In this section, we will examine how our cognitive abilities, such as thinking and language, develop. Because we are verbal animals and our language ability differentiates us from all other animals, we will look first at how this ability begins its development, which will lead us back to the nature–nurture issue. Next we will outline one of the most important theoretical contributions to psychology, Swiss psychologist Jean Piaget’s theory of cognitive development. According to Piaget, starting at birth all of us go through the same four stages of cognitive development, each of which is qualitatively different. Next we will discuss Russian psychologist Lev Vygotsky’s sociocultural approach to cognitive development. Vygotsky’s approach has recently become very popular because it emphasizes the importance of social and cultural contexts in development. Last, we will consider the question of whether intelligence declines throughout the adult portion of the life span.

Our ability to use language makes us unique. No other animal seems to be able to acquire and develop language ability as humans do. Although speechless at birth, our capacity for language begins to develop soon after. Children in different cultures learn to speak very different languages, but they all seem to go through the same sequence of stages. We will describe these stages of language acquisition and then consider the nature–nurture issue in explaining how language acquisition occurs.

Let’s begin with the newborn infant and see how language develops. Infants are speechless, but one way they communicate is through crying. Infants cry differently, for example, to indicate hunger versus pain. Crying, movement, and facial expressions allow infants to communicate fairly well. Infants also prefer baby talk (parentese), the different format of speech that adults use when talking with babies that involves the use of shorter sentences with a higher, more melodious pitch than normal speech. Actually, these exaggerated speech melodies parents use when speaking to their babies help the infants grasp the speaker’s intentions. Fernald (1993) exposed 5-month-old infants from English-speaking families to approval and prohibition phrases spoken in German, Italian, and both nonsense and regular English parentese. Even though all of this speech was gibberish to the babies, they responded with the appropriate emotion, crying when they heard prohibitions and smiling when they heard approvals. Thus, the melodious nature and not the content of parentese conveys the message to an infant.

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By two months or so, infants are making more meaningful noises such as cooing (repeating vowel sounds such as “oo” and “ah”) and laughing. Infants use cooing as their response in vocal interactions with their parents. At about six or seven months, babbling, the rhythmic repetition of various syllables, including both consonants and vowels, begins. The syllables that are babbled are not limited to the sounds that the infant hears or those from their parents’ language. However, this early babbling begins to include more and more sounds from the infant’s native language over the next six months. The infant can now also understand some words such as “mommy” and “daddy.” For example, the question “Where is mommy?” will lead the infant to look at her mother.

At about 1 year of age, infants begin to speak a few words. Their first words usually refer to their caregivers and objects in their daily environment. Sometimes, infants use a holophrase, a word that expresses a complete idea. A good example is a child going to the door and saying “bye-bye.” Vocabulary grows slowly until about 18 months, and then there is a vocabulary spurt, maybe of 100 words or more per month. This is also the period during which overextension and underextension occur. Overextension is the application of a newly learned word to objects that are not included in the meaning of the word. Underextension is the failure to apply the new word more generally to objects that are included within the meaning of the new word. A couple of examples will make these concepts clearer. A good example of overextension is children’s tendency to call any male “dada,” overextending the word and deflating the father’s ego. Underextension frequently occurs when children do not extend the categories of “dog” and “cat” to dogs and cats beyond the family’s pet dog or cat. The words are applied too narrowly. As vocabulary expands, the incidences of overextension and underextension decrease. This expansion of vocabulary reflects the influence of cognitive development; as children acquire new concepts, they learn the names that go with them.

