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

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

Some of the literature attributes the first to humans and the second to nonhuman 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 his colleagues at the University of Zurich drew up, in 2017, a review in which they evaluate the presence of g in non-human animals and explore its implications on the theories of the evolution of cognition.

• Related article: "Intelligence: the G Factor and Spearman's Bifactorial Theory"

How is the intelligence of humans and animals?

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

But What does it mean that a species has general intelligence? At the empirical level, we speak of general intelligence when the 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 a positive manifold, and it 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 short, the presence of a general intelligence in humans is represented by the factor g and finds support both in neurobiology and in 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 consists in considering 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.

Under this context, solutions to these problems would have been channeled by natural selection. An example would be that a species develops a great spatial memory when historically it has needed to find food in large and complex territories. Therefore, according to this vision, human and animal minds can be considered a set of specializations that arose to respond to specific problems in the environment.

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

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

General intelligence in nonhuman animals

The vast majority of studies that have tried 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. 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 the intelligence of this species. An example is Esther Herrmann and collaborators who, applying similar intelligence tests in chimpanzees and human children, finds that intelligence is organized differently in different species. Children's performance was best explained through three different modules, or intelligences (spatial, physical, and social). On the other hand, “chimpanzee intelligence” was better explained by two factors: a spatial one and a second that grouped both physical and social tasks).

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

On the contrary, other authors have defended the presence of general intelligence in chimpanzees after finding characteristics shared with humans. William D. Hopkins and colleagues at Georgia State University found that intelligence is, to a large extent, hereditary in chimpanzees. Furthermore, the g factor has been related to larger brains and greater cortical thickness in this species, and Beran and Hopkins found a strong correlation between g and scores on self-monitoring tasks.

Although the presence of g in great apes is still debated, These studies raise the possibility that general intelligence is not exclusive to the human species. In favor of this idea, most of the 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 a large part of the studies support the presence of general intelligence in rodents and primates leads us to consider that it has been developed in some lineages above or, perhaps at the same time, than specific adaptive capacities, theoretically easier to shape by the natural selection.

This is where a component comes into play that has been directly correlated with general intelligence: brain size. Just as specific capabilities (however sophisticated they may be) have not required a great expansion brain, it seems that those species with more general intelligence have required a significant increase in tissue cerebral.

But, What are the conditions that have led these species to possess these capacities? One proposal that tries to answer this question is the cognitive buffer hypothesis, which considers innovation and learning two main engines to develop general intelligence. Based on this idea, species whose environments often change or become unpredictable would have required a general intelligence to cope with unusual or changing ecological difficulties. Examples in favor of this theory would be the correlation between more innovative species with a greater presence of G in primates, or the fact that there has been found a higher proportion of "colonizing success" in species with more larger brains (including birds, mammals, amphibians, reptiles and fishes).

If we believe this hypothesis, the logical thing would be to wonder 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 great costs it has. The brain expansion that this type of adaptation requires entails an enormous energy cost (remember that, in humans, the brain can reach consume up to 20% of the energy required by the entire organism) which, in addition, also requires a slowing down of physical and brain development at the ontogenetic.

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

This explanation coincides with the well-known hypothesis of the social intelligence of Michael tomasello in giving importance to social learning and making 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 abilities and general intelligence. At this point, perhaps it would be more interesting and accurate to ask ourselves what skills emerged by specializations and which are the result of a subsequent adaptation thanks to the cognitive flexibility that accompanies general intelligence. In this direction, and as always in science, more comparative studies are necessary to understand when and why g evolved.