3.2 Physical Development Across the Lifespan

Prenatal development can be shaped by many influences. Teratogens are harmful agents, such as alcohol, nicotine, some medications and drugs, and some viruses, that can cross into the developing baby and cause harm. Heavy drinking during pregnancy, for example, can cause fetal alcohol syndrome, which includes lasting effects on physical growth, learning and behavior. Maternal illness, genetic mutations, hormones and other environmental factors (like the mother's nutrition and stress) also influence prenatal development.

You won't be tested on the stages of prenatal development (zygote, embryo and fetus).

Newborns come with reflexes, automatic responses that show the nervous system is developing on track. In the rooting reflex, a baby whose cheek is touched turns toward the touch and opens their mouth, which helps them find food. Other reflexes include sucking and grasping a finger placed in their palm.

Motor skills develop in a mostly fixed order: babies lift their heads, roll over, sit, crawl, stand and then walk. The timing varies a lot; some babies walk at 9 months and others at 15 months, and both are typical. Gross motor skills use large muscles (crawling, running), while fine motor skills use small, precise movements (picking up a raisin, holding a crayon). These skills depend on maturation, the biological growth that unfolds on its own schedule, and they let children become more independent.

Research with the visual cliff tested depth perception in infants. Babies were placed on a glass table with a checkerboard pattern right under the glass on one side and far below the glass on the other, creating the look of a drop. Most crawling infants wouldn't cross the 'deep' side even when their mother called them. The design was also an innovative way to measure what infants perceive without needing them to talk.

A critical period is a window early in life when certain experiences are needed for normal development. A sensitive period is a window when learning happens most easily, though it's still possible later. Language is the clearest human example: children who aren't exposed to language early in life have great difficulty mastering grammar later, and a second language learned in childhood is more likely to be spoken without an accent.

Imprinting is a dramatic example in some animals. Konrad Lorenz showed that newly hatched goslings follow and attach to the first moving object they see, normally their mother, but in his studies, Lorenz himself. This helps the young animal survive by staying near a caregiver.

Adolescence begins with puberty, the period of sexual maturation when a person becomes able to reproduce. It includes a growth spurt of rapid increases in height and weight. Primary sex characteristics are the reproductive organs themselves, which mature during puberty. Secondary sex characteristics are other physical features, such as breast development, widening hips, facial and body hair and a deepening voice. Menarche is the first menstrual period, and spermarche is the first ejaculation.

The brain also keeps maturing. The emotional limbic system develops earlier than the prefrontal cortex, which handles judgment and impulse control and continues developing into the mid-20s. This helps explain why adolescents can be more prone to risk-taking, especially around peers.

Physical abilities generally peak in early adulthood, level off and then decline gradually, at very different rates for different people. Reaction time slows, flexibility and mobility decrease, vision weakens (especially for close objects), and hearing for high-pitched sounds declines. Reproductive ability also declines; in menopause, menstruation ends and a woman can no longer become pregnant, typically around age 50. Exercise, good nutrition and staying active slow many of these changes.