Magnetic resonance imaging (MRI) -  A method for studying the brain that uses magnetic fields and radio waves to produce pictures of the brain.

Major depressive disorder -  A disorder diagnosed after at least one major depressive episode.

Major depressive episode -  A period of at least two weeks marked by sadness or irritability and loss of interest in activities. Other symptoms may include changed sleeping or eating patterns, low energy, feelings of worthlessness or guilt, difficulty concentrating, and recurrent thoughts about suicide.

Managed care -  An arrangement for health care in which an organization, such as a health maintenance organization, acts as an intermediary between a person seeking care and a treatment provider.

Manifest content -  The plot of a dream.

Massed practice -  The process of learning material over a short period; also called cramming.

Matching hypothesis -  The idea that people tend to pick partners who are about equal in level of attractiveness to themselves.

Maturation -  Genetically programmed growth and development.

Mean -  The arithmetic average of a set of scores.

Measures of central tendency -  The mean, median, and mode.

Median -  The middle score in a set when all scores are arranged in order from lowest to highest.

Medical model -  A way of describing and explaining psychological disorders as if they are diseases.

Meditation -  The practice of focusing attention.

Medulla -  A part of the hindbrain that controls essential functions that are not under conscious control, such as breathing.

Melatonin -  A hormone that regulates the sleep cycle.

Memory -  The capacity for storing and retrieving information.

Menarche -  A woman’s first menstrual period.

Menopause -  The gradual, permanent cessation of menstruation.

Mental age -  The chronological age that typically corresponds to a particular level of performance. It is used as a measure of performance on intelligence tests.

Mental hospitals -  Medical institutions that specialize in providing treatment for psychological disorders.

Mental set -  A tendency to use only solutions that have worked in the past.

Mere exposure effect -  The tendency to like novel stimuli more if one encounters them repeatedly.

Metalinguistic awareness -  The capacity to think about how language is used.

Method of loci -  The process of imagining oneself physically in a familiar place in order to remember something.

Midbrain -  The part of the brain between the hindbrain and forebrain that is involved in locating events in space and that contains a dopamine-releasing system of neurons.

Midlife crisis -  A time of doubt and anxiety in middle adulthood.

Minnesota Multiphasic Personality Inventory (MMPI) -  A test developed to help clinical psychologists diagnose psychological disorders.

Misinformation effect -  The tendency for recollections of events to be distorted by information given after the event occurred.

Mnemonics -  Strategies for improving memory.

Mode -  The most frequently occurring score in a set of scores.

Monoamine oxidase inhibitors (MAOIs) -  A class of antidepressant drugs that increase the level of norepinephrine and serotonin.

Monocular cues -  Depth perception cues that require only one eye.

Monogenic traits -  Traits determined by a single gene.

Mood disorders -  Disorders characterized by marked disturbances in emotional state, which affect thinking, physical symptoms, social relationships, and behavior.

Moral reasoning -  The reasons and processes that cause people to think the way they do about right and wrong.

Morpheme -  The smallest meaningful unit in a language.

Motivated forgetting -  The idea that people forget things they don’t want to remember; also called psychogenic amnesia.

Motivation -  An internal process that makes a person move toward a goal.

Motive -  An impulse that causes a person to act.

Motor development -  The increasing coordination of muscles that makes physical movements possible.

Muller-Lyer illusion -  Illusion in which two lines of the same length appear to be different lengths because of different diagonal lines attached to the end of each line.

Mutations -  Small changes in genes.

Myelin sheath -  The fatty coating around some axons that increases the speed of neural impulse transmission.