H

habituation The process of becoming accustomed to an object or event through repeated exposure to it, and thus becoming less interested in it.

Hayflick limit The number of times a human cell is capable of dividing into two new cells. The limit for most human cells is approximately 50 divisions, an indication that the life span is limited by our genetic program.

Head Start A federally funded early-childhood intervention program for low-income children of preschool age.

head-sparing A biological mechanism that protects the brain when malnutrition disrupts body growth. The brain is the last part of the body to be damaged by malnutrition.

health care proxy A person chosen by another person to make medical decisions if the second person becomes unable to do so.

helicopter parents The label used for parents who hover (like a helicopter) over their emerging adult children. The term is pejorative, but parental involvement is sometimes helpful.

heritability A statistic that indicates what percentage of the variation in a particular trait within a particular population, in a particular context and era, can be traced to genes.

heterogamy Defined by developmentalists as marriage between individuals who tend to be dissimilar with respect to such variables as attitudes, interests, goals, socioeconomic status, religion, ethnic background, and local origin.

heterozygous Referring to two genes of one pair that differ in some way. Typically one allele has only a few base pairs that differ from the other member of the pair.

hidden curriculum The unofficial, unstated, or implicit rules and priorities that influence the academic curriculum and every other aspect of learning in a school.

high-stakes test An evaluation that is critical in determining success or failure. If a single test determines whether a student will graduate or be promoted, it is a high-stakes test.

hikikomori A Japanese word literally meaning “pull away”; it is the name of an anxiety disorder common among young adults in Japan. Sufferers isolate themselves from the outside world by staying inside their homes for months or even years at a time.

hippocampus A brain structure that is a central processor of memory, especially memory for locations.

holophrase A single word that is used to express a complete, meaningful thought.

home schooling Education in which children are taught at home, usually by their parents.

homeostasis The adjustment of all the body’s systems to keep physiological functions in a state of equilibrium. As the body ages, it takes longer for these homeostatic adjustments to occur, so it becomes harder for older bodies to adapt to stress.

homogamy Defined by developmentalists as marriage between individuals who tend to be similar with respect to such variables as attitudes, interests, goals, socioeconomic status, religion, ethnic background, and local origin.

homozygous Referring to two genes of one pair that are exactly the same in every letter of their code. Most gene pairs are homozygous.

hookup A sexual encounter between two people who are not in a romantic relationship. Neither intimacy nor commitment is expected.

hormone An organic chemical substance that is produced by one body tissue and conveyed via the bloodstream to another to affect some physiological function.

hormone replacement therapy (HRT) Taking hormones (in pills, patches, or injections) to compensate for hormone reduction. HRT is most common in women at menopause or after removal of the ovaries, but it is also used by men as their testosterone decreases. HRT has some medical uses but also carries health risks.

hospice An institution or program in which terminally ill patients receive palliative care to reduce suffering; family and friends of the dying are helped as well.

HPA (hypothalamus—pituitary—adrenal) axis A sequence of hormone production originating in the hypothalamus and moving to the pituitary and then to the adrenal glands.

HPG (hypothalamus—pituitary—gonad) axis A sequence of hormone production originating in the hypothalamus and moving to the pituitary and then to the gonads.

Human Genome Project An international effort to map the complete human genetic code. This effort was essentially completed in 2001, though analysis is ongoing.

humanism A theory that stresses the potential of all humans for good and the belief that all people have the same basic needs, regardless of culture, gender, or background.

hypothalamus A brain area that responds to the amygdala and the hippocampus to produce hormones that activate other parts of the brain and body.

hypothesis A specific prediction that can be tested.

hypothetical thought Reasoning that includes propositions and possibilities that may not reflect reality.