Bipolar disorder: a mood disorder in which there are depressive and manic (elated) episodes.
Comorbidity: the presence of two or more disorders in a given individual at the same time.
Concordance rate: if one twin has a disorder or condition, the likelihood that the other twin also has it.
Diathesis–stress model: the notion that psychological disorders occur when there is a genetically determined vulnerability (diathesis) and relevant stressful conditions.
Dysthymic disorder: a condition involving depressive symptoms that occur much of the time over a period of at least 2 years.
Electroconvulsive therapy (ECT): a form of therapy used to treat depressed patients, in which brain seizures are created by passing an electric current through the head.
Endocrine system: a system of a number of ductless glands located throughout the body that produce the body’s chemical messengers, called hormones.
Free association: a technique used in psychoanalysis, in which the patient says the first thing that comes into his/her mind.
Learned helplessness: passive behaviour produced by the perception that punishment is unavoidable.
Major depressive disorder: a disorder characterised by symptoms such as sad depressed mood, tiredness, and loss of interest in various activities.
Meta-analysis: an analysis in which all of the findings from many studies relating to a given hypothesis are combined for statistical testing.
Neurotransmitters: chemical substances that are released at the junction between neurons (a synapse) and that affect the transmission of messages in the nervous system.
Placebo effect: positive responses to a drug or form of therapy based on the patient’s beliefs that the drug or therapy will be effective, rather than on the actual make-up of the drug or therapy.
Reinforcement: a behaviour is more likely to re-occur because the response was agreeable. Both positive and negative reinforcement have agreeable consequences.
Reliability: the extent to which a method of measurement or a research study produces consistent findings across situations or over time.
Repression: the process of forcing very threatening thoughts and memories out of the conscious mind in
Freudian theory; motivated forgetting.
Seasonal affective disorder: a disorder that nearly always involves the sufferer experiencing severe depression during winter months.
Treatment aetiology fallacy: the mistaken belief that the effectiveness of a form of treatment indicates the cause of a disorder.
Validity: the extent to which something is true. This can be applied to a measurement tool, such as a psychological test, or to the “trueness” of an experimental procedure both in terms of what goes on within the experiment (internal validity) and its relevance to other situations (external validity).