General Intelligence: what is it and how has it evolved?

We review this major concept in the study of human intelligence.

One of the most important debates when addressing the evolution of human intelligence is whether humans have developed a single general intelligence (or g) or, on the contrary, an intelligence divided into a set of specializations.

Part of the literature attributes the former to humans and the latter to non-human animals, but as always in science, not everything is so simple and there are studies that provide data against this idea.

On this debate, Judith M. Burkart and her colleagues at the University of Zurich produced, in 2017, a review in which they evaluate the presence of g in nonhuman animals and explore the implications of this on theories of the evolution of cognition.

What is intelligence like in humans and animals?

In humans, we can understand intelligence as the ability to reason, plan, solve problems or think abstractly, among other capabilities. In animals, it has been defined more by the ability to acquire knowledge of the physical or social environment and use it to solve new problems.

But But what does it mean to say that a species has general intelligence? At the empirical level, we speak of general intelligence when individuals of the species score similarly in different types of cognitive tasks (such as causal reasoning or social learning tasks), giving rise to the famous g-factor. Or, in other words, that there is a significant correlation between some scores and others.

This is what is known as positive manifold, and is the great argument in favor of the presence of g in humans. Another is the correlation of g with brain size, gray matter volume and cortical thickness, as well as school and work success, among others. In summary, the presence of general intelligence in humans is represented by the g factor and is supported by both neurobiology and life characteristics of individuals.

The alternative, or perhaps complementary, view of general intelligence is to speak of a modular intelligence. An intelligence based on specialized modules for different cognitive abilities. The evolutionary basis behind this concept is to consider these modules as cognitive adaptations to problems that have been repeated over a long period of time in the course of the evolution of a species.

In this context, solutions to these problems would have been channeled through natural selection. An example would be for a species to develop a large spatial memory when it has historically needed to find food in large and complex territories. Therefore, according to this view, human and animal minds can be considered a set of specializations that arose in response to specific environmental problems.

In the past, a very strict concept of a modular mind was defended, with modules, or independent intelligences that process information with different "input channels". This view is totally incompatible with the presence of a general intelligence in the same individual. However, many authors have recently proposed the compatibility of a general intelligence in the same individual, recently many authors propose the compatibility of these modules with a "central system" of information processing and, in turn, with a "central system" of intelligence. and, in turn, with a general intelligence.

But if this core system has only been demonstrated in humans, the key question concerning the evolution of general intelligence would be how it has emerged, during the course of human evolution, over and above the previously existing modular system. To answer this question it is necessary to probe the cognitive characteristics of nonhuman animals.

General intelligence in non-human animals

The vast majority of studies that have attempted to find g in nonhuman animals have been carried out mainly in rodents and primates, especially great apes. In rodents the presence of g appears to be quite robust, with studies examining up to 8 different tasks in mice and rats. As for non-human primates, the results have been rather mixed:

Some studies, mainly focused on chimpanzees, have found alternatives to the g factor to explain intelligence in this species. One example is that of Esther Herrmann and collaborators who, applying similar intelligence tests in chimpanzees and human children, found that intelligence is organized differently in different species. The performance of children was best explained by three different modules, or intelligences (spatial, physical and social). On the other hand, "chimpanzee intelligence" was best explained by two factors: a spatial one and a second that grouped both physical and social tasks).

Subsequent studies such as that of Herrmann and Call and Amici and collaborators found similar results (no presence of g) in chimpanzees and at the interspecific level, respectively.

In contrast, other authors have defended the presence of general intelligence in chimpanzees after finding shared characteristics with humans. William D. Hopkins and coworkers at Georgia State University found that intelligence is largely heritable in chimpanzees. In addition, the g factor has been linked to larger brains and greater cortical thickness in this species, and Beran and Hopkins found a strong correlation between g and scores on self-control tasks.

Although the presence of g in great apes remains debated, these studies raise the possibility that general intelligence is not unique to the human species.. In favor of this idea, most studies that have investigated the presence of general intelligence at the interspecific (or G) level find evidence in favor of it.

So how has general intelligence evolved?

The fact that most studies support the presence of general intelligence in rodents and primates leads us to consider that it has developed in some lineages above or, perhaps at the same time, as specific adaptive abilities, theoretically easier to shape by natural selection.

Here a component that has been found to be directly correlated with general intelligence comes into play: brain size. Just as specific abilities (however sophisticated they may be) have not required a large brain expansion, it appears that those species possessing more general intelligence have required a significant increase in brain tissue.

But, what are the conditions that have led these species to possess these capacities? One proposal that attempts to answer this question is the cognitive buffer hypothesis, which considers innovation and learning as two main drivers for developing general intelligence. Based on this idea, species whose environment tends to change or become unpredictable would have required general intelligence to cope with unusual or changing ecological difficulties. Examples in favor of this theory would be the correlation between more innovative and more G-present species in primates, or the fact that a higher proportion of "colonizing success" has been found in species with larger brains (including birds, mammals, amphibians, reptiles, and fish).

If we believe this hypothesis, it would be logical to ask why not all species have ended up developing this intelligence that would allow them to adapt to all types of environments. Well, the answer lies in the high costs involved. The brain expansion required by this type of adaptation entails an enormous energetic cost (remember that, in humans, the brain can consume up to 20% of the energy required by the whole organism) which, in addition, also requires a slowing down of physical and brain development at the ontogenetic level.

Under these conditions, only species capable of providing special and lasting care by adults to their offspring would be able to afford such a sacrifice. In this scenario, both the absence of constant predators that threaten the survival of adults and the allomaternal care (care of the offspring by, in addition to the mother, other individuals of the group) of many species, especially primates, would play an important role.

This explanation coincides with Michael Tomasello's well-known hypothesis of social intelligence in giving importance to social learning and holding it responsible, to a large extent, for brain expansion and the high cognitive capacities of the human species.

In conclusion, this review leads us to accept (or at least consider) the compatibility between specialized cognitive capacities and general intelligence. At this point, perhaps it would be more interesting and accurate to ask ourselves which abilities emerged from specializations and which are the result of a later adaptation thanks to the cognitive flexibility that accompanies general intelligence. In this direction, and as always in science, more comparative studies are needed to understand when and why g evolved.