may change as a result of whatever new condition or situation the experimenter adds. In other words, the dependent variable depends on the independent variable.

independent variable: In an experiment, the variable that is introduced to see what effect it has on the dependent variable. (Also called experimental variable.)

developmental theory: A group of ideas, assumptions, and generalizations that interpret and illuminate thousands of observations about human growth. A developmental theory provides a framework for explaining the patterns and problems of development.

operant conditioning: The learning process by which a particular action is followed by something desired (a reinforcer which makes the person or animal more likely to repeat the action) or by something unwanted (a punishment which makes the action less likely to be repeated). (Also called instrumental conditioning.)

correlation: A number between +1.0 and –1.0 that indicates the degree of relationship between two variables, expressed in terms of the likelihood that one variable will (or will not) occur when the other variable does (or does not). A correlation indicates only that two variables may be somehow related, not that one variable causes the other to occur.

longitudinal research: A research design in which the same individuals are followed over time, as their development is repeatedly assessed.

scientific observation: A method of testing a hypothesis by unobtrusively watching and recording participants’ behavior in a systematic and objective manner—in a natural setting, in a laboratory, or in archival data.

survey: A research method in which information is collected from a large number of people by interviews, written questionnaires, or some other means.

dynamic-systems approach: A view of human development as an ongoing, ever-changing interaction between the physical, cognitive, and psychosocial influences.

psychoanalytic theory: A theory of human development that holds that irrational, unconscious drives and motives, often originating in childhood, underlie human behavior.

experiment: A research method in which the researcher tries to determine the cause-and-effect relationship between two variables by manipulating one (called the independent variable) and then observing and recording the ensuing changes in the other (called the dependent variable).

conditioning: According to behaviorism, the processes by which responses become linked to particular stimuli and learning takes place. The word conditioning is used to emphasize the importance of repeated practice, as when an athlete conditions his or her body by training for a long time.

humanism: A theory that stresses the potential of all humans, who have the same basic needs, regardless of culture, gender, or background.

cross-sequential research: A hybrid research design in which researchers first study several groups of people of different ages (a cross-sectional approach) and then follow those groups over the years (a longitudinal approach). (Also called cohort-sequential research or time-sequential research.

qualitative research: Research that consider qualities instead of quantities. Descriptions of particular conditions and participants’ expressed ideas are often part of qualitative studies.

social learning theory: An extension of behaviorism that emphasizes the influence that other people have over a person’s behavior. Even without specific reinforcement, every individual learns many things through observation and imitation of other people. (Also called observational learning.)

quantitative research: Research that provides data that can be expressed with numbers, such as ranks or scales.

cognitive theory: A theory of human development that focuses on changes in how people think over time. According to this theory, thoughts shape attitudes, beliefs, and behaviors.

behaviorism: A theory of human development that studies observable behavior. Behaviorism is also called learning theory because it describes the laws and processes by which behavior is learned.