Developmental psychology examines our physical, cognitive, and social development across the life span, with a focus on three major issues:
Let's reflect now on these three development issues.
The gene combination created when our mother's egg engulfed our father's sperm helped form us, as individuals. Genes predispose both our shared humanity and our individual differences.
But it is also true that our experiences form us. In the womb, in our families, and in our peer social relationships, we learn ways of thinking and acting. Even differences initiated by our nature may be amplified by our nurture. We are not formed by either nature or nurture, but by their interrelationships-their interaction. Biological, psychological, and social-cultural forces interact.
Mindful of how others differ from us, however, we often fail to notice the similarities stemming from our shared biology. Regardless of our culture, we humans share the same life cycle. We speak to our infants in similar ways and respond similarly to their coos and cries (Bomstein et al., 1992a,b). All over the world, children of warm and supportive parents feel better about themselves and are less hostile than are the children of punishing and rejecting parents (Rohner, 1986; Scott et al., 1991). Although ethnic groups differ in school achievement and delinquency, the differences are "no more than skin deep." To the extent that family structure, peer influences, and parental education predict behavior in one of these ethnic groups, they do so for the others as well. Compared with the person-to-person differences within groups, the differences between groups are small.
Do adults differ from infants as a giant redwood differs from its seedling – a difference created by gradual, cumulative growth? Or do they differ as a butterfly differs from a caterpillar-a difference of distinct stages? Generally speaking, researchers who emphasize experience and learning see development as a slow, continuous shaping process. Those who emphasize biological maturation tend to see development as a sequence of genetically predisposed stages or steps: Although progress through the various stages may be quick or slow, everyone passes through the stages in the same order.
Are there clear-cut stages of psychological development, as there are physical stages such as walking before running? The stage theories of Jean Piaget on cognitive development, Lawrence Kohlberg on moral development, and Erik Erikson on psychosocial development propose that such stages do exist (as summarized in FIGURE 45.1). But some research casts doubt on the idea that life proceeds through neatly defined, age-linked stages. Young children have some abilities Piaget attributed to later stages. Kohlbergs work reflected a worldview characteristic of individualist cultures and emphasized thinking over acting. And adult life does not progress through a fixed, predictable series of steps. Chance events can influence us in ways we would never have predicted.
Nevertheless, the concept of stage remains useful. The human brain does experience growth spurts during childhood and puberty that correspond roughly to Piaget's stages (Thatcher et al., 1987). And stage theories contribute a developmental perspective on the whole life span, by suggesting how people of one age think and act differently when they arrive at a later age.
As we follow lives through time, do we find more evidence for stability or change? If reunited with a long-lost grade-school friend, do we instantly realize that "it's the same old Andy"? Or do people we befriend during one period of life seem like strangers at a later period? (At least one acquaintance of mine would choose the second option. He failed to recognize a former classmate at his 40-year college reunion. The aghast classmate pointed out that she was his long-ago first wife.)
Research reveals that we experience both stability and change. Some of our characteristics, such as temperament (our emotional reactivity and intensity), are very stable:
"As at 7, so at 70," says a Jewish proverb. The widest smilers in childhood and college photos are, years later, the ones most likely to enjoy enduring marriages (Hertenstein et al., 2009). While one in four of the weakest college smilers eventually divorced, only 1 in 20 of the widest smilers did so. As people grow older, personality gradually stabilizes (Ferguson, 2010; Hopwood et al., 2011; Kandler et al., 2010). The struggles of the present may be laying a foundation for a happier tomorrow.
We cannot, however, predict all of our eventual traits based on our early years of life (Kagan et al., 1978, 1998). Some traits, such as social attitudes, are much less stable than temperament, especially during the impressionable late adolescent years (Krosnick & Alwin, 1989; Moss & Susman, 1980). Older children and adolescents learn new ways of coping. Although delinquent children have elevated rates of later work problems, substance abuse, and crime, many confused and troubled children blossom into mature, successful adults (Moffitt et al., 2002; Roberts et al., 2001; Thomas & Chess, 1986). Happily for them, life is a process of becoming.
In some ways, we all change with age. Most shy, fearful toddlers begin opening up by age 4, and most people become more conscientious, stable, agreeable, and self-confident in the years after adolescence (Lucas & Donnellan, 2009; Roberts et al., 2003, 2006, 2008; Shaw et al., 2010). Many irresponsible 16-year-olds have matured into 40-year-old business or cultural leaders. (If you are the former, you aren't done yet.) Such changes can occur without changing a person's position relative to others of the same age. The hard-driving young adult may mellow by later life, yet still be a relatively driven senior citizen.