One of the challenges children face in acquiring a vocabulary is determining the meaning of words, because the context in which children hear language is often ambiguous. For instance, if a mother points out a bird flying overhead and exclaims “look at the bird,” the child has to consider many possibilities as the potential meaning of the word bird. For example, bird could refer to any object above them or any object in the sky. Research has shown, however, that children use many different types of cues to identify the speaker’s intended meaning of bird. Some researchers have argued that children are particularly good at using social cues, such as the speaker’s eye gaze, pointing, and emotional reactions to determine what the adult means when using a novel word (Baldwin & Moses, 2001; Brooks & Meltzoff, 2008; Golinkoff & Hirsh-Pasek, 2006). For instance, Tomasello, Strosberg, and Akhtar (1996) showed that children could use a speaker’s emotional reaction to determine which novel object a speaker is labeling. In their task, an experimenter told 18-month-old infants that she was going to find a “toma” (a novel object). She then picked up a novel object but then rejected it and acted disappointed. She then picked up a second novel object and acted excited. She didn’t name either of the two objects. The child was then shown both novel objects and asked to give the experimenter the “toma.” If the child was able to use the emotional reaction of the experimenter to determine which object was the toma, she should select the object that the experimenter was excited about; and most 18-month-old children were able to do so even though it was not the first object seen.

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The next step in language development is the combining of words into sentences. This begins during the vocabulary spurt between 18 and 24 months. Children engage in what is called telegraphic speech, using two-word sentences with mainly nouns and verbs. It is called telegraphic speech because the speech is like that in a telegram, concise and direct. Some examples are “Dada gone” and “Throw ball.” These two-word statements begin to be expanded, and between the ages of 2 and 5, children acquire the grammar of their native language. Children learn these rules implicitly and in a very predictable order across all cultures. How they do so returns us to the nature–nurture issue.

Children acquire language early and easily, without direct instruction, and this acquisition process seems to be the same across cultures that have very different languages. This is why there is much support for the argument that language development is a genetically programmed ability (Chomsky, 1965; Pinker, 1994). Children, however, cannot develop normal speech without exposure to human speech, and it is clear that caregivers can facilitate and enhance language development, indicating that experience definitely plays a role in language acquisition. As is usually the case with the nature–nurture issue, there is some evidence for both sides; nature and nurture provide interactive influences (Elman, Bates, Johnson, Karmiloff-Smith, Paisi, & Plunkett, 1996).

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One of the best illustrations of children’s special skill in learning a language is the existence of a critical period for acquiring it. A critical period is a time period when learning certain skills is most easily accomplished and is thought to reflect the influence of biology (brain maturation) on development. If children do not acquire a language by a certain age, usually thought to be around the time of puberty or perhaps earlier, then they will not learn it as well as younger children. For example, children who are isolated from human contact prior to puberty have difficulty learning a language, even after years of later exposure. The best known example of this is a girl known as “Genie” (Fromkin, Krasjen, Curtiss, Rigler, & Rigler, 1974). Genie was kept tied to a potty chair for most of the first 13 years of her life. During this time, Genie heard very little language and had minimal social interactions. After she was rescued, both researchers and therapists worked hard to rehabilitate her. Although she did make some linguistic progress and was able to learn several hundred words, her grammatical development never reached typical developmental levels, even after several years of trying. A similar critical period exists for children acquiring American Sign Language (ASL). Most deaf children of hearing parents are not as adept at ASL as deaf children of deaf parents because they are typically taught ASL later, since their parents are not signers themselves (Newport, 1991; Senghas & Coppola, 2001). A critical period also exists for second language learning. As you may know from your own struggles to learn a second language, children have a much easier time than adults (Birdsong & Molis, 2001; Johnson & Newport, 1989).

Language development occurs during the first few years of life when the brain and cognitive abilities, such as thinking and reasoning, are also developing. When children start talking, it is easy to start thinking of them as miniature adults, but this would be a big mistake. Their cognitive abilities are not at all like those of an adult. To see how these cognitive abilities develop, we’ll consider Piaget’s stage theory of cognitive development, which tells us how a speechless newborn develops into a cognitively complex adult.

Jean Piaget was a twentieth-century Swiss psychologist whose research on children’s thinking led to a landmark theory of cognitive development. He was named one of the twentieth century’s 20 most influential thinkers by Time magazine in 1999. Piaget started his career in France working with Theophile Simon (of Binet-Simon intelligence scale fame) standardizing intelligence tests (Hunt, 1993). However, he soon returned to Switzerland and began his research on how children think. Piaget did not conduct formal experiments. In his loosely structured interviews he instead posed problems for children to solve (he used his own three children in his early research), observed their actions carefully, and questioned them about their solutions. He was particularly interested in children’s errors, which he thought provided insight into the child’s thinking, especially into how it differed from adult thinking. He found that children of roughly the same age often gave the same wrong answers. From such data, he developed a theory of cognitive development that revolutionized our understanding of children’s thinking and its development (Piaget, 1926/1929, 1936/1952, 1983).

