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The Evolution of Human Intelligence: Increasing Importance of Domain-Specific Intelligence in the Modern Environment

Kyle R. Skottke
Rochester Institute of Technology

Human intelligence has evolved steadily over the course of thousands of generations without drastic change in the environment. However, the challenges we face in modern society have forced the independent domain of intelligence to assume the roles that other domains would have played in the primitive environment in which we evolved. There are clear genetic links that predispose people to have a larger cerebral cortex, allowing them to better deal with the challenges of the expanded work load modern life puts on our intelligence domain. Evolution has not yet had a chance to catch up to the rapid progress we have made as a society and might not due to human circumvention of natural selection.

General intelligence (g) can be described as the ability of an individual to acquire and apply knowledge. Many studies have shown that g is at least 50 percent heritable and thus, can be passed down from generation to generation (DiLalla, 2000). Evolution is the concept that the gene expression of organisms changes over time. It is thought that either natural selection or sexual selection is what drives this change in gene expression. It is clear that human intelligence has changed since the emergence of the very first hominids. While there are several genetic explanations for this change, with several genes directly involved in g, there are also environmental and migrational factors that have influenced human intelligence. The migration of people to all areas of the earth along with the industrialization of modern society has abstracted modern man from our ancestors. Since our environments are abstracted, a greater importance has been placed on cognitive ability and intelligence to allow us to function in modern society. The following is an argument for how migration and industrialization have impacted g and a discussion of some of the genes that have been associated with intelligence (Dewsbury, 2000).

General Intelligence as an Independent Domain

Two main theories concerning g are the dependent domain and the independent domain models. The domain dependent model has general intelligence as a broad category which is divided into smaller more specific intelligences, including mate selection, cheater detection module, and face recognition module. Under this model, it can be inferred that if an individual's g is high, then their ability to select and obtain a mate, detect lies, and recognize people's faces would also be higher. It also follows that someone with a low IQ would have trouble finding a mate and detecting when people were lying.

The independent domain theory states that general intelligence is only one of several psychological mechanisms, along with mate selection, the cheater detection molecule, and the face recognition module that have evolved. It is important to note that in this model, general intelligence is defined as the ability to use deductive logic and abstract thought.

I believe the independent domain model is more realistic since there have been numerous studies showing that the psychological mechanisms of the brain are unlinked. One such study was done to compare the correlation between IQ, which measures g, and the success of individuals in finding a mate, which was measured in terms of marriage. The studies found shown that very intelligent individuals (with IQs above 125, at or above the 95th percentile of the IQ distribution) are the least likely to marry of all the cognitive classes (Kanazawa, 2004). This data suggests that mate selection and general intelligence are unrelated, such as in the field independent model.

Based on the findings from the study above, it is possible to conclude that each of the psychological mechanisms is dedicated to its own area of expertise, which has been forged over thousands of generations of human evolution. The concept of dedicated psychological mechanisms denotes that each of the previously mentioned psychological mechanisms can solve adaptive problems in its own narrow domain but nowhere else (Kanazawa, 2004). This would explain why someone might have increased intelligence, but it would not be able to help in acquiring a mate, just like someone's ability to find a mate would not increase their ability to use logic and abstract thought.

Genetic analysis also supports the field independent model. One finding, "concerning specific cognitive abilities is that multivariate genetic analyses indicate that the same genetic factors largely influence different abilities" (Plomin 1999). This refers to the fact that genes involved in verbal ability are also linked to spatial ability and other cognitive abilities. Therefore, the psychological mechanism of general intelligence is not divided into smaller groups based on specific cognitive action; rather that it is a category that encompasses all cognitive ability. This supports the field independent model because the psychological mechanisms of mate selection and cheater detection are not cognitive fields, but are more hard wired gut feelings. Multivariate genetic analysis also discredits the field dependent theory because the analysis shows that verbal and spatial abilities are tied to the same genetic factors, whereas in field dependent theory, they are separate categories under g.

Origins of Dedicated Intelligence

Reverse engineering in the context of evolution, is the attempt to infer the ancestral conditions, called the environment of evolutionary adaptation, that would have made certain genetically inherited behavior-inducing modules increase their bearers' reproductive success (David, 2002). It has been inferred that prehistoric man originated in Africa based on archeological evidence. In this primitive environment, there would have been certain physiological traits that would have been more favorable to the bearer of the trait.

