10 Developmental Psychology
STEP 4 Review the Knowledge You Need to Score High
Physical development focuses on maturation and critical periods. A critical period is a time interval during which specific stimuli have a major effect on development that the stimuli do not produce at other times.
Prenatal development begins with fertilization, or conception, and ends with birth. The zygote is a fertilized ovum with the genetic instructions for a new individual normally contained in 46 chromosomes. (See Genetics and Behavior in Chapter 7.) During the first 2 weeks following conception, the zygote divides again and again, forming first a hollow ball of cells that buries itself in the wall of the uterus and then a three-layered inner cell mass surrounded by outer cells attached to the uterine lining. Different genes function in cells of the three different layers; the forming individual is now considered an embryo. During the embryonic period from the third through the eighth weeks following conception, organs start to develop as a result of differentiation and specialization of cells, and the placenta, umbilical cord, and other structures form from the outer cells. As organs develop, the embryo is particularly sensitive to environmental stimuli such as chemicals and viruses. Nutrients, oxygen, wastes, and other substances pass from the woman’s blood into and out of the developing organism through the placenta. By the end of the eighth week, the embryo has a head with partially formed eyes, limbs, and a skeleton composed of cartilage. At this point, all organs are present in rudimentary form, and the developing individual begins to resemble a human; it is a fetus, the developing human organism from about 9 weeks after conception to birth. During the fetal period, the organ systems begin to interact, bone replaces cartilage in the skeleton, and sex organs and sense organs become more refined.
Birth defects can result from a malfunctioning gene or an environmental stimulus. If the pregnant woman takes in poisonous chemicals or gets infected with a virus, developmental errors can result in birth defects that are not hereditary. Chemicals such as alcohol, drugs, tobacco ingredients, mercury, lead, cadmium, and other poisons or infectious agents, such as viruses, that cause birth defects are called teratogens. The specific nature of a birth defect depends on which structures are developing at the time of exposure. Most birth defects develop during the embryonic period and are usually more severe than problems that develop later. The critical period for eyes, ears, arms, legs, and the heart is typically the first 3 months (first trimester) of pregnancy, while the critical period for damage to the reproductive system extends across the first and second trimesters of pregnancy. The critical period for the nervous system is all three trimesters. Because of the long critical period for brain development, many kinds of brain damage can result, the most severe during the embryonic and early fetal periods. A recent example of a teratogen is the Zika virus carried by mosquitoes that caused microcepholy.
Fetal alcohol syndrome (FAS) is a cluster of abnormalities that occurs in babies of mothers who drink alcoholic beverages during pregnancy. Low intelligence, a small head with flat face, misshapen eyes, a flat nose, and thin upper lip characterize children with FAS. Intellectual impairment ranges from minor learning disabilities to severe intellectual disability. The more severe intellectual disability results from exposure of the embryo/fetus during the early months of pregnancy. Currently, FAS is the leading cause of intellectual developmental disorder in the United States. Cigarette smoking during pregnancy is associated with miscarriage, stillbirth, premature delivery, and low birth weight. Heroin- and cocaine-exposed fetuses that are born live may undergo withdrawal symptoms and may be distractable and unable to concentrate. Researchers have difficulty pinpointing other prenatal effects of cocaine and heroin because affected babies are often exposed to additional substances and situations that can account for other symptoms.
Malnutrition, as well as prescription and over-the-counter drugs, can cause birth defects. Even certain nutrients ingested in large quantities can be teratogenic. For example, high doses of vitamin A can cause heart, nervous system, and facial defects. Viruses such as rubella can pass into the placenta and cause birth defects. During the first trimester of pregnancy, rubella can cause cataracts, deafness, and heart defects; during later weeks effects include learning disabilities, speech and hearing problems, and Type 1 diabetes.
Behavior of the Neonate
At birth, neonates, or newborn babies, are equipped with basic reflexes that increase their chances of survival. A sequence of rooting, sucking, and swallowing reflexes enables the neonate to get food. Rooting is the neonate’s response of turning his or her head when touched on the cheek and then trying to put the stimulus into his or her mouth. What touches the newborn’s cheek is frequently a nipple. Sucking is the automatic response of drawing in anything at the mouth. Swallowing is a contraction of throat muscles that enables food to pass into the esophagus without the neonate choking. The lack of some reflexes in a neonate can indicate possible brain damage to neuropsychologists. Among those they test are the grasping reflex, when the infant closes his or her fingers tightly around an object put in his or her hand, and the Moro or startle reflex, in which a loud noise or sudden drop causes the neonate to automatically arch his or her back, fling his/her limbs out, and quickly retract them. As the infant matures, developing voluntary control over behaviors, many of the reflexes disappear.
