Contemporary theories about intelligence can be divided into two classes: psychometric and cognitive types. The quantitative approach to intelligence is better reflected in psychometric theories, of which Spearman’s is an early example. In contrast, cognitive theories are both qualitative and quantitative. Following Spearman and even his predecessor, Galton, A. R. Jensen (2006) is perhaps the chief advocate of general intelligence or “psychometric g” as it is described in current literature. His evidence for “g” goes beyond factor analysis and seeks validity in reaction time studies of elementary mental processes. He is poised to launch a movement for finding a “super G,” or an all-inclusive general ability, picking up where Galton left off (Jensen, personal communication, March 2008).

More sophisticated statistical methods have been employed since Spearman, although these are primarily derived from factor analysis for identifying hierarchical strata of mental abilities. For example, Carroll’s (1993) theory is a proposal for three such strata: Stratum I (narrow abilities); Stratum II (broad abilities); and Stratum III, a general ability—Jensen’s psychometric g is in the third one. Stratum II contains Cattell’s fluid (Gf) and crystallized intelligence (Gc) (Hunt, (1997)). Stratum I includes specific skills such as reading words and arithmetic knowledge. However, the usefulness of these statistical results within a diverse cultural context has been questioned because the data sets were based on North American participants who were administered familiar North American tests.

Gf and Gc is a popular way to divide intelligence. As advanced by Cattell, and later by Horn, fluid intelligence is the ability to deal with novel intellectual problems, whereas crystallized intelligence is the ability to apply learned solutions to new problems (Hunt, (1997)). The problem, however, is agreeing on a standard battery for fluid and crystallized intelligence.

The psychometric approach to general intelligence has continued to advance. A recent classification of abilities has been proposed. It comprises verbal, perceptual, and image rotation abilities with general intelligence or g at its top. However, like all psychometric classification of intelligence, it has a common weakness. “The weakness of psychometric models is related to their strength. They stand on an impressive mathematical model of analysis of a given set of tests, without any clear stance about what the tests should be in the first place” (Hunt, personal communication, April, 2008).

Intelligence as cognitive processing is a common base for cognitive theories of intelligence. Such theories also advance the idea that intelligence has multiple categories. For example, both Sternberg and Gardner view intelligence as neither a single nor biologically determined factor, but as a number of domains that represent the interaction of the individual’s biological predispositions with the environment and cultural context. Das’s PASS theory is a further advance in this direction (Das et al., 1994).

The theory of multiple intelligences, developed by Gardner (1999), proposes seven separate kinds of intelligences comprising linguistic, logical-mathematical, spatial, musical, bodily-kinesthetic, interpersonal, and intrapersonal domains. There have also been two recent additions: naturalistic and existential intelligence. Although these nine types of intelligence are highly popular outside the community of psychologists because of their intuitive appeal, the proposal seems to lack empirical support. One may wonder why nine should be the limit. Earl Hunt reflects the opinions of many when he remarks that the theory of multiple intelligences cannot be evaluated by the canons of science until it is made specific enough to generate measurement models. Thus, if one cannot operationalize the concept of intelligence, it cannot be evaluated.

Sternberg’s triarchic theory (Sternberg, (2005)) proposes three components of intelligence. The first relates to the internal world of the individual and specifies the cognitive mechanisms that result in intelligent behavior; its components are concerned with information processing. Learning how to do things (and actually doing them) is the essential characteristic of the second component of Sternberg’s theory. This component is also concerned with the way people deal with novel tasks and how they develop automatic routine responses for well-practiced tasks. The third component of the theory is concerned with practical intelligence. More recently, Sternberg has expanded the three dimensions of intelligence, adding to these a measure of creativity. This revision led to his theory of successful intelligence, a theory that is still evolving.

The PASS theory of intelligence (Das et al., 1994) proposes that cognition is organized into three systems and four processes. The first system is the planning system, which involves the executive functions responsible for controlling and organizing behavior, selecting and constructing strategies, and monitoring performance. The second is the attention system, which is responsible for maintaining arousal levels and alertness and ensuring focus on relevant stimuli. The third system is an information processing system that employs simultaneous and successive processing to encode, transform, and retain information.

Simultaneous processing is engaged when the relationship between items and their integration into whole units of information is required. Examples include recognizing figures such as a triangle within a circle versus a circle within a triangle, or the difference between “he had a shower before breakfast” and “he had breakfast before a shower.” Successive processing is required for organizing separate items in a sequence, for example, remembering a sequence of words or actions exactly in the order in which they had just been presented. These four processes are functions of four areas of the brain: Planning is broadly located in the front part of our brains, the frontal lobe. Attention and arousal are a function of the frontal lobe and the lower parts of the cortex, although some other parts are also involved in attention as well. Simultaneous processing and successive processing occur in the posterior region, or the back of the brain. Simultaneous processing is broadly associated with the occipital and the parietal lobes, while successive processing is associated with the frontal-temporal lobes.

The four processes of the PASS theory can be assessed in a psychometric test battery, the Das-Naglieri Cognitive Assessment System, published in 1997. These tests have been used for understanding, assessment, and intervention with regard to educational problems (mental retardation, reading disability, autism, and attention-deficit), cognitive changes in ageing, and decision making in management.