One such example that survives to this day is the taste preference for foods that are higher in fat and sugar (Emery & Clayton, 2004). It can be assumed that during the origin of man, people who had taste buds that would have made them more inclined to eat fatty foods would have been much healthier and had a better chance at surviving and passing on their genes than individuals that had a predisposition to prefer foods low in nutrition.

Prehistoric man evolved during the Pleistocene epoch, about 1.6 million to 10,000 years ago, a period characterized by constancy and continuity (Kanazawa, 2004). Our ancestors were hunter-gatherers who would have spent their whole lives on the African savanna. Not only this, but for many generations, the environment remained stable, which is why evolution was able to take place. Evolution is a very slow process, which occurs over a long period of time through the mechanisms of natural selection and sexual selection.

The consistency of the Pleistocene epoch would have promoted evolution in a specific direction without much deviation. One key aspect of this era was that more children were being born than were supportable by the environment. There had not been the advent of modern agriculture to mass produce food, and there was no domestication of animals. It is primarily because of this aspect of the Pleistocene epoch that evolution was able to shape the human race through natural selection. Since only some of the population was able to survive and reproduce each generation, it follows that the individuals best suited to the environment would survive and pass on their traits, since most traits are at least 50 percent heritable.

However, in the past 1000 years, we have come a long way from our roots in Africa. It is safe to say that not even the most basic thing in our lives, the natural landscape, resembles the landscape that our ancestors lived in. This means that many of the psychological mechanisms that evolved over the backdrop of an ever constant savannah environment are now useless given the modern environment. For example, in America with the large abundance of food that we have, the inborn predisposition to foods high in sugar and fat is no longer an adaptive advantage, but might actually shorten the life span of an individual by increasing the risk of obesity and heart disease.

Not only has the availability of food changed, but many other aspects of life, including personal interactions have changed. The business related society we live in fosters many different types of relationships and interactions that primitive man would not have dealt with and would not have had a mechanism to cope with. These interactions include short encounters with strangers and relatively unknown people, which the business environment relies heavily upon. The fact that we were not adapted to deal with impersonal relationships like this explains why some people have better people skills, since it is not a universal adaptation.

Since we live in an environment that is far abstracted from the African savannah, even the simplest of things that we take for granted relies on our function of g. Changes have been made so rapidly in society that the natural selection mechanism of evolution has not had a chance to catch up to the progress we have made technologically. Thus, the part of the brain that is able to process abstract thought is used to help us navigate and cope with our "foreign" environments. There is no physiological mechanism for humans to know how to operate an elevator or how to travel through a city filled with skyscrapers, which is why g has become increasingly important over the past millennia; we have further developed g through our dependence on it. A study was done which supports the idea that intelligence can develop over time. Newborn chicks which have small brains are able to correctly perceive partially occluded objects as one object. However, human babies were likely to perceive it as two separate objects (Langer, 2004). This study demonstrates that intelligence develops over time, and if intelligence can develop over time in one individual, it is possible for intelligence to develop over time for an entire population. Since intelligence can develop and because popular views on intelligence and types of intelligence are changing, there has also been increased pressure to revise methods of testing intelligence; for which the most common measure is currently modifications of the Wechsler intelligence test (Esters, 1999).

The Molecular Biology of Intelligence

There have been several examples where one specific gene has been linked to loss of function or disease. Sickle Cell Anemia, Hemophilia, and PKU are examples of a mutation in a single gene causing loss of function of a protein that leads to disease. However, intelligence is not dimorphic, there is a broad range of intelligence and there are several genes that govern g. These multiple-gene systems are often referred to as quantitative trait loci (QTLs), which can, contribute interchangeably and additively like probabilistic risk factors (Plomin, 1999).

Historically genetics has utilized mutants to identify the location and function of genes. Due to the slow reproductive rates of humans and obvious ethical issues, it is much harder identify, isolate, and study human mutations. One of the few chances scientists had to look at gene mutations affecting intelligence was in the case of Pakistani families isolated in Yorkshire. For social reasons the Pakistani families interbred and as a result the homozygosity of the group increased. This means that there were more cases of two recessive mutated alleles occurring that were once covered up by a dominant wild type allele. As a result, there was a very high proportion of the group that had microcephaly compared to the rest of the population. Microcephaly is a condition which leaves the patient with an abnormally small head and brain.

