During the era dominated by psychometric theories, the study of intelligence was influenced most by those investigating individual differences in people’s test scores. In an address to the American Psychological Association in 1957, the American researcher Lee Cronbach, a leader in the testing field, decried the lack of common ground between psychologists who studied individual differences and those who studied commonalities in human behaviour. Cronbach’s plea to unite the “two disciplines of scientific psychology” led, in part, to the development of cognitive theories of intelligence and of the underlying processes posited by these theories. (See also pedagogy: cognitive theories.)

Fair assessments of performance require an understanding of the processes underlying intelligence; otherwise, there is a risk of arriving at conclusions that are misleading, if not simply wrong, when evaluating overall test scores or other assessments of performance. Suppose, for example, that a student performs poorly on the verbal analogies questions in a psychometric test. One possible conclusion is that the student does not reason well. An equally plausible interpretation, however, is that the student does not understand the words or is unable to read them in the first place. A student who fails to solve the analogy “audacious is to pusillanimous as mitigate is to __” might be an excellent reasoner but have only a modest vocabulary, or vice versa. By using cognitive analysis, the test interpreter is able to determine the degree to which the poor score stems from low reasoning ability and the degree to which it results from not understanding the words.

Underlying most cognitive approaches to intelligence is the assumption that intelligence comprises mental representations (such as propositions or images) of information and processes that can operate on such representations. A more-intelligent person is assumed to represent information more clearly and to operate faster on these representations. Researchers have sought to measure the speed of various types of thinking. Through mathematical modeling, they divide the overall time required to perform a task into the constituent times needed to execute each mental process. Usually, they assume that these processes are executed serially (one after another) and, hence, that the processing times are additive. But some investigators allow for parallel processing, in which more than one process is executed at the same time. Regardless of the type of model used, the fundamental unit of analysis is the same—that of a mental process acting upon a mental representation.

A number of cognitive theories of intelligence have been developed. Among them is that of the American psychologists Earl B. Hunt, Nancy Frost, and Clifford E. Lunneborg, who in 1973 showed one way in which psychometrics and cognitive modeling could be combined. Instead of starting with conventional psychometric tests, they began with tasks that experimental psychologists were using in their laboratories to study the basic phenomena of cognition, such as perception, learning, and memory. They showed that individual differences in these tasks, which had never before been taken seriously, were in fact related (although rather weakly) to patterns of individual differences in psychometric intelligence test scores. Their results suggested that the basic cognitive processes are the building blocks of intelligence.

The following example illustrates the kind of task Hunt and his colleagues studied in their research: the subject is shown a pair of letters, such as “A A,” “A a,” or “A b.” The subject’s task is to respond as quickly as possible to one of two questions: “Are the two letters the same physically?” or “Are the two letters the same only in name?” In the first pair the letters are the same physically, and in the second pair the letters are the same only in name.

The psychologists hypothesized that a critical ability underlying intelligence is the rapid retrieval of lexical information, such as letter names, from memory. Hence, they were interested in the time needed to react to the question about letter names. By subtracting the reaction time to the question about physical match from the reaction time to the question about name match, they were able to isolate and set aside the time required for sheer speed of reading letters and pushing buttons on a computer. They found that the score differences seemed to predict psychometric test scores, especially those on tests of verbal ability such as reading comprehension. Hunt, Frost, and Lunneborg concluded that verbally facile people are those who are able to absorb and then retrieve from memory large amounts of verbal information in short amounts of time. The time factor was the significant development in this research.

A few years later, Sternberg suggested an alternative approach that could resolve the weak relation between cognitive tasks and psychometric test scores. He argued that Hunt and his colleagues had tested for tasks that were limited to low-level cognitive processes. Although such processes may be involved in intelligence, Sternberg claimed that they were peripheral rather than central. He recommended that psychologists study the tasks found on intelligence tests and then identify the mental processes and strategies people use to perform those tasks.