Life requires both stability and change. Stability provides our identity. It enables us to depend on others and be concerned about the healthy development of the children in our lives. Our trust in our ability to change gives us our hope for a brighter future. It motivates our concerns about present influences and lets us adapt and grow with experience.
Nothing is more natural than a species reproducing itself. And nothing is more wondrous. With humans, the process starts when a woman's ovary releases a mature egg-a cell roughly the size of the period at the end of this sentence. The woman was born with all the immature eggs she would ever have, although only 1 in 5000 will ever mature and be released. A man, in contrast, begins producing sperm cells at puberty. For the rest of his life, 24 hours a day, he will be a nonstop sperm factory, with the rate of production-in the beginning more than 1000 sperm during the second it takes to read this phrase-slowing with age.
Like space voyagers approaching a huge planet, the 200 million or more deposited sperm begin their race upstream, approaching a cell 85,000 times their own size. The relatively few reaching the egg release digestive enzymes that eat away its protective coating (FIGURE 45.2a). As soon as one sperm penetrates that coating and is welcomed in (Figure 45.2b), the egg's surface blocks out the others. Before half a day elapses, the egg nucleus and the sperm nucleus fuse. The two have become one. Consider it your most fortunate of moments. Among 200 million sperm, the one needed to make you, in combination with that one particular egg, won the race. And so it was for innumerable generations before us. If anyone of our ancestors had been conceived with a different sperm or egg, or died before conceiving, or not chanced to meet the partner or ... the mind boggles at the improbable, unbroken chain of events that produced you and me.
Fewer than half of all fertilized eggs, called zygotes, survive beyond the first 2 weeks (Grobstein, 1979; Hall, 2004). But for you and me, good fortune prevailed. One cell became 2, then 4-each just like the first-until this cell division had produced some 100 identical cells within the first week. Then the cells began to differentiate-to specialize in structure and function. How identical cells do this - as if one decides "I'll become a brain, you become intestines!" - is a puzzle that scientists are just beginning to solve.
About 10 days after conception, the zygote attaches to the mother's uterine wall, beginning approximately 37 weeks of the closest human relationship. The zygote's inner cells become the embryo (FIGURE 45.3a) . The outer cells become the placenta, the life-link that transfers nutrients and oxygen from mother to embryo. A healthy and well-nourished mother helps form a healthy baby-to-be. Over the next 6 weeks, the embryo's organs begin to form and function. The heart begins to beat.
For 1 in 270 sets of parents, though, there is a bonus. Two heartbeats will reveal that the zygote, during its early days of development, has split into two. If all goes well, two genetically identical babies will start life together some 8 months later (Module 14).
By 9 weeks after conception, an embryo looks unmistakably human (Figure 45.3b). It is now a fetus (Latin for "offspring" or "young one"). During the sixth month, organs such as the stomach have developed enough to give the fetus a good chance of survival if born prematurely.
At each prenatal stage, genetic and environmental factors affect our development. By the sixth month, microphone readings taken inside the uterus reveal that the fetus is responsive to sound and is exposed to the sound of its mother's muffled voice (Ecklund-Flores, 1992; Hepper, 2005). Immediately after birth, emerging from living 38 or so weeks underwater, newborns prefer her voice to another woman's or to their father's (Busnel et al., 1992; DeCasper et al., 1984, 1986, 1994). They also prefer hearing their mother's language. If she spoke two languages during pregnancy, they display interest in both (Byers-Heinlein et al., 2010). And just after birth, the melodic ups and downs of newborns' cries bear the tuneful signature of their mother's native tongue (Mampe et al., 2009). Babies born to French-speaking mothers tend to cry with the rising intonation of French; babies born to German-speaking mothers cry with the falling tones of German. Would you have guessed? The learning of language begins in the womb.
In the 2 months before birth, fetuses demonstrate learning in other ways, as when they adapt to a vibrating, honking device placed on their mother's abdomen (Dirix et al., 2009). Like people who adapt to the sound of trains in their neighborhood, fetuses get used to the honking. Moreover, 4 weeks later, they recall the sound (as evidenced by their blasé response, compared with reactions of those not previously exposed).
