Dark adaptation -  The process by which receptor cells become more sensitive to light.

Decay theory -  A theory stating that memory traces fade with time.

Decentration -  The ability to focus simultaneously on several aspects of a problem.

Decision-making -  The process of weighing alternatives and choosing among them.

Declarative memory -  The remembering of factual information. Declarative memory is usually considered explicit.

Deductive reasoning -  The process by which a particular conclusion is drawn from a set of general premises or statements.

Defense mechanisms -  Behaviors that protect people from anxiety.

Deindividuation -  The tendency of people in a large, arousing, anonymous group to lose inhibitions, sense of responsibility, and self-consciousness.

Deinstitutionalization -  The trend toward providing treatment through community-based outpatient clinics rather than inpatient hospitals.

Delta waves -  The type of brain waves present when a person is deeply asleep.

Delusions -  False beliefs that are held strongly despite contradictory evidence.

Dementia -  A condition characterized by several significant psychological deficits.

Dendrite -  A fiber that extends from a neuron. It received signals from other neurons and sends them toward the cell body.

Dendritic trees -  Highly branched fibers extending from neurons.

Denial -  A defense mechanism that involves refusing to acknowledge something that is obvious to others.

Dependent variable -  The variable that is observed in an experiment and that may be affected by manipulations of the independent variable.

Descriptive statistics -  Numbers that researchers use to describe their data so it can be organized and summarized.

Development -  The series of age-related changes that occurs over the course of a person’s life span.

Developmental norms -  The median ages at which children develop specific behaviors and abilities.

Diabetes -  A condition caused by a deficiency of insulin.

Diagnosis -  The process of distinguishing among disorders.

Diagnostic and Statistical Manual of Mental Disorders (DSM)  -  A reference book used by psychologists and psychiatrists to diagnose psychological disorders.

Dialectical reasoning -  A process of going back and forth between opposing points of view in order to come up with a satisfactory solution to a problem.

Dichromat -  A person who is sensitive to only two of the three wavelengths of light.

Difference threshold -  The smallest difference in stimulation that is detectable 50 percent of the time. This threshold is also called the just noticeable difference, or jnd.

Diffusion of responsibility -  The tendency for an individual to feel less responsible in the presence of others because responsibility is distributed among all the people present.

Discriminative stimulus -  In operant conditioning, a cue that indicates the kind of consequence that’s likely to occur after a response.

Disease model of addiction -  The idea that addiction is a disease that has to be medically treated.

Disorganized type -  A subtype of schizophrenia characterized by disorganized behavior, disorganized speech, and emotional flatness or inappropriateness.

Displacement -  A defense mechanism that involves transferring feelings about a person or event to someone or something else.

Display rules -  Norms that tell people whether, which, how, and when emotions should be displayed.

Dissociative amnesia -  A disorder characterized by an inability to remember extensive, important personal information, usually about something traumatic or painful.

Dissociative disorders -  Disorders characterized by disturbances in consciousness, memory, identity, and perception.

Dissociative fugue -  A disorder in which a person suddenly and unexpectedly leaves home, fails to remember the past, and becomes confused about his or her identity.

Dissociative identity disorder -  A disorder in which a person fails to remember important personal information and has two or more identities or personality states that control behavior. It is also called multiple personality disorder.

Dissonance theory -  A theory that proposes that people change their attitudes when they have attitudes that are inconsistent with one another.

Distributed practice -  The practice of learning material in short sessions over a long period. It is also called the spacing effect.

Divergent thinking -  A style of thinking in which people’s thoughts go off in different directions as they try to generate many different solutions to a problem.

Dopamine -  A neurotransmitter involved in voluntary movement, learning, memory, and emotion.

Double-blind -  A procedure in which neither the subjects nor the experimenter knows which subjects belong to the experimental and control groups.

Drive reduction theories of motivation -  Ideas that suggest people act in order to reduce needs and maintain a constant physiological state.

Drug therapy -  Treatment that involves the use of medications. It is also called pharmacotherapy.

Dysthymic disorder -  A disorder involving depressed mood on a majority of days for at least two years.