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Piaget’s cognitive theory incorporated two of his interests, biology and philosophy. He assumed that cognitive development stems from a child’s adaptation to the environment, and that children attempt to promote their survival by trying to learn about their environment. This means that a child is an active seeker of knowledge and gains an understanding of the world by operating in it. The child organizes this knowledge into what Piaget called schemes (now called schemas), which are frameworks for our knowledge about people, objects, events, and actions. Remember, we discussed these in Chapter 5. Schemas are the basic units of our knowledge that allow us to organize and interpret information about our world. In our long-term memories, we have schemas for concepts (such as books or dogs), events (such as going to a restaurant or to the dentist’s office), and actions (such as riding a bicycle).

According to Piaget, cognitive adaptation involves two processes, assimilation and accommodation, both of which impact the development of schemas and thus learning. Assimilation is the interpretation of new experiences in terms of our existing schemas; accommodation is the modification of current schemas to allow for new experiences. Our earlier example of overextension—when infants call all men “dada”—would represent a child’s attempt to assimilate. Children learn, however, that they need to accommodate and change their schemas. A child has only one father, but there are many men in the world. It is through accommodation that the number and complexity of a child’s schemas increase and learning occurs. In accommodation, either new schemas are created for information that doesn’t fit into one’s present schemas or existing schemas are modified to include the new information (such as for father and men).

Piaget also proposed that major changes in children’s thinking occur in stages. Each stage permits only certain kinds of thinking and involves qualitatively different cognitive functioning. Piaget further assumed that all children go through the same stages in the same order. He proposed four stages, outlined in Table 7.2. As you learn about each stage, realize that you will be changing your own schemas to accommodate all this new information about Piaget’s theory. Then, after you have finished reading about the stages, look back at Table 7.2. You should easily be able to assimilate the stage descriptions into your modified schemas for Piaget’s theory.

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The sensorimotor stage. In the sensorimotor stage, from birth to about age 2, infants learn about the world through their sensory and motor interactions with it. Beginning with the simple reflexes that we discussed earlier, infants come to know the world by looking, listening, sucking, grasping, and manipulating. Infants less than 8 to 12 months old lack object permanence, the knowledge that an object exists independent of perceptual contact with it. For example, young infants do not understand that a toy continues to exist even if they can no longer see it. Object permanence develops over the first 2 years of life. Very young infants will not search for a toy that vanishes; but at about 4 to 8 months, they will sometimes search for it, especially if it is only partially hidden. At 8 to 12 months, they will search for a toy even if it is completely hidden, indicating that they realize that the toy still exists even if they cannot see it. Children continue to develop their understanding of object permanence and have a fairly complete understanding by 2 years of age. Similarly, symbolic representation of objects and events starts to develop during the latter part of the sensorimotor stage. Infants begin to use words as symbols to represent known objects at around 18 months. By 18 to 24 months, infants use telegraphic speech, which represents continuing development of symbolic representation.

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The preoperational stage. In the preoperational stage, from age 2 to age 6, children’s thinking becomes more symbolic and language-based, but remains egocentric and lacks the mental operations that allow logical thinking. Preoperational children can pretend, imagine, and engage in make-believe play. They have the ability to use one thing to represent another. Preoperational children might pretend that a broom is a horse to ride, or that their finger is a toothbrush. They no longer need to be interacting with an object to think about it. For example, they now can point to a picture of a dog and say “doggie” or crawl around and pretend to be a dog by barking like a dog. Word learning also continues at a rapid pace, and children have learned thousands of words by the end of the preoperational stage. Children also learn to produce narratives, descriptions of past events that have the structure of a story. However, preoperational children’s thinking still has major limitations. Let’s first consider what it means that their thinking is egocentric.

