Intelligence is a concept that is believed to be made up of a set of complex behaviors. It is described as the ability to problem solve, to perceive objects in one’s environment, to understand spatial relationships, to remember, classify, calculate, and reason. It is analogous with cognition, a mental quality comprising one’s ability to learn from experience, to adapt to new situations, acquire knowledge, and deal with abstract concepts. It has been found to be clearly associated with the integrity and functioning capacity of one’s central nervous system functioning (the brain) and is made up of several anatomically related domains (e.g., arousal and alertness, visual spatial processing, attention, memory, executive functioning, etc.).
Raymond Cattell suggested that intelligence consists of two parts, “fluid” intelligence (one’s innate basic reasoning that is supposedly independent of culture and educational background) and “crystallized” intelligence (the information and skills one learns through education, experience, and socialization).
Many scientists and educators believe that intelligent behavior is not restricted to human behavior; that is, it can be exhibited by other species. However, it is assumed to be limited by the absence of language. Language is defined as a culture’s representation of ideas and objects in symbolic form that is used classify and store information in memory. Only the species of Homo sapiens has developed the ability to string together several sounds (phonemes) to produce new meanings. Language is considered a critical component of higher-order intelligence because without it, one could not measure and define the more complex aspects of intellectual behavior.
Integral to the current use of the term intelligence is the intelligence quotient (IQ). An individual’s IQ is commonly measured and quantified by paper and pencil, individually administered IQ tests, as well as by the appraisal of adaptive behavior (based on how a person functions in an everyday setting).
Evolution of the Brain
In all species, the brain is the organ most associated with “intelligence.” Much of the human brain’s evolution has occurred within the past 5 million years, during which time it has increased 5 times from its original size. The modern human brain weighs between 1,000 and 2,000 g.
Current theorists believe that our earliest bipedal ancestor was the East African Australopithecus. Anthropological evidence indicates that its brain weighed about 400 g (the same size as a modern chimpanzee’s). They evolved into the species Homo habilis, a toolmaking hominid characterized by dental patterns similar to modern humans’. Homo habilis developed a significantly larger brain than its predecessor’s (believed to weigh one half the size of modern man’s brain), which was thought to be directly related to their scavenger lifestyle. This type of lifestyle would have been more reliant on the organism’s sensory, motor, and social skills than that of the Australopithecus lifestyle. It is thought that Homo habilis continued to develop its skill in designing and implementing tools until it changed from scavenger to hunter and migrated out of Africa.
These new behaviors were exhibited by Homo erectus, a species characterized by a larger brain and the behavior of traveling. Fossilized remains of Homo erectus have been found in Africa, Java, and Peking. As a hunter-gatherer living in a complex sociological society, survival of Homo erectus was strongly influenced by their ability to read another’s behavior and adjust their own based on experiential and motivational factors.
The role of preadaption may explain the continued development of brain size in early man. Once certain intellectual behaviors (such as extended travel and toolmaking) had developed that could enable a species to take advantage of their environment, it opened the door for new cognitively superior developments to emerge. Dean Falk suggested that the development of the upright posture of Australopithecus contributed to the development of new changes in the venus blood flow of the brain. Until that point, apes had reached an upper limit to the growth of their brains due to the oxygen consumption and generation of heat in the brain. Exercise (like that involved in travel) put the species at risk for overheating the brain, until they developed a more widely dispersed venus blood flow system that served as a cooling system and allowed for the advent of extensive movement. Once upper limits of growth were selected out, other influences were free to contribute to brain growth (e.g., increased social development, climate adaptation, diet changes, increased skill in movement, larger bodies, mate selection).
Scientific discoveries of this type have contributed to the theory that the anatomical increase in brain size is responsible for producing more sophisticated behavioral abilities. It would stand to reason, then, that the more “sophisticated” the species, the more specialized central nervous system (CNS) functional domain areas. That is, as the human brain has evolved in size, it has actually developed more functions, which allows, in turn, more advanced skills to evolve.
Western Versus Eastern Views on Intelligence
The Western concept of intelligence can be traced back to ancient Greece, when Plato described three aspects of a human’s soul as comprising intellect, emotion, and morality (later renamed as cognition, affect, and conation). Cicero appears to have coined the term “intelligentsia,” and St. Augustine defined intelligence as quickness of understanding and acuteness of discernment. Prior to the late 1800s, when Alfred Binet developed a paper-and-pencil test to measure it, intelligence was considered a philosophical concept and could not be easily quantified.