Sternberg began his study with the analogies cited earlier: “lawyer is to client as doctor is to __.” He determined that the solution to such analogies requires a set of component cognitive processes that he identified as follows: encoding of the analogy terms (e.g., retrieving from memory attributes of the terms lawyer, client, and so on); inferring the relation between the first two terms of the analogy (e.g., figuring out that a lawyer provides professional services to a client); mapping this relation to the second half of the analogy (e.g., figuring out that both a lawyer and a doctor provide professional services); applying this relation to generate a completion (e.g., realizing that the person to whom a doctor provides professional services is a patient); and then responding. By applying mathematical modeling techniques to reaction-time data, Sternberg isolated the components of information processing. He determined whether each experimental subject did, indeed, use these processes, how the processes were combined, how long each process took, and how susceptible each process was to error. Sternberg later showed that the same cognitive processes are involved in a wide variety of intellectual tasks. He subsequently concluded that these and other related processes underlie scores on intelligence tests.

A different approach was taken in the work of the British psychologist Ian Deary, among others. He argued that inspection time is a particularly useful means of measuring intelligence. It is thought that individual differences in intelligence may derive in part from differences in the rate of intake and processing of simple stimulus information. In the inspection-time task, a person looks at two vertical lines of unequal length and is asked to identify which of the two is longer. Inspection time is the length of time of stimulus presentation each individual needs in order to discriminate which of the two lines is the longest. Some research suggests that more-intelligent individuals are able to discriminate the lengths of the lines in shorter inspection times.

Other cognitive psychologists have studied human intelligence by constructing computer models of human cognition. Two leaders in this field were the American computer scientists Allen Newell and Herbert A. Simon. In the late 1950s and early ’60s, they worked with computer expert Cliff Shaw to construct a computer model of human problem solving. Called the General Problem Solver, it could find solutions to a wide range of fairly structured problems, such as logical proofs and mathematical word problems. This research, based on a heuristic procedure called “means-ends analysis,” led Newell and Simon to propose a general theory of problem solving in 1972. (See also Thought: Types of thinking.)

Most of the problems studied by Newell and Simon were fairly well structured, in that it was possible to identify a discrete set of steps that would lead from the beginning to the end of a problem. Other investigators have been concerned with other kinds of problems, such as how a text is comprehended or how people are reminded of things they already know when reading a text. The psychologists Marcel Just and Patricia Carpenter, for example, showed that complicated intelligence-test items, such as figural matrix problems involving reasoning with geometric shapes, could be solved by a sophisticated computer program at a level of accuracy comparable to that of human test takers. It is in this way that a computer reflects a kind of “intelligence” similar to that of humans. One critical difference, however, is that programmers structure the problems for the computer, and they also write the code that enables the computer to solve the problems. Humans “encode” their own information and do not have personal programmers managing the process for them. To the extent that there is a “programmer,” it is in fact the person’s own brain.

All of the cognitive theories described so far rely on what psychologists call the “serial processing of information,” meaning that in these examples, cognitive processes are executed in series, one after another. Yet the assumption that people process chunks of information one at a time may be incorrect. Many psychologists have suggested instead that cognitive processing is primarily parallel. It has proved difficult, however, to distinguish between serial and parallel models of information processing (just as it had been difficult earlier to distinguish between different factor models of human intelligence). Advanced techniques of mathematical and computer modeling were later applied to this problem. Possible solutions have included “parallel distributed processing” models of the mind, as proposed by the psychologists David E. Rumelhart and Jay L. McClelland. These models postulated that many types of information processing occur within the brain at once, rather than just one at a time.

Computer modeling has yet to resolve some major problems in understanding the nature of intelligence, however. For example, the American psychologist Michael E. Cole and other psychologists have argued that cognitive processing does not accommodate the possibility that descriptions of intelligence may differ from one culture to another and across cultural subgroups. Moreover, common experience has shown that conventional tests, even though they may predict academic performance, cannot reliably predict the way in which intelligence will be applied (i.e., through performance in jobs or other life situations beyond school). In recognition of the difference between real-life and academic performance, then, psychologists have come to study cognition not in isolation but in the context of the environment in which it operates.