Hallucinations -  Sensory or perceptual experiences that happen without any external stimulus.

Hallucinogens -  Drugs that cause sensory and perceptual distortions.

Health psychology -  A branch of psychology that focuses on the relationship between psychosocial factors and the emergence, progression, and treatment of illness.

Heritability -  A mathematical estimate that indicates how much of a trait’s variation in a population can be attributed to genetic factors.

Heuristic -  A general rule of thumb that may lead to, but doesn’t guarantee, a correct solution to a problem.

Hierarchical classification -  The ability to classify according to more than one level.

Hierarchy of needs theory -  The idea, proposed by Abraham Maslow, that people are motivated by needs on four levels. Maslow believed people pay attention to higher needs only when lower needs are satisfied.

Higher-order conditioning -  In classical conditioning, the process by which a neutral stimulus comes to act as a conditioned stimulus by being paired with another stimulus that already evokes a conditioned response.

Hindbrain -  Portion of the brain consisting of the medulla, the pons, and the cerebellum.

Hindsight bias -  The tendency to interpret the past in a way that fits the present.

Hippocampus -  A part of the limbic system involved in memory.

Histogram or bar graph -  A plot that shows how data are distributed.

Histrionic personality -  A personality type characterized by a desire to be the center of attention and the tendency to be self-focused, excitable, highly open to suggestion, very emotional, and dramatic.

Histrionic personality disorder  -  A disorder characterized by attention-seeking behavior and shallow emotions.

Homeostasis -  Maintenance of a state of physiological equilibrium in the body.

Hormones -  Chemicals that are produced in glands and released into the bloodstream, involved in regulating body functions.

Humanism -  A school of thought that encourages seeing people’s lives as those people would see them.

Humanistic therapies -  Therapies aimed at helping people accept themselves and free themselves from unnecessary limitations.

Hypnosis -  A procedure in which suggestions are made to a person.

Hypochondriasis -  A disorder in which a person has constant fears of having a serious disease.

Hypothalamus -  A part of the forebrain that helps to control the pituitary gland, the autonomic nervous system, body temperature, and biological drives.

Hypothesis -  A testable prediction of what is going to happen given a certain set of conditions.