Egocentrism is the inability to distinguish one’s own perceptions, thoughts, and feelings from those of others. This means that a preoperational child cannot perceive the world from another person’s perspective. For example, preoperational children don’t realize what they are doing when they block the view of the television. They assume that another’s view is the same as their view. Egocentric behavior does not stem from selfishness or a lack of consideration. Preoperational children just have not developed the cognitive ability to see another person’s view. It is important for parents to realize this cognitive limitation in their preoperational children. If not, they may misinterpret their children’s behavior in a negative way, leading to unjust punishment of the child.

Next, to understand what Piaget meant by the lack of mental operations that allow a child to think logically, let’s consider conservation. Some grasp of conservation marks the end of the preoperational stage and the beginning of the concrete operational stage. Conservation is the knowledge that the quantitative properties of an object (such as mass and number) remain the same despite changes in appearance. Simply put, the quantitative properties of an object do not change with a change in appearance. There are many Piagetian conservation tests, but a well-known one is the liquid/beakers problem (see Figure 7.2). In this test, the child is first shown two identical short, fat beakers with equal amounts of liquid in each. With the child watching, the liquid in one of the beakers is poured into a taller, thinner beaker. Then the child is asked if the two beakers have the same amount of liquid or if one has more liquid than the other. If the child understands conservation, then he can explain why the two differently shaped beakers have an equal amount of liquid in them. No liquid was taken away or added. However, a preoperational child will say that the two beakers have different amounts and most often that the taller, thinner beaker has more liquid. Like egocentric thinking, the failure to understand conservation illustrates one of Piaget’s main points—a child is not like a miniature adult with less information. A child’s way of thinking is very different, and how it is different depends upon the child’s stage of cognitive development.

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A major reason why a preoperational child does not understand conservation is that the child lacks an understanding of reversibility—the knowledge that reversing a transformation brings about the conditions that existed before the transformation. As adults, you and I realize that you could easily pour the liquid in the taller beaker back into the shorter beaker to return to the starting state. A preoperational child does not understand this reversibility operation. A preoperational child’s thinking also reflects centration—the tendency to focus on only one aspect of a problem at a time. In the liquid/beakers problem, for example, the child may only focus on the heights of the beakers and conclude that one has more because it is taller. Obviously, both the height and width of the beakers need to be considered in order to make a correct judgment. Other Piagetian conservation tests in addition to the liquid/beakers problem are illustrated in Figure 7.2. Find a preoperational child around 3 to 4 years of age and try these tests. The child’s responses will not only amaze you but will also give you a much better understanding of the cognitive limitations of the preoperational stage of development.

The concrete operational and formal operational stages. During the concrete operational stage, from about age 6 to 12, children gain a fuller understanding of conservation and other mental operations that allow them to think logically, but only about concrete events. Different forms of conservation are developed at different times. For example, conservation of continuous quantity, number, and mass are acquired rather early, but conservation of length is more difficult and is acquired later in the concrete operational stage (Vasta, Miller, & Ellis, 2004). In addition to conservation operations, concrete operational children develop other mental operations that allow them to reason logically, such as transitivity (if A > B, and B > C, then A > C) and seriation (the ability to order stimuli along a quantitative dimension, such as a set of pencils by their length).

However, all of these operations are limited to reasoning logically about concrete events. For example, transitivity is limited to having the actual objects present, such as three sticks of different lengths. Children wouldn’t be able to solve the transitivity problem without the sticks physically present. Similarly, concrete objects (such as beakers of liquid) would need to be present to solve the conservation of continuous quantity problem. This means that the reasoning of concrete operational children is tied to immediate reality (what is in front of them and tangible) and not with the hypothetical world of possibility. They cannot deal with what-if and if-then problems and abstract thinking. They also do not engage in systematic deduction to solve a problem, but rather use a haphazard trial-and-error strategy.

In the formal operational stage, starting at age 12 or so, children gain the capacity for such hypothetical-deductive thought. According to Piaget, this capacity allows adolescents to engage not only in hypothetical thought but also in systematic deduction and tests of hypotheses, what could easily be referred to as scientific thinking. To understand the difference in thinking between concrete and formal operational children, Piaget used several scientific thinking tasks (Inhelder & Piaget, 1958).

