dark adaptation: the gradual process through which the eyes adjust from a change in illumination from light to low light intensity.

Darwin (1809-1882): a hugely influential naturalist, who proposed that all species evolve through natural selection so that traits that enhance survival are passed on.

daydream: a visionary fantasy experienced while awake, especially one of happy, pleasant thoughts, hopes, or ambitions.

debriefing: an ethical procedure that occurs at the end of a study, whereby participants are given as much information as possible about the study, are given the option to discuss  their experience of the study, to ensure that participants leave the experiment in the same emotional state as they entered.

decay: the loss of information in memory over a long period of time.

deception: in research, the intentional misleading and misinforming of participants with regard to the aim of the study.

decibels (dB):a measure of volume (sound intensity) .decision-making: reasoning that involves considering and choosing different options.

declarative knowledge: memory for facts (semantic knowledge) and events (episodic knowledge).

deduction/deductive reasoning: the logical process of drawing a particular conclusion from a set of general principles.

defence mechanism: psychological strategies as part of Freudian psychoanalytic theory, that are used to distort or deny reality, in order to cope with anxiety and/or a situation which an individual feels is difficult to cope with.

deindividuation: a process through which group members cease to view themselves as individuals. Individual identity is replaced with identification with a group.

delinquency: criminal/antisocial activity.

delusion: unfounded and irrational beliefs held despite contrary evidence. Characteristic of mental disorders such as schizophrenia, can be manifested in delusions of grandeur (believing that one is famous or powerful) or delusions of persecution (believing that one is being chased or followed).

demand characteristic: cues in an experiment that reveal information to participants about the aim and expected outcome, thereby influencing their behavior and subsequently confounding the results.  

dementia: disorder characterised by considerable deterioration in cognitive function, for instance in loss of memory. Different types of dementia include corticial dementias (e.g. Alzheimers disease) and sub-cortical dementias (e.g. Huntingtons disease).

demographic: a socioeconomic or similar factor that defines a certain group or area.

dendrites: branched fibres at the end of the cell body of a neuron that receive incoming impulses

denial: a defence mechanism , whereby an individual may denies or rejects some aspect of reality.

deoxyribonucleic acid (DNA): the molecule which forms basis of heredity. DNA holds all genetic information on the chromosomes.

dependent personality disorder: a form of personality disorder, whereby an individual is heavily reliant upon others and demonstrates feelings of inadequacy and helplessness when alone.

dependent variable (DV): in an experiment, the values of the variable that change as a result of manipulation of the independent variable.

depression (unipolar disorder): a type of mood disorder, characterised by persistent feelings of great sadness, hopelessness, worthlessness, guilt and a loss of interest in activities.

deprivation: a condition of having too little of something.

depth/distance (visual) perception: the capability to view the world three-dimensionally, utilising monocular and binocular cues to appraise depth and distance between objects.

descriptive statistics: the description and summation of sets of scores in statistics.  

determinism: the assumption that all behaviour has specific causes.

developmental psychology: also known as human development. It is the scientific study of the processes which underlie and control growth and change in behaviour over time.  

deviant behavior: behavior that is a recognised violation of social norms.?

diagnosis: the identification and classification of a psychological disorder.

Diagnostic and Statistical Manual (DSM): a multi-axial manual used for the classification, definition and description of mental health disorders.

diathesis-stress model: an explanation of mental disorders based on a combination of genetic vulnerability (diathesis) and environmental influences.

dichotic listening: utilised in attention research, whereby a different auditory message is simultaneously presented to each ear. Participants are required to repeat one of the messages whilst ignoring the other.

diencephalon: a part of the forebrain, containing the thalamus and the hypothalamus.

diffusion of responsibility: occurs in groups when an individual feels less responsibility because accountability is diffused amongst the group. Evident in emergency situations, whereby the larger the number of bystanders, the less responsibility each bystander feels.

digit span: a test of short-term memory, whereby participants are presented with a series of digits and asked to repeat them. Average digit span is 7 +/- 2.

directional hypothesis: states which of the two condition means will be larger, most often used, one tailed T-test.

discovery learning: a Piagetian belief that children learn through self-discovery, aided by a teacher providing suitable materials, thereby stimulating intrinsic satisfaction.

discrete variable: measurement using of a discrete category (eg. Gender) as opposed to a continuous score (e.g height, weight, intelligence).

discrimination: unequal and unlawful treatment based upon race, colour, creed, religion, sex, national origin, age, disability, veteran status, or sexual orientation.

disengagement theory: mutual process of disengagement in activities expected by the individual and by society.

disorganised speech: one of the positive symptoms of schizophrenia, a disturbance whereby speech is disjointed and incoherent.  

displacement: forgetting in short-term memory, as a result of to new incoming information replacing the previous contents

dispositional attribution: when behavior is attributed to internal factors that are directly controllable by a person, e.g. an individuals effort or ability, as opposed to external factors (situational attributions), such as the weather or bad luck.

dissociative disorder: Is a condition, often caused by trauma, in which a person disconnects from a full awareness of self, time, or external circumstances as a defence against unpleasant realities or memories.

distal cause: a factor which has an indirect effect on behaviour, such as previous experiences in similar situations.

divided attention: the ability to divide our attentional processing between more than one task.

dizygotic twins (non-identical twins): twins that develop from different zygotes (eggs) and only share about fifty percent of their DNA.

door-in-the-face technique: a technique used to induce compliance, whereby individuals are first asked a large favour, followed by a smaller favour, which is more likely to be followed.

dopamine: a chemical neurotransmitter in the brain, important for learning and the experience of pleasure and reward.

dopamine hypothesis: argues that schizophrenia is based on over-activity of synapses that depend on dopamine.

double-bind theory: a theory of schizophrenia proposed by Bateson, which argues that faulty communication patterns within the family contribute to the onset of schizophrenia.

double-blind design: a form of experimental control, whereby both the subject and experimenter are kept uninformed about the purpose of the experiment, to reduce any forms of bias (in particular, experimenter bias).

Down's syndrome: a chromosomal disorder that is characterised by low IQ levels.

dreaming: a stage of sleep typified by the experience of visual imagery and rapid eye movements (REM).

drive reduction theory (of motivation): Hull's proposal that all behaviour is motivated and that motivation stems from the satisfaction of homeostatic drives (e.g. hunger and thirst). Stimuli (e.g. food and water) that decreases the drives subsequently reinforce the behaviour that led to them.

drug treatments: treatment of psychological disorders that are based on biological explanations of abnormal behavior. Treatment includes anti-anxiety drugs, anti-depressant drugs and anti-bipolar drugs.

dysfunctional: functioning incorrectly or abnormally.

dyslexia: 'developmental dyslexia' is used to explain difficulties with written and spoken language (across differing levels of intellect) that occurs as a result of development, whilst acquired dyslexia?occurs as a result of a stroke or similar injury, whereby language skills are impaired.