Smart Brains: what is general intelligence and why should we study it?


For over 100 years there has been documented a rather remarkable phenomenon that someone who excels at one thing tends to excel at another. This may seem intuitive at first but let’s think about that for a second: why would being good at maths mean you are good at art? Or being good at discriminating between two musical pitches mean you are also good at science? This was first studied by Charles Spearman who, after testing school children on a wide range of both academic and non-academic abilities, found that those who are better at one thing tend also to be better at others (i.e. they maintain a relative ranking among their peers between tasks). This phenomena became known as Spearman’s g for “general intelligence” which proposes that all forms of intellectual ability share some sort of factor, be it mentally or neurologically, the strength of which determines your relative ability to perform tasks. Picture1In this image g is at the top and under it are groups of abilities (G1-G3; e.g. use of numbers, space, and memory), and under these are specific tasks (T1-6; e.g. spotting patterns, noticing differences, adding numbers). Since the original study this has been replicated numerous times across ages, cultures, contexts and using multiple sets of tests, and the evidence is overwhelming: abilities correlate.

Why this is true is less certain, but some progress has been made towards understanding what goes on in the brain to underlie this. One of the most likely explanations is that g relates to the speed at which brain cells communicate. As this is a factor which would affect the brain at all levels, it is possible that small differences between individuals would cause differences in information processing. This has been supported with a huge amount of evidence suggesting that general intelligence correlates with reaction times, such as responding to a particular stimulus. Thus the genes and upbringing which build a faster brain may just be building a better and faster system in general, and this manifests as correlations in mental abilities.

The idea is far from without criticism, however, as different theorists have postulated different models which range from there being a few “types” of intelligence up to hundreds. A well-known alternative is Cattell’s model of fluid and crystallised intelligence. The former is involved in problem solving, and the latter in knowledge which is accrued by the use of the former over your life. G is also criticised for being a statistical artifact and not a biopsychological phenomenon (i.e. nothing to do with the brain); that choosing to use statistical tools which look for a single factor to explain your observations means you’re more likely to find one. Despite this, measures of g (or close approximations such as IQ tests) have been found to predict job performance, income, health, and susceptibility to certain mental disorders.

This whistle-stop tour of Spearman’s g was intended to be just that: a tour. There are many a good review and webpage out there which delve into the topic is much greater detail (a quick google search for “general intelligence” or “spearman’s g” which return you many). What I felt was missing however was information on what is known in non-human animals and what this can tell us about the nature of general intelligence. What I present next is again by no means everything which is known but it will illustrate a few trends and hopefully inspire you to read on about this fascinating phenomenon.


In our closest relatives, the primates, innovation, social learning, tool use, removal of food from enclosures such as cases and skins (extraction), and social manipulation was found to correlate across 62 species. Whilst another study found differences in the abilities of 7 species, this suggests that a well-rounded intellect is at least a general feature of the primates. So what about other mammals? Most work has been done in rodents and has largely supported the human evidence. The abilities of mice tested on navigation, aversion to negative stimuli, distinguishing between smells, and associating a sound with a reward all correlated. Another study found that similar abilities were also correlated with the ability of mice to hold information in their working memory, suggesting that the rapid use of information is a key part of intelligence. Whilst a single study found that location learning, working memory, the ability to direct away from a goal and the ability to detect novelty did not all correlate, it has largely been found that abilities do correlate in mice. Support also comes from dogs tested on their ability to learn new rules and recognise new objects, and the speed of their reactions.


In our feathered cousins most work has been done by looking at vocalisations, such as how birdsong relates to learning. Songbirds learn to sing at least in part from a tutor, such as their father, and the number of notes a zebra finch sings has been shown to relate to how fast they get to grips with a food location task. The total number of songs a song sparrow sings, however, has been shown to be inversely correlated with their ability to learn the spatial arrangement of food (i.e. the more songs they have the slower they learn). This suggests that at least in song sparrows, investment in song-learning and other forms of learning may be traded off against each other. Other work in birds has revealed mixed results with some and no correlations in abilities depending on the tasks involved.

What does this tell us so far?

It is interesting to note how conclusive the human evidence is, that in primates and other mammals it’s near enough solid, but that in birds it’s all over the place. What does this mean? It could be telling us that fundamentally the mental abilities of mammals and birds went in different directions. Namely that mammals became well-rounded flexible problem solvers whilst birds because more specialised. Both approaches have their benefits. Being highly flexible means you are more likely to survive abrupt changes to your physical and social environment. If early mammals lived, and continue to live, in an unstable environment for one reason or another then perhaps having a brain geared towards general flexibility would be advantageous. Conversely, different bird species may have divided very quickly into small niches with particular demands and thus would be better suited to being strong at the particular abilities needed for their lifestyle without regard for others. However, drawing any conclusions is very difficult as only a limited number of species have been looked at in this regard, and whilst primates are well studied we don’t have examples from the full range of either mammals or birds, never mind reptiles, amphibians, fish or the plethora of invertebrates! It is interesting however that so far there seems to be a difference between the mammals and the birds.

What can be gained from studying this in non-human animals?

Studying non-human animals brings us insights which studying just humans can’t. Firstly, the ability to manipulate systems to gain a greater understanding of how they work. Tentative evidence regarding brain size and genetics exists in humans but with other animals the specific substructures of the brain and their involvement in the different parts of learning can be studied: what makes them different? How does that translate into the differences we observe? We can study the fine-scale genetic, biochemical, physiological, and anatomical changes involved to create a much fuller picture of learning in general. It is also known that issues during early life such as nutritional deprivation or poor parental health can affect someone’s abilities in adulthood. With this observational evidence we can look at the development of other animals to work out when and why early life and other periods are so important.

Secondly, evolution. Positive correlations in mental abilities create problems for those models of brain and behaviour that suggest the brain contains a series of systems which specialise in independent functions. For example there would be a separate system for detecting changes in the environment, another for learning to speak, and another for learning what’s good to eat. By studying non-human animals we can learn more about differences in how these abilities are organised both within and between individuals, as well as within and between species. Thus the “mind” as we understand it may have evolved differently multiple times and for different functions in different groups of animals.

Finally, and perhaps most importantly, for a greater understanding of one of many fascinating aspects of nature. Studying more abilities in more animals only adds to the wealth of knowledge that we as a species are so good at. The scientific-ape that we are always strives to learn more and that in itself is surely the greatest reason to delve further in our attempt to unravel the nature of general intelligence.

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