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In one of these tasks, children or adolescents are shown several flasks of what appear to be the same clear liquid and are told that one combination of two of these liquids would produce a blue liquid. The task is to determine the combination that would produce the blue liquid. The concrete operational children just start mixing different clear liquids together haphazardly. The formal operational children, however, proceed very differently. They develop a systematic plan for deducing what the correct combination must be by determining all of the possible combinations (hypotheses for the correct combination) and then systematically evaluating each one. To accomplish this plan, they systematically mix the liquid in one beaker with each of the other liquids. If none of these combinations produced the blue liquid, they deduce that the liquid in that beaker is not relevant to the sought-after combination and then proceed to test each of the other clear liquids in the same manner until they find the correct combination.

Formal operational adolescents can also evaluate the logic of verbal statements without referring to concrete situations; the concrete operational child can only do so with concrete evidence. For example, in one formal operational study, the experimenter asked whether a statement about some colored poker chips was true, false, or uncertain (Osherson & Markman, 1975). When the experimenter hid a chip in his hand and asked about the statement, “Either the chip is red or it is not red,” the formal operational children realized that the statement was true regardless of the color of the hidden chip, but the concrete operational children were uncertain of the statement’s truth status. The formal operational children understood the disjunctive logic of the statement, and the concrete operational children did not. Concrete operational children also have difficulty with propositional logic that contradicts reality (Moshman & Franks, 1986). For example, concrete operational children would judge the following reasoning to be faulty, “If cats are bigger than horses and horses are bigger than mice, then cats are bigger than mice,” because the first relationship does not hold in real life. Concrete operational children are tied to the realistic truth of the content (what is) in their logical reasoning, but formal operational children are not.

Evaluation of Piaget’s theory. Recent research has shown that cognitive development seems to proceed in the general sequence of stages that Piaget proposed (Lourenco & Machado, 1996). This means that Piaget’s theory seems to have captured the general nature of cognitive development accurately. However, there are many issues with the specifics of Piaget’s stage theory. For example, recent research has demonstrated that rudiments of many of Piaget’s key concepts (such as object permanence) may begin to appear at earlier ages than Piaget proposed. Infants and young children may be more cognitively competent than Piaget theorized. Piaget’s tests for the understanding of concepts may have been too complex and thus missed partial knowledge of the concept. For example, Piaget’s test for object permanence required infants to reach for a hidden object. A complete understanding required the infants to search for the object after several invisible (hidden) movements. Later research that involved tracking infants’ eye movements has found that infants (as young as 3 months) continue to stare at the place where the object disappeared from sight, indicating some degree of object permanence (Baillargeon, 1987).

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More recent research on the formal operational stage also makes it clear that not all people reach this stage of thinking, especially in cultures that do not emphasize such thinking, and that those that do reach the stage may not always use such thinking (Dasen, 1994; McKinnon & Renner, 1971). For example, people in non-Western cultures do not usually do well on the specific scientific reasoning tasks used by Piaget, but they do very well and demonstrate formal operational thought on comparable tasks involving content that they are familiar with and that is significant within their culture (Vasta, Miller, & Ellis, 2004). Even Piaget, late in his life, realized that there were limitations on achieving formal operations (Piaget, 1972).

Other cognitive developmental researchers question whether Piaget’s characterization of distinct stages of development is correct. In particular, the information-processing approach to cognitive development questions the existence of stages and argues that development is continuous and not composed of distinct stages. So how do information-processing developmental psychologists explain the growth in children’s cognitive abilities? They attribute this growth to developmental changes in children’s information-processing abilities—how they take in, store, and use information. The information-processing approach uses a computer metaphor to describe children’s thinking. Just as a computer’s ability to solve problems is affected by memory and requires specific processing steps, children’s problem-solving ability involves similar information processing. Information-processing researchers study factors that affect such processing. For example, developmental improvements in speed of processing (Kail, 1991), storage capacity (Pascual-Leone, 1989), and knowledge base (Schneider, 1993) have all been found to influence improvements in children’s memory and thinking. As children grow older, they become increasingly more adept at information processing.

