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According to Garrett and Hough (2022), individual intelligence is a measurable construct of one's ability to reason, understand, and profit from experience, often reflected as an intelligence quotient (IQ). The IQ test determines how cognitive functioning shapes mental abilities such as judgment, comprehension, and reasoning (Ganuthula & Sinha, 2019). However, there is a challenge in defining intelligence because of how abstract the concept is. Originally, intelligence tests were designed to determine child performance by calculating their mental age, but these tests now apply to adults across broader categories of performance (Garrett & Hough, 2023). Most intelligence testing occurs in school settings, but different methods for measuring IQ range depend on subtype. For example, one IQ test may focus on working memory, while another primarily determines processing speed. The most widely used IQ test is the Weschler Adult Intelligence Scale III (WAIS-III), which measures an individual's mental ability performance across comprehension, processing speed, perceptual organization, and working memory (Ganuthula & Sinha, 2019). Ganuthula and Sinha (2019) also reference other measurements of intelligence, such as the SAT and the Stanford-Binet IQ test, which measure cognitive abilities within the frame of educational reasoning.

The pathway of defining intelligence varies depending on who you ask. Some researchers believe that intelligence found through nonverbal tests is the best indicator of "pure" intelligence because it offers the advantage of being inclusive of all cultures (Garrett & Hough, 2022). On the other hand, some claim that intelligence stems from aspects of the environment rather than educational stimulants found through conventional intelligence tests (Garrett & Hough, 2023). How intelligence is defined depends on how it is measured and the social constructs that influence its existence. The structure of intelligence is split into two viewable categories, lumpers, and splitters, which remain controversial in the realm of biological understanding of the entity (Garrett & Hough, 2022). Giofre and colleagues (2019) definelumpersas those who view intelligence as a single factor and definesplittersas those who view it as several factors. The view of lumpers pertains to the idea that while separate abilities make up intelligence, one single factor is enough to determine overall intelligence (Garrett & Hough, 2022). In contrast, splitters then feel as though the several abilities range in importance of each other, unable to be confined to a singular factor.

Ability plays a significant role in defining and determining intelligence. Garrett and Hough (2022) talk about how different abilities or tasks, such as spatial or verbal ones, activate different portions of the brain and other brain areas are common within a combination of tasks. While brain size accounts for a slight difference in people's intelligence, the amount of grey and white matter is a strong intelligence indicator (Garrett & Hough, 2022). However, vast amounts of grey matter do not hold value without the white matter connections that make up the brain's communication network (Garrett & Hough, 2022). Penke and team (2012) show evidence that lower brain-wide white matter tract integrity can negatively impact general intelligence by reducing processing speed. Therefore, regarding brain structure and intelligence, factors such as white matter seem essential to maintaining the connective network surrounding task abilities.

This concept then provides us with a view into how the different subtypes of intelligence work independently and cohesively together. Spearman essentially created the statistical method of factor analysis to analyze the multitude of mental ability tests to gain a more robust understanding of how different cognitive abilities interplay (Garrett & Hough, 2022). Additionally, Garrett and Hough (2022) outline three major cluster components of intelligence via an individual's capabilities: spatial, linguistic, and logical-mathematical. However, while these abilities impact the activation of the brain and measurement process of intelligence, there are other subtypes of intelligence worth noting.

Growing old is inevitable; with that, an individual's intelligence becomes compromised due to expected declines in cognitive abilities. Fluid intelligence relies on working memory, abstract thinking, and processing speed, whereas crystalized accounts for vocabulary, analogies, and general knowledge learned through experience and education (Bajpai et al., 2022). Unfortunately, as we age, our brain begins to lose coordination in the default mode network, which controls our level of preparedness for action, because of a decline in white matter connections (Garrett & Hough, 2022). However, just because this health decline is happening does not mean that practicing learned skills cannot help regain some lost ability. Intellectual disability is another aspect of intelligence that researchers aim to better understand through genetic, hereditary, and environmental factors. An extra 21st chromosome is a precursor for determining Down syndrome, which results in lower IQ measurement, while autism spectrum disorder is a set of neurodevelopmental disorders that impact intelligence (Garrett & Hough, 2022). It is essential to remember that different subtypes of intelligence exist, with some being influenced by the brain's structure and others through hereditary or genetic predispositions. Overall, intelligence encompasses the actionable capabilities of individuals verbally, spatially, logically, and emotionally, all of which vary depending on the network of connections within the brain and how they interplay with other internal and external factors of that person.