Human Development


Adolescence is a time of rapid physical change during which sexual maturation, rapid growth, and many physiological and hormonal changes occur, some of which are different for males and females.

Adolescence is a human life stage spanning the transition between childhood and adulthood. It involves rapid physical and emotional change. The World Health Organization considers adolescents to be individuals of ages 10 to 19 years. In physiological terms, adolescence can be defined as the period beginning with secretion of pituitary and hypothalamic hormones that stimulate development of reproductive organs. Adolescence ends when full adult height is reached. Often adolescence is defined more narrowly as puberty, which physiologically is the first several years of adolescence. Puberty is the stage of development when the human body becomes capable of reproduction. Puberty ends in females when they experience their first menstrual period and in males with their first ejaculation of viable sperm. In North America girls typically hit puberty at around 11 to 12 years and boys at 12 to 13 years. The onset of puberty varies depending on genetics of the parents, as well as environmental factors, such as nutrition and overall health. Adolescents living in areas in which there is access to balanced nutrition may undergo puberty earlier than adolescents in areas with minimal nutrition. Physical changes of adolescence continue for many years after puberty.

Adolescence also includes significant cognitive developments, such as the development of systematic thinking or strategic thinking about problem-solving. These rapid transformations typically affect emotions and behaviors as individuals adapt to their developing bodies. The onset of adolescence can vary significantly across individuals, both within and between different cultures.

Adolescence involves very different changes in females and males. In both, these are hormone-driven effects leading to sexual maturation. In both sexes, the maturing hypothalamus secretes rising levels of gonadotropin-releasing hormone. Gonadotropin-releasing hormone (GnRH) triggers the secretion by the pituitary of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In females, FSH causes ovarian follicles to develop. Ovarian follicles secrete estrogen, progesterone, inhibin, and some androgen. Androgen and estrogen are classes of hormones, while progesterone and inhibin are more specific. Estrogens, particularly estradiol, and androgens stimulate most physical changes in females undergoing puberty. Onset of puberty begins with breast development, or thelarche, triggered by estrogen, progesterone, and prolactin. Breast development continues until approximately age 20. After thelarche comes pubarche. Triggered by androgens, pubarche marks the growth of pubic and axillary hair (hair under the armpit) and the development of sebaceous and axillary glands. Androgens also stimulate the libido (sex drive). Menarche is next—the first menstrual period. Sufficient fat stores are required to support pregnancy. Menstruation is not reached in females with little body fat (typically less than 17 percent) and typically ceases in females with less than 22 percent body fat. Estradiol triggers many other changes, including growth of ovaries and secondary sex organs, as well as rapid increase in height. The pelvis widens, and fat is deposited in breasts, buttocks, thighs, hips, labia majora, and the mons pubis. Progesterone prepares the uterus for possible pregnancy.

Puberty in Females

Stage of Puberty Thelarche Pubarche Menarche
Physical changes Breast development
  • Growth of pubic and axillary hair
  • Development of sebaceous and axillary glands
  • Stimulation of libido
  • First menstrual period
  • Growth of ovaries and secondary sex organs
  • Increase in height
  • Widening of pelvis and fat is deposited in breasts, buttocks, thighs, hips, labia majora, and mons pubis
  • Preparation of the uterus for pregnancy
Hormonal trigger Estrogen, progesterone, prolactin Androgens Estradiol and progesterone

Puberty in females is defined by three stages: thelarche, pubarche, and menarche. Each involves a series of physical and physiological changes, each the result of additional hormone triggers.

Although testes in males release testosterone during the early months of fetal development, they soon become dormant. Eventually the hypothalamus secretes hormones that initiate the process of puberty. The hormones stimulate the pituitary, which secretes hormones that stimulate activity in the testes. The hormones cause the testes to begin producing testosterone again. Beginning with puberty, reproductive development and function are controlled by the brain-testicular axis—hormone connections between the pituitary, hypothalamus, and gonads. These mechanisms are similar to those in females, although they drive different changes. As in females, rising GnRH levels trigger the pituitary to secrete FSH and LH. In males, these hormones stimulate testicular cells, triggering testes enlargement, the initial change in puberty. LH stimulates production of androgens, particularly testosterone. FSH stimulates release of androgen-binding proteins, which are critical to testosterone's function. Testosterone prompts many major changes in males. It stimulates spermatogenesis and the development of secondary sex characteristics. First, the scrotum and testes enlarge. Next, the penis. Pubic, axillary, and facial hair begin to grow. Skin becomes thicker and secretes more sebum (oil), which can produce acne. Sweat glands develop in many body regions. Reproductive structures—ducts and accessory glands—enlarge so that erections occur often; ejaculation may take place during sleep. Growth rate surges, muscle mass increases, and the larynx grows, which causes males' voices to deepen. Metabolic rate increases, and the libido is stimulated.

Changes in Adolescence

During adolescence, females and males both go through rapid and distinctive changes in physical development as they transition from childhood to adulthood.