We should mention two other major criticisms of Piaget’s stage theory. One is that Piaget did not sufficiently consider the impact of culture and social environment on cognitive development (Miller, 2011; Segall, Dasen, Berry, & Poortinga, 1990). The second is that Piaget’s stage theory of cognitive development ends with adolescence and the development of formal operations, instead of continuing through to adulthood. Although Piaget did not address these issues, other developmental psychologists have done so. We will discuss the first issue in the next section when we examine the work of Russian psychologist Lev Vygotsky, whose theory did emphasize the sociocultural aspects of cognitive development. The second issue will be addressed in the section after that when we examine the question of what happens to intelligence from adolescence to old age. Does it decline as we age, especially in late adulthood? This discussion will allow us to examine two major research methods used by developmental psychologists, cross-sectional studies and longitudinal studies.

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Lev Vygotsky was a Russian developmental psychologist who was a contemporary of Piaget. Both were born in 1896, but Vygotsky died of tuberculosis at a very young age, 37, and did not have the opportunity to finish developing his theory. As with Piaget’s work, there was little interest in the Western world in Vygotsky’s work until the 1960s. Vygotsky’s approach has become especially popular recently, however, because of its sociocultural emphasis on development.

Vygotsky (1930, 1933, 1935/1978, 1934/1986) stressed that cognitive abilities develop through interactions with others and represent the shared knowledge of one’s culture. The social aspects of Vygotsky’s approach are straightforward. We are social animals, and therefore much of our learning occurs within social interactions. In brief, we learn from other people—our parents, siblings, friends, teachers, and others. Vygotsky proposed that culture impacts both the content and the processes of the child’s cognitive development, because a child’s cognitive development occurs within this cultural context. Now that we have a general idea of Vygotsky’s theory, let’s take a look at two of his major theoretical concepts—the zone of proximal development and scaffolding.

In Vygotsky’s theory, the zone of proximal development is the difference between what a child can actually do and what the child could do with the help of others. In Vygotsky’s terms, this is the difference between the levels of actual development and potential development. It means that there are thinking skills that the child can display with the help of others but cannot perform independently. It also leads to a style of teaching called scaffolding. In scaffolding, the teacher adjusts the level of help in relation to the child’s level of performance, while directing the child’s learning progress toward the upper level of the child’s zone of proximal development. The teacher gauges the amount of assistance necessary based on the learner’s needs. The learning is structured in steps so that the child learns to achieve each step independently, but is guided and supported by the teacher throughout the learning process.

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To illustrate these two concepts and Vygotsky’s theory, let’s consider the example of a child trying to solve a jigsaw puzzle (Berger, 2006). A child may appear not to be able to solve the puzzle. However, Vygotsky would say that this particular problem-solving task could be within the child’s zone of proximal development, but that she could not achieve it on her own. She needs a teacher to scaffold the task for her. How might this scaffolding proceed? The teacher would break the task down into manageable units; for example, the teacher might ask the child just to look for pieces for a particular section of the puzzle with specific suggestions about the size, shape, and colors of the relevant pieces. If this doesn’t work, the teacher might actually place a few pieces in their proper places or move a few relevant pieces to their correct orientations, so their relevance is more obvious to the child. Throughout this scaffolding process, the teacher must be totally supportive of the child’s progress and sensitive to how much help the child needs to progress toward solving the puzzle and how best to direct her to succeed in the next step of the solution process. After solving the puzzle, the teacher might have the child do it again, but this time with less guidance. Soon the child will be able to complete the puzzle independently. The teacher builds a scaffold to enable the child’s learning. Once the learning is achieved, the scaffold is no longer necessary.

As recommended with Piaget’s tests for the various types of conservation, find a young child and try to teach her how to solve a jigsaw puzzle using Vygotsky’s scaffolding method. It will not only give you a better understanding of this approach but will also lead you to understand the social aspects of learning that Vygotsky stressed in his theory.