The physical size of specific regions of the brain can have tremendous effects on an individual's g. One gene that was linked to the smaller brain size in the microcephaly patients was named microcephalin. This gene was determined to be active only during the fetal stages of development.

Another gene linked to smaller brain size is found in nearly all members of the animal kingdom. The Asp gene is responsible for forming spindle fibers during cell division. The spindle fibers act to separate homologous pairs of chromosomes so that there will be genetic material for both of the daughter cells. In the Pakistani patients that had microcephaly, half of them had two defective copies of the Asp gene. When both copies of the Asp gene are defective, spindle formation and chromosome separation are substantially slowed. This results in much slower growth of the brain, a smaller brain, and therefore a much lower g.

In addition to the rare opportunity to study field mutation in Yorkshire, QTL studies have isolated insulin-like growth factor-2 receptor (IGF2R) as a gene on chromosome 6 which is linked to intelligence because it has, "been shown to be especially active in brain regions most involved in learning and memory" (Plomin, 1999, p. 789). Of the two alleles that are possible, it was found that a group of children with high IQ had twice the frequency of one allele as compared to the group of children with low IQ. More studies are needed to show the direct role that this gene plays in determining intelligence, but it is important to note that genes determining intelligence do exist and can be passed on to offspring.

Brain Regions and Neurotransmitters Associated With Intelligence

It is a long established fact that the neocortex is not only the newest portion of the brain to have developed, but has also gotten bigger over time (Emery & Clayton, 2004). The dorsal thalamus is another region of the brain which functions as a relay or gateway into the cortex. A study was conducted by K. K. Glendenning in which the dorsal thalamus of placental mammals was examined for the presence of g-aminodecarboxylase immunoreactive neurons. The presence of more of these neurons results in the increased uptake of GABA, a type of neurotransmitter which acts to slow activity in the brain. It was found that the systematic increase in inhibitory receptors in the dorsal thalamus of placental mammals is correlated with the increase in the development of the cortex of placental mammals (Glendenning, 1998).

Using the findings of this experiment in conjunction with the changing environments man has found himself in since the origin of Homo sapiens, it can be reasoned that evolution has favored individuals with the ability to better regulate their brain activity (Fischman, 1993). The ability to regulate the brain is a clear advantage when living in groups. If an individual was not able to control primal urges and behave in accordance with the social norms of the times, whether prehistoric or modern, this individual would be exiled from the group. Although being exiled from the group does not sound like a terribly important thing in modern society, it would have been devastating to prehistoric man. If early humans were exiled, they had little chance of reproduction and their own chances of survival were dramatically decreased (Kamil, 2004).

With the exception of mass murderers and rapists in modern society, most people are generally able to regulate their primal urges to levels that are acceptable. The fact that most people now posses this trait of increased sensitivity to GABA suggests that it is such an important trait that it has evolved and remained a favorable trait over several millennia. This would mean that early humans who possessed less regulational control over their brains would be ostracized and would not pass their genes to the next generation, resulting in the trait for lower GABA sensitivity fading away.


It is clear that as a species, Homo sapiens exhibits a broad range of intelligences, of which heritability can account for 50 percent. The fact that there are genes which control intelligence provides a platform for evolution and natural selection to act upon. During the early development of the human race, there would have been many more challenges in finding food and reproducing than we face today, which would have made the mechanisms of natural selection much more important. In today's society, nearly anyone can survive and reproduce, which negates the principles of natural selection, which only function if there are more individuals born than a given environment can support. In essence, since we have effectively eliminated hunger and disease, we have removed any pressure from the environment to keep adapting and become more fit.

Instead, since we have removed ourselves from the "native" landscape and challenges of Africa, we have become more dependent on the psychological mechanism of general intelligence than previously in the history of mankind. While there have been definite changes in the brain that account for increased intelligence of humans over apes and other mammals such as the presence of more GABA receptors and brain growth genes, much of the intelligence that we use is partially due to environmental factors. Since we are removed from the environment in which we evolved, the g region of the brain is forced to account for much more than it would otherwise have to. For example, if a caveman were to walk into a modern kitchen, he would have no idea what to do with anything in the room. None of the other psychological mechanisms are designed to deal with microwaves or toasters; only general intelligence is adapted to handle situations that are new and abstract.

