Increasing evidence over the past several years of research indicates that multiple structures in the brain are associated with intelligence and cognitive function at the molecular level. The most crucial factors of human intelligence are reasoning, problem-solving, and learning. To further explore the relationship between human brains and intelligence, careful consideration of the structure of human intelligence would be needed. Research over the past years has shown how brain size is also the deciding factor for intelligence in a person. This review paper will explore how the parts of the brain, dopamine receptors, and quantity of white matter along with lobes of the brain play an important role in defining one’s intelligence.
The human brain is very complex and hence, it is important to learn about the structure of the brain. The brain is one of the most complex parts of the human body. It is known for controlling and coordinating all the functions of the various organ systems. The human brain weighs three pounds and is responsible for functions like memory, behavior, emotional responses, intelligence, speech, movement, control, and coordination. The brain is a part of the central nervous system (CNS) along with the spinal cord, and lies entirely within the skull. The brain can be divided into three main parts: the cerebellum, the cerebrum, and the brain stem.
The cerebellum is the part of the brain associated with control and coordination, that has extensive connections with the cerebrum and the spinal cord. In contrast to the hemispheres of the cerebellum (known as the cerebellar hemispheres), the right cerebellar hemisphere is associated with the movements of the right side of the body, whilst the left side of the cerebral hemispheres is associated with the movement of the left side of the body.
The rostral and largest part of the brain is the cerebrum . In the middle, it is divided into two cerebral hemispheres separated by a deep sagittal fissure. The right cerebellar hemisphere receives sensations from and also controls the total left side of the body, whereas the left cerebral hemisphere is concerned with sensation and movements of the right side of the body. The remaining part of the brain forms the brain stem. The brain stem forms the stalk from where the cerebral hemispheres and cerebellum rise .
The relationship between brain size, the volume of the brain, and intelligence, both in humans and among different species, has never been particularly well-defined until the research published in the last few decades. Humans still believe that they have exceptional cognitive abilities and the highest thinking power among the animalia kingdom in terms of brain size, as well as thinking. Both of these common assumptions are incorrect. Mammals such as blue whales and elephants have bigger brains in size than humans, and humans have an equal brain to body mass ratio as mice.
To resolve the brain size and intelligence correlation conflict, scientists have crafted a 3rd measure of brain size known as the encephalization quotient (EQ). Encephalization quotient is the ratio of actual brain mass relative to the anticipated brain mass for an animal’s size (based on the thought that larger animals require slightly less brain matter relative to their size compared to very small animals). It also shows the relative brain size based on the ratio of brain mass for a particular size of organism. Humans lead the rankings in EQ, with an EQ of “7.5” far surpassing the dolphin’s 5.3 and that of a mice 4.8 . This shows that the humans with the highest brain mass accounts for the highest encephalization quotient.
Regions of the Brain Involved in Intelligence
The development of neuroimaging techniques and other computational software tools have enabled neuroscientists and researchers to investigate the human brain in vivo as well. The evidence obtained from neuroimaging studies and pictures suggests that no single brain region has an exclusive effect on intelligence . Literature Reviews in the previous years have suggested the parieto-frontal cortex theory, which indicates the parietal cortex and frontal cortex as the two regions of the brain involved in intelligence. The parieto-frontal theory has been supported by many neuro-imaging studies.
Out of all the structural neuroimaging studies done previously, 40% of them showed that both frontal and parietal lobes are involved in intelligence . Studies pertaining to the usage of diffusion tensor imaging (DTI) have shown that structural organization of white matter (WM) at the minimal networking level and its developmental trajectory plays an important role in intelligence . However, to date, network-level underlying mechanisms have stated that intelligence is derived from and is also associated with the white matter (WM) structural and functional levels.
There is also proof that gray matter (GM) is also associated with intelligence. The gray matter and its association with the structural framework of intelligence has not been researched in-depth. Other parts of the brain like the cerebellum that could potentially contribute to intelligence are often ignored or overlooked.
