In this chapter we explore what language is and how psychologists study it, scientific principles of reasoning and problem solving, and the concept of intelligence.

Language consists of a system of symbols and rules for combining them to produce messages or meanings. Language is symbolic, has a rule-governed structure, and is generative (its symbols are combined to generate messages). Language uses sounds, written signs, or gestures to refer to objects, events, ideas, and feelings. With language we can talk about not only the present but the past, future, and imaginary events, a feature of language called displacement. Language has both a surface and a deep structure. The surface structure is the way symbols are combined (through rules of grammar called syntax). The deep structure refers to the underlying meaning. Semantics are the rules for connecting symbols to what they represent. Phonemes are the smallest units of sound in a language, while morphemes are the smallest units of meaning. The development of language seems to be both biological and psychological. All infants, regardless of culture or society, vocalize the entire range of phonemes found in the world's languages. There is evidence for a sensitive period in childhood during which language is most easily learned. Learning through imitation, rewards, and punishments, also seems to be important in the development of language, though it cannot completely explain the development of language skills. A second language is best learned and spoken most fluently when learned during the sensitive period of childhood. Language seems to affect thinking processes. Benjamin Lee Whorf, in fact, argued that language determines what we are capable of thinking, though most modern linguists disagree with that assertion. We do seem to think in propositions, statements that express fact. Propositions consist of concepts, basic units of semantic memory. Many concepts in turn are defined by prototypes, the most typical and familiar members of a category.

We reason through both deductive (top-down) and inductive (bottom-up) reasoning. Sometimes, though, we run into stumbling blocks in reasoning. These stumbling blocks include being distracted by irrelevant information, failing to apply deductive rules, and belief bias, the tendency to abandon logical rules in favor of one's own personal beliefs. To solve a problem, we must first frame it. Following that, we generate potential solutions, test the solutions, and evaluate the results. Problem-solving schemas help us to select information and solve problems. Algorithms are formulas for solving problems, while heuristics are general problem-solving strategies that we employ to solve problems. Means-ends analysis, a heuristic which can involve generating subgoals, moves from an initial state toward a desired goal state. The representativeness heuristic is used to determine whether a new stimulus is a member of a particular class or category. The availability heuristic can bias our perceptions by focusing on what is available in our memories. Similar to belief bias is the confirmation bias, by which we search only for information that supports our beliefs. Yet another problem with problem solving is functional fixedness, which involves being unable to think about alternative uses for an object. In contrast, divergent thinking and incubation can aid problem solving.

Intelligence is a concept that refers to individual differences in the ability to acquire knowledge, to think and reason effectively, and to deal adaptively with the environment. The modern intelligence testing movement was begun by French psychologist Alfred Binet. He assumed that mental abilities developed with age and that the rate at which people gain mental competence is a characteristic of the person and is stable over time. German psychologist William Stern developed the idea of the intelligence quotient (IQ), which was originally expressed as the ratio mental age/chronological age x 100. In the United States, Lewis Terman and David Wechsler developed the most prominent IQ tests. Other tests that measure mental abilities are achievement tests, which measure learning, and aptitude tests, which measure potential for future learning and performance.

For a psychological test to be recognized by the scientific community it must show acceptable levels of reliability, validity, and standardization. Reliability refers to consistency of measurement and includes test-retest reliability, internal consistency, and interjudge reliability. Validity refers to whether a test is measuring what it is supposed to measure and includes construct validity, content validity, and predictive validity. Standardization refers to controlled testing procedures and the collection of norms.

Psychologists use two major approaches in studying the nature of intelligence. The psychometric approach examines the structure of intellect and competencies involved in test performance. The cognitive process approach studies specific thought processes underlying mental competencies. Charles Spearman argued for one general factor that he called "g" which he argued underlies all mental abilities. Thurstone argued for seven distinct primary mental abilities. Cattell and Horn argued that intelligence is of two types, crystallized and fluid. Crystallized intelligence consists of the ability to apply existing knowledge while fluid intelligence consists of the ability to deal with novel situations. In that tradition, Howard Gardner proposed a theory of multiple intelligence. Emotional intelligence, which combines elements of Gardner's theory, refers to the abilities to read and respond to others' emotions appropriately, to motivate oneself, and to be aware of and to control one's own emotions. Robert Sternberg, a leading proponent of the cognitive process approach, developed a triarchic theory of intelligence consisting of planning and regulating task behavior, executing behavioral strategies, and encoding and storing information.

Heredity and environment both influence intelligence. The reaction range for intelligence, or any trait for that matter, refers to the range of possibilities that the genetic code allows. Asian-Americans, on average, score somewhat above the White American mean on IQ tests. Hispanic-Americans score, on average, roughly the same as White Americans. African-Americans score, on average, 12-15 IQ points below the White American average. The mean differences between African-Americans and White Americans have declined over the past 25 years as greater educational and vocational opportunities for African-Americans have emerged, suggesting an environmental component to group differences in intelligence. Gender differences in cognitive abilities have also been discovered. Males perform better on spatial tasks and tasks involving mathematical reasoning. Females perform better on tests of perceptual speed, verbal fluency, mathematical calculation, and precise manual tasks. Self-beliefs and stereotypes seem to influence test performance. About 3-5 percent of the U.S. population is mentally retarded, or cognitively disabled, although with educational enrichment and support many of these individuals can learn to live independently. The intellectually gifted also need special educational opportunities.