Although thousands of years have passed since mankind migrated from Africa and populated the vast expanses of the world, there has been insufficient time for evolution to take effect and modify us to better fit our new environments. Some people believe that in the future we will evolve so that our brains are larger and our cognitive abilities will dramatically increase. The chances of evolving to this level are slim because even if we were given sufficient time to let genetic changes accumulate, there is no pressure to weed out the less intelligent individuals, who actually procreate more than individuals with high IQ's (Kanazawa, 2004). Since we are now in a state of limbo where there is no evolutionary pressure, unless there is a dramatic change in the environment, or we are not able to support as many people living on the earth, I believe that the evolution of human intelligence will cease.

Peer Commentary

What About the Role Sexual Strategies Play in the Evolution of Human Intelligence?

Lisa C. Burt
Rochester Institute of Technology

Skottke pointed out how domain-specific intelligence becomes more important as the environment becomes more modern. The review of the evolution of intelligence discussed how the human brain has grown with the changes industrialization brought on and with the increasing demands of the modern world. I agree that human intelligence has evolved to meet the demands of today's society; however, I have to wonder what role sexual strategies play when it comes to procreation, relative to the evolving human mind. Skottke developed the idea that industrialization has engulfed the changes in intelligence and the human mind has evolved domain-specific intelligence. I feel a more dominant aspect of evolution has to do with sexual strategies. Skottke made the point that most people with high IQs do not marry and therefore do not procreate. With this in mind, those working in today's modern society are those with average intelligence or lower.

Sexual strategies have also been evolving. It has been shown that people look for a mate similar to themselves. Usually, people procreate with someone with the same intelligence they have. Therefore, intelligence has evolved in the sense that similar intelligences have procreated together.

Sexual strategies can be seen and have been studied in animals. Competition begins in mate selection and moves all the way up into industries. It can be argued that people learn their competitive drives and their work ethic because of sexual strategies. Those who work hard for what they want, those who do not give up, and those who are seen as the strongest, more often than not get the mate they want. This can also be seen in society. The same characteristics and the same traits, mark the personality of the most successful people in the modern world. Intelligence also can be largely valued through a person's status. Procreation can play a role in the evolution of human intelligence along with the demands of a changing world. Without people mating with others of the same intelligence, there would be no growth, just as without the increase in the need for greater intelligence in society, there would be no need for intelligence to evolve like it has. I agree that there has been a need for domain-specific intelligence because of the changes in modern society, but I also feel that without sexual strategies and without the importance of mate selection, the evolution of human intelligence would have been more limited.

Peer Commentary

Stagnation Could Cause Extinction

Alexandria K. Cherry
Rochester Institute of Technology

Let me first make the point I feel the independent domain model for intelligence has a firm basis. This seems to be the most logical and well-developed model so far on intelligence. Nevertheless, to say that human intelligence has evolved to the point of stagnation is quite absurd. There is absolutely no evidence in the paper, or anywhere else for that matter, that can show that the human brain has ceased in its growth or is on its way to doing so.

Intelligence has always seemed to be one of humanity's greatest weapons against its enemies, whether the problem be of survival or reproduction. The idea there are many types of intelligence to help us with these problems seems appropriate. The other idea that these different elements have developed independently over several generations also seems just. Yes, selection has been very kind to humanity in the adaptation of our brains and thus our intelligence to better help us with our need to survive. It has taken us so far as to change our little make-shift huts to buildings that touch the sky, and to change our raw meat into seven-course meals.

The problem arises in the conclusion. With this growth and expansion, the need to work for survival is greatly lessened. This does not, however, mean that the evolution of human intelligence will cease. The major reason was stated: Evolution takes a very long time. There have been many places even in written history in which people living then felt that there would be no need to work for survival because they had achieved it. That was obviously not the case, though. History has both highs and lows. There are several instances in which a new disease or new threat was discovered. Why is this the time in which there will be no other threat?