Early Eastern views of intelligence were also grounded in philosophy, although their understanding originally evolved from the Indian concepts of wisdom and prudence. Over time, this concept of intelligence (aka buddhi) was expanded to encompass human emotion, societal values, and interpersonal relationships.
History of Intelligence
Scientific research aimed at quantifying and identifying human intelligence began as early as 1869, when Francis Galton studied individual differences in the biological underlay of cognitive functioning by measuring reaction time and determined that all humans possessed a “general” intellectual ability.
This modern, Western concept of quantifiable intelligence scores was further developed and popularized during the 1910s by German psychologist Wilhelm Stern, and the continued work of Lewis Terman. They were responsible for expanding the intellectual concept of “mental age” into a quotient by computing a formula that divided mental age by chronological age and multiplying by 100.
The idea that a mathematical score could represent one’s overall intellectual functioning was truly expanded by the infamous French psychologist Alfred Binet and his colleague Theodore Simon, with their development of an empirical measure of mental retardation (Binet-Simon Intelligence Scale, 1905). They were the first to use age standards in measuring intelligence, and their scale was designed to compare a mentally retarded individual’s score with that of their normal, same-age peers.
Both Henry H. Goddard and Lewis M. Terman revised Binet’s original intelligence scale and adapted it for use within the United States in 1916, with several revisions following in the years of 1937, 1960, 1972, and 1986. To date, it is still one of the most popular intelligence assessment measures used.
Meanwhile, as Binet and his predecessors continued to perfect their empirical measure of human intelligence, a Swiss biologist named Jean Piaget developed a theory that depicted intelligence not as a “general” ability, but rather as a continuum of human development and biological adaptation. He described the way a child typically perceives, understands, and operates in their environment as a cognitive process that progresses through time as a result of the reorganization of psychological brain structures. Piaget theorized that intellectual development occurred throughout four childhood stages, starting at birth, and each stage allowed for the construction of increasingly complex intellectual processes. Because he believed intellectual ability could be impacted by both biological and genetic factors, he thought it to be malleable.
It wasn’t until the late 1930s that scientists began to describe intelligence as not just a single general ability, but as a cluster of cognitive processes (e.g., information processing, attention, planning, forming strategies, etc.) that, as Piaget had suggested, could grow and develop over time and were influenced by both cultural and social factors.
Louis L. Thurstone postulated that intelligence was actually made up of as few as seven independent mental abilities that operated independently, and in 1939, American psychologist David Wechsler developed the Wechsler-Bellevue Intelligence Scale test in an attempt to tease out and measure these different underlying mental abilities by providing distinct scores for each area of functioning.
In 1983, Howard Gardner added to this theory, claiming the concept of multiple intelligences could be further broken down into cognitive components that could describe specific behavior. However, when these distinct functions were taken in whole, they would better describe the overall cognitive performance of an individual. He added the specific concepts of musical, bodily kinesthetic (athletic ability, dancing), logical-mathematical (numerical ability), linguistic (verbal ability), spatial (orientation in space map reading, visual arts), interpersonal (understanding the behavior of others, empathy, sympathy) and intrapersonal (understanding oneself) functioning to the theory of intelligence.
Soon after, William Sternberg expanded the theory of intelligence to encompass the sociocultural context in which humans behave and interact. He redefined intelligence as a purposeful, goal-oriented behavior that could not be understood outside of an individual’s environment, including a person’s practical knowledge as measured by their success in the real world, not by a generic, quantifiable number.
Qualitative and Quantitative Intelligence Measurement
Intelligence is extremely difficult to identify and measure, as it is a complex process that involves many types of behavior. Intelligence tests were primarily designed to allow for an objective, systematic observation of an individual’s intellectual performance in a controlled situation. However, it is obvious that no single IQ test or theory can fully identify the full spectrum of intelligence, particularly the individual’s ability to use both environmental and personal resources to adapt to the world. Despite that, standardized intelligence scores are used in many industrialized societies as a basis for selection, placement, and decision making in educational and employment arenas across an individual’s life span. They also play an enormous role in the decisions of allocation of the society’s resources and opportunities. Although IQ tests cannot tap into every aspect of intellectual functioning, a good intelligence test should be both reliable and valid and ought to include both clear and ambiguous items that vary in difficulty. Commonly used measures of intelligence provide scientifically collected age- and/or education-adjusted comparative data that are normally distributed amongst the population. Currently, the average IQ as measured by the Wechsler Adult Intelligence Scale is a standard score of 100 (+15), where mentally retarded people score below 70, and gifted at or above 130.
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