There are similarities and differences between the theory of successful intelligence including the triarchic theory and the PASS theory. Both theories have empirical support in several research studies and both have an appeal to practical problems. The theory of successful intelligence attempts to include creativity and broader issues in intelligence but only the PASS theory has a psychometric measure. In keeping with contemporary trends, brain functions are also linked to the PASS processes in some recent studies.

The significance of brain studies awaits further discussion in the broader context of the biology of intelligence. The biology of intelligence is concerned with explaining how intelligence is related to specific areas of the brain and the connections between them. A brain network for general intelligence, involving the parietal and frontal lobes, has been recently suggested by Jung and Haier (2007). Their parieto-frontal integration theory attempts to explain individual differences in reasoning. According to Earl Hunt it also holds much promise for explaining individual differences in intelligence. However, it still considers intelligence as a general ability, and it does not appear to explain how emotions impact reasoning.

Goleman (1995), the researcher who popularized the term emotional intelligence, describes it as including such things as being able to motivate oneself and persist in the face of frustrations; to control impulse and delay gratification; to regulate one’s moods; and to keep distress from swamping the ability to think, empathize, and hope. More recently, however, emotional intelligence has been researched within the framework of abilities to carry out accurate reasoning about emotions and the ability to use emotions and emotional knowledge to enhance thought. A four-branch model has been advanced comprising accurate perceiving of emotion, using emotions to facilitate thought, understanding emotion, and managing emotions.

Emotional intelligence is not necessarily a part of intelligence as it is defined in Euro-American psychology. It is not like reasoning about abstract relationships, memory, and processing information. Consider compassion, a term that is commonly used in the East as a desirable emotional trait. Compassion is a blend of friendship and kindness, combined with a feeling of nonenmity. Similar adaptive emotions that promote well-being include an attitude of not expecting a return for kindness, a habit of reflection and detachment, and a belief that nothing is permanent. Such ideas are closer to Goleman’s concept of emotional intelligence than the four-branch model.

It is commonly believed that the contribution of heredity to IQ is between 50% and 80%. However, even identical twins from the same household, reared together, do not have a perfect correlation in regard to IQ, as they should for sharing identical genetic material. Even if one accepts that the correlation is .85, it is high, but not perfect. More importantly, other factors have not been considered, such as literally sharing the same womb, that may have contributed to that high correlation. When other contributing factors, such as home environment (17%) along with maternal environment were included, the estimated contribution of heredity dropped down to 48%. The implication of these previously overlooked influences is important—there is room to intervene and enhance intelligence.

One should not assume while accounting for the percentages of contribution that the effect of environmental factors is unidirectional; that is, it can only add to genetic endowment. As a matter of fact, it works both ways: It is bi-directional. The environment plays an active role in regulating gene expression, an important determinant neglected in previous studies of heritability of IQ. Researchers now know much more about the conditions that favor the expression of only specific genes and about epigenetic changes. Epigenetics is about regulation of various gene functions, including gene expression, that are brought about by heritable, but potentially reversible, changes that can occur due to a number of cultural and environmental influences. For example, it helps one to understand why identical (monozygotic) twins may be different concerning health and disease and also why during pregnancy the mother’s diet influences epigenetic changes. There could be numerous environmental effects, including maternal behavior, that cause some epigenetic traces.

The earlier research that assigned proportions of genetic and environmental contribution to IQ—nature and nurture—ignores the fact that the influence of one on the other is bidirectional. The correct question to ask is how genes and environment work together to influence not only intelligence, but a host of other conditions associated with diseases, such as Alzheimer’s. Simply put, one should accept that functions of genes are altered by environmental influences and vice versa.

Understanding Gene × Environment interactions has progressed rapidly: A handful of genes have shown significant association to intelligence and can actually be measured. Thus, it is feasible to identify those environmental factors that interact with genetic makeup. Generalist genes have been proposed, suggesting that the same genes affect several cognitive abilities and disabilities.

Behavior genetics has changed the parameters of measuring heritability but, in a broader perspective, world events have forced us to look at the sociopolitical problems of IQ testing. Consider the two important cultural factors that lower intelligence: the absence of a literacy environment and inadequate provision of health care for children even in some affluent countries. There is, then, room for intervention to enhance intelligent behavior and a reason to believe that intelligence is malleable.

In this brief presentation on intelligence a few important topics have been excluded. Social intelligence and its failures is an emergent topic; it is explained by using brain-imaging studies that focus on how humans interpret their own and other’s desires, intentions, and feelings. Significant difficulties in doing so are observed in autism and schizophrenia, as shown by Chris and Uta Frith.

The identification of gifted individuals, especially in verbal and mathematical ability, has much practical value. Lubinski and Benbow (1992) have shown that based on Scholastic Aptitude Test results obtained at age 13, a majority of those individuals who were in the top 1% have become writers and scientists. This is less surprising than the fact that among them, a “tilt” toward verbal or mathematical ability at that early age has tended to produce writers or scientists, respectively.

Racial and gender differences in intelligence have not been considered in this brief article, although this is an age-old topic that remains controversial. Cross-cultural difference in intelligence is another important consideration now that many countries are concerned with multinational communities and business companies. These differences, however, matter much more in personality and motivation than in the area of academic intelligence. Finally, the Euro-American concept of intelligence needs to be broadened by considering Asian and African cultures. A list of words commonly related to intelligence in many cultures includes acquired knowledge, conscience, and a pragmatic understanding of the self and of the life situation.

Emotional Intelligence; Intelligence Quotient; Intelligence Testing.