Human expansion into places of unknown terrain is an example. Humanity has enough intelligence to move into outer space and explore other worlds. Maybe this is not the end of the growth of intelligence but just the beginning. Several studies have shown that intelligence is still rising. If intelligence is still on the rise, then what evidence is there that it will slow and eventually stop?

This paper makes a conclusion that is unfounded and unsupported. Thus far, there has been no evidence that human intelligence has ceased its growth.

Peer Commentary

Possible Confounding Factors in Marriage and Intelligence

Rebecca M. Light
Rochester Institute of Technology

Kyle R. Skottke's paper, "The Evolution of Human Intelligence: Increasing Importance of Domain-Specific Intelligence in the Modern Environment," provides an exemplary look into domain-specific intelligence and its change over time. However, I think his hasty rejection of the domain dependent model is flawed. Domain dependent intelligence, as described in the paper, holds that intelligence encompasses certain personality characteristics; mate selection, cheater detection module, and face recognition are all influenced according to intelligence. He rejects this theory based on a study done on individuals with an IQ higher than 125. Of all the cognitive classes, the study showed that this percentile was least likely to marry. Skottke stated, "This data suggests that mate selection and general intelligence are unrelated..."

This certainly does appear to disprove the domain specific model. However, perhaps there is a graphical tendency of intelligence that is not strictly linear. As intelligence reaches a certain point, it could be argued that likelihood of getting married decreases in more of a bell-shaped manner. This would suggest that there is a relationship between intelligence and mate selection. The same can be applied to those of exceptionally low IQs and any trend in mate selection that can be observed with them. The exact test results of the study would need to be examined at greater length before completely rejecting the domain specific model.

There are many factors that could contribute to the decreased likelihood of marriage in those with extremely high intelligence. The social habits of those with high IQs could have an influence on the number of possible mates to choose from. Those individuals that dedicate themselves to their work may encounter fewer opportunities to engage in social activities with the opposite sex. Beyond social habits, the personality traits in general could make it difficult for intelligent individuals to meet, connect, pursue, engage and eventually marry a potential mate. They may have trouble expressing their emotions, never telling the object of their desire how they feel.

Standards for mates might be higher than those with lower intelligence levels. Those with extremely high IQs may typically desire qualities in mates that are rare or unrealistic. For example, the desire to marry someone that is of equal or greater intelligence would make it a challenge for someone who is a genius to find another genius to marry. This requirement, in combination with any other standard makes it increasingly difficult to let alone find a mate, regardless of if the feelings are reciprocated. This also presents another challenge; perhaps others view those with an extremely high intelligence as utterly undesirable. There could be an aspect of their personality that prevents the love being reciprocated. Many geniuses might experience dismal cases of unrequited love.

The marriage beliefs of intelligent individuals may also cause declined marriage levels in highly intelligent individuals. Perhaps it is the general logic of intelligent individuals that marriage is unnecessary. This could be observed in highly intelligent individuals that take on a lifelong partner or engage in lifelong friendships and affairs. For example, Wilkie Collins never married Caroline Graves, although they lived together like man and wife during their adult life. Wilkie Collins also took on a lover, Martha Rudd, with whom he fathered three children. He was highly intelligent and simply did not believe in the constraints of marriage. He supported a sort of polygamy, and showed un unwillingness to limit himself to one woman only.

It could be a characteristic that highly intelligent individuals are homosexual. Leonardi DaVinci never married, probably because he was attracted to men, which was not socially acceptable. Even today, the homosexual marriage debate presents a challenge to those individuals of high intelligence who would like to get married, but are unable to because they are attracted to members of the same sex.

It is a rash assumption to state that intelligence and mate selection is completely unrelated aspects of human cognition. Before the domain dependent theory is completely eliminated, the possible confounding variables ought to be fully examined and tested. Patterns and trends in the characteristics and behaviors of those with high intelligence could provide new insight into their mate selection process.

