adolescent growth spurt: rapid increase in the individual’s height and weight during puberty resulting from simultaneous release of growth hormones, thyroid hormones, and androgens. Males experience their growth spurt about two years later, on average, than females

adrenarche: an increase in the production of androgens by the adrenal cortex that usually occurs during the eighth or ninth year of life and typically peaks at around 10 to 14 years of age and is eventually involved in the development of pubic hair, body odor, skin oiliness, and acne

amygdala: part of the limbic system in the brain, which is involved with emotions and emotional responses and is particularly active during puberty

anorexia nervosa: an eating disorder characterized by self-starvation. Affected individuals voluntarily under-eat and often over-exercise, depriving their vital organs of nutrition. Anorexia can be fatal

axons: fibers that extend from the neurons and transmit electrochemical impulses from that neuron to the dendrites of other neurons

bed-sharing: when two or more people sleep in the same bed

binge-eating disorder: an eating disorder characterized by recurrent episodes of eating large quantities of food (often very quickly and to the point of discomfort); a feeling of a loss of control during the binge; experiencing shame, distress, or guilt afterwards; and not regularly using unhealthy compensatory measures (e.g., purging) to counter the binge eating. It is the most common eating disorder in the United States

body dissatisfaction: negative subjective evaluation of the weight and shape of one’s own body, which may predict the onset, severity, and treatment outcomes of eating disorders

body image: a person’s idea of how his or her body looks

bulimia nervosa: an eating disorder characterized by binge eating and subsequent purging, usually by induced vomiting and/or use of laxatives

cephalocaudal: refers to growth and development that occurs from the head down

circumcision: the surgical removal of the foreskin of the penis

colostrum: the first secretion from the mammary glands after giving birth, rich in antibodies

cortex: the outer layers of the brain in humans and other mammals. Most thinking, feeling, and sensing involves the cortex

co-sleeping: a custom in which parents and their children (usually infants) sleep together in the same room

dendrites: fibers that extend from neurons and receive electrochemical impulses transmitted from other neurons via their axons

dopamine: a neurotransmitter in the brain that plays a role in pleasure and the reward system; increases in the limbic system and later in the prefrontal cortex during adolescence

estrogen: primary female sex hormone that is responsible for the development and regulation of the female reproductive system and secondary sex characteristics

failure to thrive: decelerated or arrested physical growth (height and weight measurements fall below the third or fifth percentile or a downward change in growth across two major growth percentiles) and is associated with abnormal growth and development

fine motor skills: physical abilities involving small body movements, especially of the hands and fingers, such as drawing and picking up a coin. The word “fine” in this context means “small”

frontal lobes: the parts of the brain involved in impulse control, planning, and higher-order thinking; still developing in adolescence

gonad: a sex organ that produces gametes; specifically, a testicle or ovary

gonadarche: refers to the earliest gonadal changes of puberty. In response to pituitary gonadotropins, the ovaries in girls and the testes in boys begin to grow and increase the production of sex steroids, especially estradiol and testosterone

gross motor skills: voluntary movements including the use of large muscle groups such as the arms and legs. The word “gross” in this context means “big”

immunization: a process that stimulates the body’s immune system by causing the production of antibodies to defend against attack by a specific contagious disease

infantile marasmus: starvation due to a lack of calories and protein

kwashiorkor: also known as the “disease of the displaced child,” results in a loss of appetite and swelling of the abdomen as the body begins to break down the vital organs as a source of protein

limbic system: structures in the brain (including the amygdala) that involve processing emotional experience and social information and determining rewards and punishments; develops years before the prefrontal cortex

malnutrition: a condition that results from eating a diet in which one or more nutrients are deficient

masturbation: sexual self-stimulation, usually achieved by touching, stroking, or massaging the male or female genitals until this triggers an orgasm

melatonin: sleep hormone whose levels rise later at night and decrease later in the morning for teens, compared to children and adults

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

milk anemia: an iron deficiency in infants who have been maintained on a milk diet for too long

motor skills: the word “motor” refers to the movement of the muscles. Motor skills refer to our ability to move our bodies and manipulate objects

muscle dysmorphia: sometimes called “reverse anorexia,” this is an obsession with being small and underdeveloped; extreme concern with becoming more muscular

myelin: a coating of fatty tissues around the axon of the neuron

myelination: an aspect of brain maturation in which more myelin is formed around the axons of neurons, thereby increasing neural transmission

neurons: nerve cells in the central nervous system, especially in the brain

neurotransmitters: brain chemicals that carry information from the axon of a sending neuron to the dendrites of a receiving neuron

percentile: a point on a ranking scale of 0 to 100. The 50th percentile is the midpoint; half of the infants in the population being studied rank higher, and half rank lower

perception: the process of interpreting what is sensed

pincer grasp: a developmental milestone that typically occurs at 9 to 12 months of age; the coordination of the index finger and thumb to hold smaller objects; represents a further development of fine motor skills

prefrontal cortex: the area of the cortex at the very front of the brain that specializes in anticipation, planning, and impulse control

primary sex characteristics: the parts of the body that are directly involved in reproduction, including the vagina, uterus, ovaries, testicles, and penis

proximodistal: development that occurs from the center or core of the body in an outward direction

pruning: the process by which unused connections in the brain atrophy and die

puberty: the period of rapid growth and sexual development that begins in adolescence

reflexes: the inborn behavioral patterns that develop during uterine life and are fully present at birth. These are involuntary movements (not learned) or actions that are essential for a newborn’s survival immediately after birth and include: sucking, swallowing, blinking, urinating, hiccuping, and defecating

secondary sex characteristics: physical traits that are not directly involved in reproduction but that indicate sexual maturity, such as a man’s beard or a woman’s breasts

sensation: the interaction of information with the sensory receptors

serotonin: “calming chemical,” a neurotransmitter in the brain involved with the regulation of mood and behavior; serotonin levels increase in the limbic system during adolescence

sexually transmitted infections (STIs): diseases spread by sexual contact, including syphilis, gonorrhea, genital herpes, chlamydia, and HIV/AIDS

spermarche: a boy’s first ejaculation of sperm. Erections can occur as early as infancy, but ejaculation signals sperm production. Spermarche may occur during sleep (nocturnal emission or “wet dream”) or via direct stimulation

sudden infant death syndrome (SIDS): a situation in which a seemingly healthy infant, usually between 2 and 6 months old, suddenly stops breathing and dies unexpectedly while asleep

synapses: the intersection between the axon of one neuron to the dendrites of another neuron

synaptic pruning: the selective elimination of non-essential synapses and the strengthening of important neural connections

testosterone: the primary male sex hormone that plays a key role in developing male reproductive tissues such as testes and prostate and promoting secondary sexual characteristics such as increased muscle and bone mass and the growth of body hair. Females also produce testosterone, but at a lower level than males

transient exuberance: the great, but temporary increase in the number of dendrites that develop in an infant’s brain during the first two years of life