marasmus A disease of severe protein-calorie malnutrition during early infancy, in which growth stops, body tissues waste away, and the infant eventually dies.

massification The idea that establishing institutions of higher learning and encouraging college enrollment can benefit everyone (the masses).

massive open online course (MOOC) A course that is offered solely online for college credit. Typically, tuition is very low, and thousands of students enroll.

maximum life span The oldest possible age that members of a species can live under ideal circumstances. For humans, that age is approximately 122 years.

mean length of utterance (MLU) The average number of words in a typical sentence (called utterance, because children may not talk in complete sentences). MLU is often used to indicate how advanced a child’s language development is.

menarche A girl’s first menstrual period, signaling that she has begun ovulation. Pregnancy is biologically possible, but ovulation and menstruation are often irregular for years after menarche.

menopause The time in middle age, usually around age 50, when a woman’s menstrual periods cease and the production of estrogen, progesterone, and testosterone drops. Strictly speaking, menopause is dated one year after a woman’s last menstrual period, although many months before and after that date are menopausal.

metacognition “Thinking about thinking,” or the ability to evaluate a cognitive task in order to determine how best to accomplish it, and then to monitor and adjust one’s performance on that task.

middle childhood The period between early childhood and early adolescence, approximately from ages 6 to 11.

middle school A school for children in the grades between elementary and high school. Middle school usually begins with grade 6 and ends with grade 8.

midlife crisis A supposed period of unusual anxiety, radical self-reexamination, and sudden transformation that was once widely associated with middle age but that actually had more to do with developmental history than with chronological age.

mirror neurons Cells in an observer’s brain that respond to an action performed by someone else in the same way they would if the observer were actually performing that action.

modeling The central process of social learning, by which a person observes the actions of others and then copies them. (Modeling is also called observational learning.)

monozygotic (MZ) twins Twins who originate from one zygote that splits apart very early in development. (Also called identical twins.) Other monozygotic multiple births (such as triplets and quadruplets) can occur as well.

Montessori schools Schools that offer early-childhood education based on the philosophy of Maria Montessori, which emphasizes careful work and tasks that each young child can do.

morality of care In Gilligan’s view, moral principles that reflect the tendency of females to be reluctant to judge right and wrong in absolute terms because they are socialized to be nurturing, compassionate, and nonjudgmental.

morality of justice In Gilligan’s view, moral principles that reflect the tendency of males to emphasize justice over compassion, judging right and wrong in absolute terms.

moratorium An adolescent’s choice of a socially acceptable way to postpone making identity-achievement decisions. Going to college is a common example.

morbidity Disease. As a measure of health, morbidity usually refers to the rate of diseases in a given population—physical and emotional, acute (sudden) and chronic (ongoing).

mortality Death. As a measure of health, mortality usually refers to the number of deaths each year per 1,000 members of a given population.

motor skill The learned abilities to move some part of the body, in actions ranging from a large leap to a flicker of the eyelid. (The word motor here refers to movement of muscles.)

mourning The ceremonies and behaviors that a religion or culture prescribes for people to employ in expressing their bereavement after a death.

multifactorial Referring to a trait that is affected by many factors, both genetic and environmental, that enhance, halt, shape, or alter the expression of genes, resulting in a phenotype that may differ markedly from the genotype.

multifinality A basic principle of developmental psychopathology that holds that one cause can have many (multiple) final manifestations.

multiple intelligences The idea that human intelligence is composed of a varied set of abilities rather than a single, all-encompassing one.

myelination The process by which axons become coated with myelin, a fatty substance that speeds the transmission of nerve impulses from neuron to neuron.