Piaget’s description of intellectual development stops in adolescence with the onset of formal operations (hypothetical thought and systematic deduction), but it is important to examine what happens to intelligence across the various stages of adulthood from youth to old age. Do our cognitive abilities severely decrease across adulthood, especially in old age? The attempt to answer this question illustrates the differences between two major research methods in developmental psychology, cross-sectional studies versus longitudinal studies. In a cross-sectional study, people of different ages are studied and compared with one another at a single point in time. In a longitudinal study, the same people are studied over a long period of time. This involves collecting data periodically on the same people as they age. Longitudinal studies assess changes in people over time, whereas cross-sectional studies assess differences among age groups at a particular point in time. We will examine the use of both of these developmental research methods to answer the question about intelligence across the life span, learning the advantages and disadvantages of each.

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The cross-sectional method. The early studies on this question about intelligence across the life span used the cross-sectional method. These studies used representative samples of people of various ages and consistently found that intelligence declined with age. Later studies, however, used the longitudinal method. When the same people were retested over a period of years, researchers found that intelligence did not decline with age, but remained rather stable and possibly increased until very late in life when it showed a decline. Now think about why there were two different answers to the intelligence question. First, consider the nature of a cross-sectional study and the possible problems with this method. A cross-sectional study compares people not only of different ages but also of different generations. This difference in generations can lead to what are called cohort effects—people of a given age (cohorts) are affected by factors unique to their generation, leading to differences in performance among generations. For example, there were significant differences in education and educational opportunities for the various generations across the twentieth century. Earlier generations generally received less education, which could certainly account for the intellectual decline observed in the cross-sectional studies. So, why would a researcher use the cross-sectional method, given such possible cohort effects? The cross-sectional method is far less time-consuming and less expensive than the longitudinal method. In addition, there is no need for continual retesting, as there is in longitudinal research.

The longitudinal method. Now consider the longitudinal research method. Although there is no possibility of cohort effects when using the longitudinal method, it is time-consuming and expensive, and repeated testing has to be conducted. In addition, another problem arises. Participants may discontinue their participation, move far away, or die. This means that the sample changes across time, which could have an impact on the research findings if those who disappear from the sample are unlike, in some relevant characteristic, those who stay. How might a changing sample lead to the misleading finding that intelligence remains fairly stable? Here’s one explanation—those who survived to be tested at the older ages may have been the most intelligent and healthiest participants, those whose intelligence would be the most likely not to decline. This would also mean that the participants whose intelligence was likely to decline may have no longer been in the study. Given the shortcomings of both methods (see Table 7.3), it has proven rather difficult to get a clear answer to this question of intelligence across the life span.

The type of intelligence that is being tested is also important and further complicates the search for an answer. Remember from the last chapter, in our discussion of types of intelligence, that we differentiated fluid intelligence and crystallized intelligence. Crystallized intelligence refers to accumulated knowledge, verbal skills, and numerical skills that increase with age; fluid intelligence involves abilities, such as abstract thinking and logical problem solving, that decrease with age (Horn, 1982). This difference may help to explain why scientists make their major contributions early, but historians and philosophers make theirs later in their careers.

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The Seattle Longitudinal Study was a major attempt to answer the question of whether or not intelligence declines with age (Schaie, 1994, 1995). It was a large-scale longitudinal study of various intellectual abilities (such as inductive reasoning, word fluency, and perceptual speed) across the entire span of adulthood. It started in 1956 with more than 5,000 participants being tested every 7 years through 1998. The study was actually a combination of the longitudinal and cross-sectional methods, since groups of new participants were added periodically. This allowed the researchers to examine the possible shortcomings of both methods.

In general, the researchers found that most intellectual abilities decline somewhat by age 60, but the decline is not great until a person reaches age 80 or more (except for abilities largely dependent upon speed of processing, which clearly declines as we age). There are large individual differences, however. Those who suffer the least decline seem to be those who have stayed healthy, are in the higher socioeconomic categories, and are still involved in intellectually stimulating environments. In brief, it seems that if we work to stay healthy and cognitively stimulated, we won’t suffer major deficits in our cognitive abilities until rather late in adulthood.

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