Sounds are not the only stimuli fetuses are exposed to in the womb. In addition to transferring nutrients and oxygen from mother to fetus, the placenta screens out many harmful substances, but some slip by. Teratogens, agents such as viruses and drugs, can damage an embryo or fetus. This is one reason pregnant women are advised not to drink alcoholic beverages. A pregnant woman never drinks alone. As alcohol enters her bloodstream, and her fetus', it depresses activity in both their central nervous systems. Alcohol use during pregnancy may prime the woman's offspring to like alcohol and may put them at risk for heavy drinking and alcohol use disorder during their teens. In experiments, when pregnant rats drank alcohol, their young offspring later displayed a liking for alcohol's taste and odor (Youngentob et al., 2007, 2009).
Even light drinking or occasional binge drinking can affect the fetal brain (Braun, 1996; Ikonomidou et al., 2000; Sayal et al., 2009). Persistent heavy drinking puts the fetus at risk for birth defects and for future behavior problems, hyperactivity, and lower intelligence. For 1 in about 800 infants, the effects are visible as fetal alcohol syndrome (FAS), marked by lifelong physical and mental brain abnormalities (May & Gossage, 2001). The fetal damage may occur because alcohol has an epigenetic effect: It leaves chemical marks on DNA that switch genes abnormally on or off (Liu et al., 2009).
Babies come with software preloaded on their neural hard drives. Having survived prenatal hazards, we as newborns came equipped with automatic reflex responses ideally suited for our survival. We withdrew our limbs to escape pain. If a cloth over our face interfered with our breathing, we turned our head from side to side and swiped at it.
New parents are often in awe of the coordinated sequence of reflexes by which their baby gets food. Thanks to the rooting reflex, when something touches their cheek, babies turn toward that touch, open their mouth, and vigorously root for a nipple. Finding one, they automatically close on it and begin sucking - which itself requires a coordinated sequence of reflexive tonguing, swallowing, and breathing. Failing to find satisfaction, the hungry baby may cry - a behavior parents find highly unpleasant and very rewarding to relieve.
The pioneering American psychologist William James presumed that the newborn experiences a "blooming, buzzing confusion," an assumption few people challenged until the 1960s. But then scientists discovered that babies can tell you a lot-if you know how to ask. To ask, you must capitalize on what babies can do-gaze, suck, turn their heads. So, equipped with eye-tracking machines and pacifiers wired to electronic gear, researchers set out to answer parents' age-old questions: What can my baby see, hear, smell, and think?
Consider how researchers exploit habituation-a decrease in responding with repeated stimulation. We saw this earlier when fetuses adapted to a vibrating, honking device placed on their mother's abdomen. The novel stimulus gets attention when first presented. With repetition, the response weakens. This seeming boredom with familiar stimuli gives us a way to ask infants what they see and remember.
An example: Researchers have used visual preference to "ask" 4-month-olds how they recognize cats and dogs (Quinn, 2002; Spencer et al., 1997). First, they showed the infants a series of images of either cats or dogs. Then they showed them hybrid cat-dog images (FIGURE 45.4). Which of those two animals do you think the infants would find more novel (measured in looking time) after seeing a series of cats? It was the hybrid animal with the dog's head (and vice versa if they previously viewed dogs). This suggests that infants, like adults, focus first on the face, not the body.
Indeed, even as newborns, we prefer sights and sounds that facilitate social responsiveness. We turn our heads in the direction of human voices. We gaze longer at a drawing of a face-like image (FIGURE 45.5). We prefer to look at objects 8 to 12 inches away. Wonder of wonders, that just happens to be the approximate distance between a nursing infant's eyes and its mother's (Maurer & Maurer, 1988).
Within days after birth, our brain's neural networks were stamped with the smell of our mother's body. Week-old nursing babies, placed between a gauze pad from their mother's bra and one from another nursing mother, have usually turned toward the smell of their own mother's pad (MacFarlane, 1978). What's more, that smell preference lasts. One experiment capitalized on the fact that some nursing mothers in a French maternity ward applied a balm with a chamomile scent to prevent nipple soreness (Delaunay-El Allam, et al., 2010). Twenty-one months later, their toddlers preferred playing with chamomile-scented toys! Their peers who had not sniffed the scent while breast feeding showed no such preference. (This makes one wonder: Will adults who as babies associated chamomile scent with their mother's breast become devoted chamomile tea drinkers?
