What is Intelligence?

1 What is Intelligence?

The most succinct definition of intelligence is the ability to solve problems. But to be more precise it can be defined as 

a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly and learn from experience. It is not merely book learning, a narrow academic skill, or test-taking smarts. Rather, it reflects a broader and deeper capability for comprehending our surroundings - "catching on," "making sense" of things, or "figuring out" what to do."

 From New Scientist

2 IQ Tests - What are the measuring? 

A century ago, psychologist Charles Spearman observed that individuals who do well on one mental test tend to do well on all types of mental tests.  He reasoned that all tests must therefore tap into some deeper, general ability and he invented a statistical method called factor analysis to extract this common factor from correlations across all tests. He defined a general factor of intelligence known as the "g factor".  In essence, g equates to an individual's ability to deal with cognitive complexity.

In the 1970s psychologist Arthur Jensen discovered that g aligns with diverse features of the brain, from relative size to processing speed. In all human groups - and in other species too - most cognitive variation comes from variation in g.

So, g operates as a proficiency at mentally manipulating information, which facilitates learning, reasoning, and solving problems.  At the physiological level, differences in g reflect differences in the brain's overall efficiency or integrity. The genetic roots of g are emerging from the joint actions of hundreds if not thousands of genes, themselves responding to different environments.

Having a high g rating is useful, but not a virtue. It is especially handy when one is faced with complex tasks, but it is also associated with lower rates of health-damaging behaviour, chronic illness, post-traumatic stress disorder, Alzheimer's and premature death. A high g helps an individual get ahead socioeconomically, but it has little connection with emotional well-being or happiness. Neither does it correlate with conscientiousness, which is a big factor in whether someone fulfils their intellectual potential.

3 Quantifying intelligence

In 1904, Alfred Binet developed the IQ test to find a practical way to identify children who needed support in elementary school. The test consisted of 30 short, objective questions on tasks such as naming an everyday object and identifying the heavier of two items. His invention worked and spawned massive intelligence-testing programmes on both sides of the Atlantic, used to test applicants for jobs, military recruits, college applicants etc. 

The most comprehensive IQ tests combine scores from areas such as comprehension, vocabulary and reasoning to give an overall IQ. IQ tests are the most technically sophisticated of all psychological tests and undergo the most extensive quality checks before publication. 

Intelligence tests are calibrated so that, at each age, the IQ average score is 100 and 90 per cent of individuals score between IQ 75 and 125. The typical IQ difference between strangers is 17 points and it is 12 between full siblings. So what makes some people smarter than others? And how can we change our score?

4 Older and wiser 

The brain is a physical organ and no less subject than any other to ageing, illness and injury. Normally, the aptitude for learning and reasoning increases quickly in youth, peaks in early adulthood, and then declines slowly thereafter and drops precipitously before death. The good news is that some important abilities resist the downturn.

Some IQ researchers distinguish between tests of fluid intelligence (gF) and crystallised intelligence (gC). The first assess on-the-spot learning, reasoning and problem solving; the second assess the crystallised fruits of our previous intellectual endeavours, such as vocabulary in one's native language and broad cultural knowledge. During youth, gF and gC rise in tandem, but they follow different trajectories thereafter. All gF abilities decline together, perhaps because the brain's processing speed slows down with age. However, most people's gC abilities remain near their personal peak into old age because they reside in the neural connections that gF has laid down over a lifetime of learning and practice. Of course, age-related memory loss will affect an individual's ability to recall, but exactly how this affects intelligence is not yet known.

This has practical implications. On the positive side, robust levels of gC buffer the effects of declining gF. Older workers are generally less able to solve novel problems, but they can often compensate by calling upon their larger stores of experience, knowledge and hard-won wisdom. But gC can also disguise declines in gF, with potentially hazardous results. For example, health problems in later life can present new cognitive challenges, such as complex treatments and medication regimes, which individuals with ample gC may appear to understand when actually they cannot cope.

There are ways of slowing or reversing losses in cognitive function. The most effective discovered so far is physical exercise, which protects the brain by protecting the body's cardiovascular health. Mental exercise, often called brain training, is widely promoted, but it boosts only the particular skill that is practised – its narrow impact mirroring that of educational interventions at other ages. Various drugs are being investigated for their value in staving off normal cognitive decline, but for now preventive maintenance is still the best bet – avoid smoking, drinking to excess, and head injuries.

5 Nature and nurture 

Each of us is the embodiment of our genes and the environment working together from conception to death. To understand how these two forces interact to generate differences in intelligence, behavioural geneticists compare twins, adoptees and other family members. The most compelling research comes from identical twins adopted into different homes – individuals with identical genes but different environments – and non-kin adopted into the same home – unrelated individuals sharing the same environment. These and other studies show that IQ similarity most closely lines up with genetic similarity.

More intriguingly, the studies also reveal that the heritability of intelligence – the variation due to genes – steadily increases with age. Heritability is less than 30 per cent before children start school, rising to 80 per cent among western adults. In fact, by adolescence, separated identical twins answer IQ tests almost as if they were the same person and adoptees in the same household as if they were strangers.