Peer Commentary

Emotional Intelligence and Its Role in Evolutionary Selection

Lauren A. McCarthy
Rochester Institute of Technology

Skottke stated that modern society has caused humans to rely more on "cognitive ability and intelligence," but he neglected to expand on a very important skill for survival in the modern environment--social intelligence, also known as emotional intelligence. Someone with a high emotional intelligence would be better prepared to get along with others and avoid conflict in a group. Through evolution and natural selection, those with a lower emotional intelligence would have been excluded from a group and less likely to survive and reproduce compared with their peers who were able to work in a group and hence benefit from the advantages a group provided for defense, food gathering, and general welfare. Although this skill would have been very important when human were less independent, it is still applicable in today's society, as those with a low emotional intelligence are less likely to form connections with others. These important connections can help foster career promotions, discourage a potential attacker (because one is not a loner), and even provide friendships that reduce the risk for depression.

Spearman, who first determined the existence of general intelligence, argued that a high score on one facet of intelligence often correlated with a high score on another facet. Gardner, on the other hand, used the argument of a brain damaged person or a savant who has a specific talent that does not correlate with high scores in any other areas of intelligence. I agree with Skottke's assertion that the domain-independent model is the only appropriate model, because only this model takes into account the different types of intelligence that people possess, leading to their greater chance for survival and reproduction. Therefore, even now after natural selection has acted on the population, we have people not only with high intelligence and good spatial reasoning, but also people who are skilled in other areas such as art and photography. The domain-dependent model relies on correlations in high scores of intelligence, but this could simply be the result of linked genes. Given that intelligence is 50% heritable, it would be logical to see a correlation between linked genes and the g factor. This does not suggest, however, that all traits such as mate selection and intelligence are also correlated.

Skottke stated that there has been a noted increase in the size of the dorsal thalamus and cortex as compared with our earlier ancestors. He did not, however, explore the idea that there could be a direct correlation between the size of the dorsal thalamus and intelligence. People who are deficient in GABA often suffer from panic attacks or an over-firing of neurons inhibiting a person from concentrating. Overproduction of GABA, however, has been found to have sedative effects. Therefore, it is important for optimum performance that GABA be neither deficient nor excessive. Perhaps those with a larger dorsal thalamus also have a slight excess of GABA production, and conversely those with a small dorsal thalamus have deficient GABA production.

In his conclusion, Skottke argued that evolution cannot occur now because "there is no pressure to weed out the less intelligent," but it seems that he is only thinking about the population of developed countries rather than of developing countries. People in developing countries compete for available food and resources, and hence those less able to provide for themselves or their families are less likely to survive and reproduce. In a developing country, not only one's general intelligence but also one's emotional intelligence is important to survival. Those with low emotional intelligence would more often be ostracized from a group, which would threaten their welfare and safety and hence their chances for reproduction.

Author Response

Natural Selection as the Mechanism for Evolution

Kyle R. Skottke
Rochester Institute of Technology

In her commentary, Burt brought into question the importance of sexual selection in the evolution of intelligence. This is a valid question, because Darwin proposed two methods of evolution, natural selection and sexual selection. Although this is an interesting perspective for exploring intelligence evolution, I chose to focus mainly on the natural selection aspect and sexual selection was largely ignored because of space limitations.

In her commentary, Cherry disputed the cessation of intelligence evolution. When I stated that there might not be any further evolution of intelligence, it was mainly based on the lack of natural selection that we face in modern society. That is not to say that there will not be further abstractions of our environment that will force us to develop g further, but merely that there is no basis to expect further evolution of intelligence based on the traditional natural selection mechanism.

In her review, Light suggested that the domain-dependent model should be given more consideration than it received in my paper. Light stated that a bell-shaped curve could account for the lower marriage rates of high-intelligence individuals. This is an interesting proposal; however, I still favor the domain-independent model, because g was also compared to cheater detection and face recognition. I find it hard to believe that the same bell curve applies to these modules as well; it seems much more likely that they are independent.

In her commentary, McCarthy advocated emotional intelligence and suggested that it should be more heavily considered throughout the paper. It is true that social skills and emotional intelligence are important, but it is widely accepted that they were equally important to our primitive ancestors. Emotional intelligence appears to be a trait that has remained important throughout evolution, which is why I did not include it when discussing factors that would have caused a need for increased intelligence, such as moving into an unfamiliar environment.

There are still several aspects of this paper that are contestable. Experts argue whether domains of the brain are dependent or independent as well as whether or not we will experience any more major changes in intelligence. Although there are still uncertainties, I believe that the views expressed in this paper are the most plausible given available knowledge.


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