Introduction
Through this essay, I will introduce intelligence scientifically as a component of many different traits and defining it in context of research on intelligence in humans. It has many different definitions and can be spilt into statistical values, such as g and IQ, or specific traits that contribute to overall intelligence, such as cognitive ability, fluid and crystallised intelligence, memory, or verbal reasoning. the nature vs nurture debate is controversial and currently being investigated in research, and I have looked into how much intelligence is affected by the genetic code. My initial hypothesis was that genetics affect intelligence to a very large extent, and the environment does not have a significant effect.
What is intelligence?
Intelligence is commonly defined as the capacity to acquire and apply knowledge. However, defining intelligence has been debated amongst neuroscientists over time, as intelligence covers much in the brain. In 1904, Charles Spearman introduced general intelligence, (Spearman, 1904) also known as g . G factor (g) is a statistical measure of intelligence and is used in research to account for variance in IQ scores across the normal distribution. It can be estimated from a sum of intelligence test results; they will often produce different scores, so g can be used as an average. G factor is independent of IQ (the intelligence quotient) as g factor tests for different types of intelligence and is an average measure of intelligence, which is then gathered from a number of IQ tests. IQ tests are used to give a value for intelligence which can be measured, but they can be subjective or biased, so many tests must be used to calculate g. The most common IQ test is the Raven’s Progressive Matrices Test (RAPM) (Raven, 1941). In research, scientists look for the factors that will cause variance of intelligence, which is the difference between IQ scores, shown when IQ is modelled as a normal distribution (see Figure 1). With a mean value 100 and a standard deviation 15, IQ test scores can be collected into three general groups. Low intelligence is often modelled as a score below 85, average intelligence is between 85 and 115 (68% of the population falls into this category) and high intelligence of 115 and above. In research, however, more extreme values are used to show more difference between high and low intelligence.
Intelligence can be split into many different types, such as cognitive ability, verbal reasoning, memory, processing speed and others. Cognitive ability generally describes academic success in subjects such as maths, science, or English, so this can be used to represent intelligence in studies (some of which I refer to later in this essay). The concept of multiple kinds of intelligence was mainly introduced by L.L Thurstone in 1938 (Thurstone, 1938). Since, many alternate views on intelligence have been published within research papers, with some being more influential, such as Gardener’s ‘Theory of Multiple Intelligences’ (Gardener, 1962) , which includes the controversial ‘emotional intelligence’ or Guilford’s introduction of around 150 different types of intelligence (Guilford, 1967), that are organized and classified into a multitude of categories . Intelligence can be seen as a wide spectrum of different traits, as many believe it is not limited to academic success or extensive vocabulary, and can include the ability to talk to people, empathize and understand emotions, or write and present musical compositions. General intelligence is used to summarize the mental capacity of an individual, although it does not encompass anomalies, such as savants.
Raymond Cattell’s theory of intelligence (Cattell, 1971) states that general intelligence (g) can be divided into fluid or crystallised intelligence. Fluid intelligence refers to the basic capacity of g, where working memory is used, with a higher short-term memory also being linked to higher fluid intelligence. Crystallised intelligence however is developed through experience and knowledge; with long-term memory indicating the vocabulary and knowledge of the person, developed over time. The long-term memories are, however located across different areas of the brain, in order to accurately record multiple aspects of the memory (such as sight in the visual cortex and sound located in various parts of the cerebrum). This means that, when testing intelligence, the crystallised intelligence may be reflected through the use of long-term memories, which are active across many areas of the brain. If crystallised intelligence can be shown through very specific structural areas in the brain, then the genes linked to the formation and development of these areas can also be investigated and marked as being linked to intelligence.
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Studies often simplify intelligence into a particular trait to measure and investigate how the trait is represented in the brain. For example, verbal fluency IQ tests could be used to give an estimate of intelligence, if a study wanted to look at how genes affect the brain’s ability to receive and process information. Splitting up the concept of intelligence is key during a controlled experiment to ensure a study produces reliable and significant results.
What is the study of genetics?
DNA is one of the most important polymers in any living organism, as it is the information that builds all of life. Specific sections of DNA on a particular locus of a chromosome are called genes, and the genes separate the functions of the genome into organised areas that serve one particular or a particular set of functions; depending on the type of gene. Pleiotropic genes are those that can serve multiple functions, and some phenotypes are described as polygenetic, which means a number of genes all contribute to the phenotype. Intelligence is an example of a polygenetic trait, and hence finding specific genes that you can measure in people to directly correlate against intelligence would be very difficult. By initially studying how the brain works, and how functioning can differ with more and less intelligent individuals, we can find which genes affect certain parts of the brain and then that will show the extent to which genetics affect intelligence.
How can the structure of the brain relate to intelligence?
The brain is composed of a network of neurons that relay information by passing electric currents and chemical signals through the connections between the cells, meaning a certain stimulus can be responded to. Neurons can also store information in the form of memory, with long-term memory a group of neurons with specific connections in order to replicate the pattern of the original experience. Structurally, neurons have four main components, each performing their individual functions inside of the brain (Lodish, et al., 2000). The main cell body contains the nucleus, along with all other eukaryotic organelles. Dendrites are the parts of the cell that receive the signals from other neurons, and they can also synthesise the remaining neural proteins. Axon and axon terminals are the long structures that connect neurons through the brain, perform a process called action potential to transfer information. Axons are protected by glial cells and myelin fats that insulate and protect the axon. The genes which code for particular structures in the brain can then be linked into intelligence, potentially showing how important genes are to intelligence. For example, the insulation of axons (known as myelination) which can make electric signals flow easier and quicker through the brain (NewScientist, 2018).
What research has been done to locate parts of the brain which are significant to intelligence?
For scientists aiming to locate the areas of the brain which are stimulated through different kinds of intelligence. As a very broad concept, intelligence is located across many areas of the brain. The parietal frontal integration theory (Haier & Jung, 2007) shows a variety of areas of the brain which have been shown to affect intelligence. These areas include a network of brain tissue that underlie intelligence, with many locations in frontal and parietal lobes; hence the name the parieto-frontal integration theory (PFIT). The PFIT also shows how information flows through the brain into different areas. Information first enters the back of the brain in sensory perception, then into areas which associate memory against the information, then into the frontal lobes for processing and finally into motor and speech areas if actions must be performed. The brain networks for each area are also described as being integrated into each other, emphasising how the connections between them are also significant to intelligence, a found example of which is the arcuate fasciculus. PFIT is important to be used as it can differentiate different types of intelligence from similar general intelligence scores, as two individuals may have the same average intelligence but with unique cognitive abilities that could be considered a different type of intelligence (such as verbal fluency and mathematical reasoning). These unique individuals can then be separated as they would likely have some different PFIT areas to account for their different areas of specialised intelligence. This technique is useful to differentiate between different types of intelligence in the brain, and being able to understand where intelligence comes from will allow for more effective research.
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