Surprisingly, most family environments are equally effective for nurturing intelligence – the IQ of an adult will be the same almost regardless of where he or she grew up, unless the environment is particularly inhumane. Studies on the nature of nurture offer a clue as to why this is. All children enter the world as active shapers of their own environment. Parents and teachers experience this as their charges frustrate attempts to be shaped in particular ways. And increasing independence gives young people ever more opportunities to choose the cognitive complexity of the environments they seek out. The genetically brighter an individual, the more cognitively demanding the tasks and situations they tend to choose, and the more opportunities they have to reinforce their cognitive abilities.

Given that an individual's ability to exploit a given environment is influenced by their genetic endowment, and given that "better" family environments tend not to produce overall increases in IQ, it is not surprising that attempts to raise low IQs by enriching poor school or home environments tend to disappoint. Narrow abilities can be trained up, but g apparently cannot. This makes sense if g is an overall property of the brain. That does not mean intensive early educational interventions lack positive effects: among other things they may reduce rates of teenage pregnancy, delinquency and school dropout. Besides, even if we cannot boost low intelligence into the average range, we do know how to help all children learn more than they currently do and achieve more with the intelligence they have.

6 Different types of intelligence

Consider the engineer's superior spatial intelligence and the lawyer's command of words and you have to wonder whether there are different types of intelligence. This question was debated ferociously during the early decades of the 20th century. Charles Spearman, on one side, defended the omnipotence of his general factor of intelligence, g. On the other, psychologist Louis Thurstone argued for seven "primary abilities", including verbal comprehension (in which females excel) and spatial visualisation (in which males excel). Thurstone eventually conceded that all his primary abilities were suffused with the same g factor, while Spearman came to accept that there are multiple subsidiary abilities in addition to g on which individuals differ.

This one-plus-many resolution was not widely accepted until 1993, however. It was then that American psychologist John B. Carroll published his "three stratum theory" based on a monumental reanalysis of all factor analysis studies of intelligence. 

At the top is a single universal ability, g. Below this indivisible g are eight broad abilities, all composed mostly of g but each also containing a different "additive" that boosts performance in some broad domain such as visual perception or processing speed. These in turn contribute to dozens of narrower abilities, each a complex composite of g, plus additives from the second level, together with life experiences and specialised aptitudes such as spatial scanning.

This structure makes sense of the many differences in ability between individuals without contradicting the dominance of g. For example, an excellent engineer might have exceptional visuospatial perception together with training to develop specialist abilities, but above all a high standing on the g factor. The one-plus-many idea also exposes the implausibility of multiple-intelligence theories eagerly adopted by educators in the 1980s, which claimed that by tailoring lessons to suit the individual's specific strength – visual, tactile or whatever – all children can be highly intelligent in some way.

7 Physical Attributes of the brain

How is intelligence affected by the brain's physiology? 

7.1 Brain Mass 

It was once thought that a heavier brain means higher intelligence because it would have more neurones ("grey matter") and axons (white matter). However, "more" doesn't mean "better" as shown by the Kleiber's Law diagram, in which humans are at the apex of intelligence. Further increase in brain mass entails more increase in body weight such that the add-on brain cells are allocated for neural housekeeping chores (such as controlling more muscle fibres) unrelated to intelligence.

7.2 Neural Connections

The human brain contains about 120 billion neuronal cells, with an approximately equal number of non-neuronal cells (glial cells). These cells pass signals to each other via as many as 1000 trillion synaptic connections. The communication networks (at about 2% of our body weight) consume about 20% of the energy that we expend at rest. In newborns, it is an astounding 65% so infants rely on parental care for survival. Further increase in neural connections will demand more resource at the expense of the other biological processes in our body.

7.3 Transmission Time

The transmission time between neurones depends on the travelling distance. That's why the motion of elephants appears to be rather slow. The transmission time is also determined by the speed of the signal. It is found that thicker axons carry signals faster. This advantage is negated by greater consumption of energy and occupies more space (the same trade-off as for more neural connections).

7.4 Neurone Density

It was discovered recently that unlike other mammals, cortical neurones in primates enlarge very little as the brain increases in size. This kind of packing strategy allows for a greater number of cortical cell as brains enlarge; and it also permits faster communication, because the cells pack more closely. However, thermal motion triggers random firing of the action potential if the axons or neurones become too small.

7.5 Evolutionary Limits

Restricted by the above-mentioned constraints, it is doubtful that a major evolutionary leap could lead to a smarter brain. The evolution of brains started about 600 million years ago. Since then all kinds of animals have now evolved to a point where the brain circuits have arrived at a similar design for running tasks such as vision, smell, navigation, etc. Such evolutionary convergence usually suggests that a certain anatomical or physiological solution has reached maturity so that there may be little room left for improvement. If our brains were bigger, we would think slower and too much energy would be required. If our brains had increased neural connections or faster transmission times, it would again cost too much energy. And if our neurone density increased, the transmission signals would be too noisy.