Neonates’ adaptive behavior is not limited to reflexes; they also show behaviors that facilitate social interactions. Shortly after birth, infants respond to the human face, voice, and touch. They show a preference for the voice and odor of their mothers. Their vision is best for objects normally about the distance from the infant’s eyes to the caretaker’s face. They can track objects with their eyes when they are only a few days old. Infants can distinguish among different colors, and they prefer certain complex patterns, such as the human face. Newborns also prefer sweet and salty tastes. Their sense of hearing is well developed at birth and typically the dominant sense during the baby’s first months. As structures in the eye and brain develop during infancy, visual acuity (clarity of vision) and depth perception improve, so that sight normally becomes a more dominant sense sometime during the second half of the baby’s first year.
How do psychologists know this information if babies can’t talk? Psychologists depend on gazes, sucking, and head turning measured by sophisticated computerized equipment in response to changes in stimuli to reveal abilities of infants. For example, when infants are shown a stimulus for the first time, they gaze at it for a length of time. With repeated presentations of that stimulus, they look away sooner. If a new stimulus is presented and the infant can remember and discriminate between the two stimuli, the infant will look at the new stimulus longer than if he or she perceives no difference between the first and second stimulus. These are called habituation studies. Habituation is decreasing responsiveness with repeated presentation of the same stimulus.
The First 2 Years
An infant’s physical development during the first 2 years is amazing. Brain development proceeds rapidly from the prenatal period, during which about 20 billion brain cells are produced, through the baby’s first 2 years, during which dendrites proliferate in neural networks, especially in the cerebellum, then in occipital and temporal lobes as cognitive abilities grow. Body proportions change as the torso and limbs grow more quickly so that the head is less out of proportion to body size. Physical development of the musculoskeletal system from head to tail, and from the center of the body outward, accompanies nervous system maturation to enable the baby to lift its head, roll over, sit, creep, stand, and walk, normally in that order. Maturation, motor and perceptual skills, motivation, and environmental support all contribute to development of new behaviors. During childhood, proliferation of dendrites proceeds at a rapid rate, especially in the frontal cortex.
The next growth spurt comes in adolescence, following a dramatic increase in production of sex hormones. The defining feature of puberty is sexual maturation, marked by the onset of the ability to reproduce. Primary sex characteristics, reproductive organs (ovaries and testes) start producing mature sex cells, and external genitals (vulva and penis) grow. So do secondary sex characteristics—nonreproductive features associated with sexual maturity—such as widening of hips and breast development in females; growth of facial hair, muscular growth, development of the “Adam’s apple,” and deepening of the voice in males; and growth of pubic hair and underarm hair in both. Girls begin their growth spurt about two years before their first menstrual period (menarche), typically at age . Early maturation of females can put them at a social disadvantage, whereas early maturation of males can put them at a social advantage. Boys start their growth spurt about 2 years later than girls but about 2 years before ejaculation of semen with viable sperm.
During adolescence, changes in the brain include selective pruning of unused dendrites with further development of the emotional limbic system, followed by frontal lobe maturation. Maturation of the prefrontal/frontal cortex involved in judgment and decision making continues into early adulthood. This disconnect between the physical maturation and the mental maturation can lead to challenges with risky behavior as the prefrontal cortex has not had adequate time to develop.
By our mid-20s, our physical capabilities peak, followed by first almost imperceptible, then accelerating, decline. According to evolutionary psychologists, peaking at a time when both males and females can provide for their children maximizes chances of survival for our species. Decreased vigor, changes in fat distribution, loss of hair pigmentation, and wrinkling of the skin are changes associated with advances in age. In females at about age 50, menopause—cessation of the ability to reproduce—is accompanied by a decrease in production of female sex hormones. Men experience less frequent erections and a more gradual decline in reproductive function as they age. Typically, as adults age, the lenses of their eyes thicken, letting less light reach the retina and worsening vision for near objects; the ability to detect high-pitched sounds decreases; and sensitivity to tastes, odors, and temperature may decrease. Neural processes slow and parts of the brain begin to atrophy. Physical signs of aging can be slowed, and to some extent reversed, if we stay physically, mentally, and socially active and have a healthy diet. Heart disease, stroke, cancer, Alzheimer’s, and other degenerative diseases are often terminal diseases of old age. Since they do not interfere with reproductive success, genes involved in these diseases do not incur any selective pressures. On average, men die about 4 years earlier than women.