The parts of the brain involved in perceiving intelligence usually comprise the cortex, frontal lobe, and parietal lobe, and dopamine receptors. In a study done by Louise Ridley, in which he compares a normal brain and a genius brain, there are certain differences in the above-stated brain regions which distinguish a normal brain from a genius’s brain . The studies show:
A normal brain has a 50:50 ratio of long and short connections whereas a genius brain has a majority of either type of connections. Short connections indicate an aptitude based on one interest, whereas long connections suggest aptitude in many interests as well as the ability to see problems from new angles and perspectives .
Minicolumns are present in the frontal lobes of the brain. The genius’s brain has a dense concentration of “minicolumns”, also known as the brain’s “microprocessors”, which help in powering the thought processes of the brain. However, the normal brain has an average of 80-120 minicolumns .
Research through the past several decades shows that the brains of geniuses have fewer dopamine receptors present in the thalamus. Dopamine inhibits neuronal signals, canceling out information it deems worthless. The shortage of receptors such as dopamine receptors in geniuses might mean that they are able to come up with a typical solution to a problem that normal brains tend to disregard .
The parietal lobe: Einstein’s brain was relatively smaller than a normal brain but the parietal lobe was larger than average. This shows that the parietal role plays an active role in intelligence 
Larger Brains Are “Smarter” Across Species and Evolutionary Time
Early evidence has supported the notion that, within the genus Homo, evolutionary constraints have generally been selected for larger brain size relative to body size over time.
Charles Darwin writes in The Descent of Man, “As the various mental faculties gradually developed themselves the brain would almost certainly become larger. No one, I presume, doubts that the large proportion which the size of man’s brain bears to his body, compared to the same proportion in the gorilla or orang, is closely connected with his higher mental powers” (Darwin 1871, p. 37).
For a long time, researchers and scientists have tried to correlate the size of the brain with the level of intelligence. The first of this work was done by Galton in the year 1869, which focused on brain size approximated by measures of head size .
Scientists Tramo and Gazzaniga in the year 1999 have found definite and positive correlations between magnetic resonance imaging (MRI) measures of brain volume, and intelligence.37 neuroimaging studies shown by Mc Daniel in 2005, demonstrates a small, yet consistent relationship between whole brain volume and psychometric measures of intelligence (r = .33). Moreover, the relationship between brain size and IQ appears to be rather equally distributed across tissue types, with unweighted mean correlation values of .31 for white matter volume and .27 for gray matter volume .
This relationship between total brain volume and intelligence is emerging through an evolutionary level, basic level, and now also at a nano-level. Two of the prime candidate genes that have been identified so far, microcephalin and ASPM, appear to play an important role in the regulation of brain size .
It has been proved in research that brain size plays an important role in intelligence. From an evolutionary standpoint, the brain has evolved ranging from organisms such as mice, cat, lion, bear, chimpanzee, and dolphin, until humans. As we have a tendency to see from the tiniest to the biggest primates, for instance from mouse to humans, we are able to see which way the scale of the brain has inflated afterward, defining the amount of intelligence in every of them.
When speaking of the uncertain relationship between brain size and cognitive abilities between different species, a few questions arise. Can brain size predict anything about intelligence amongst humans or is it just a myth? Does having a large brain mean that one is smarter and more intelligent than another human with a smaller brain size?
Some studies suggest that the answer is yes. A recent genome-wide association study, which included around 20,000 human subjects along with their brain scans, was widely reported by the media to have discovered an “IQ gene”. As per the results of the experiment, certain variations in the HMGA2 gene, which is responsible for coding a protein that helps regulate DNA transcription and cell growth, can increase intracranial volume as well as an enhanced IQ .
Neuroscientists and researchers also believe that the brain’s computational capacity is determined by the complexity of cellular and molecular organization of neural connections, along with neuronal ends or synapses. This hypothesis is supported by findings and research that shows that intelligence is more correlated with frontal lobe and grey matter volume (denser in neuronal cell bodies and synapses), than sheer brain size.
MRIs and several other technologies have enabled to compare brain sizes of living humans and to find the relationship between brain volume and/or size and IQ . Although having a large brain size is somewhat predictive of having smart thinking, intelligence and higher cognitive abilities probably depend much more on how efficiently different parts of your brain communicate with each other. Researchers generally agree that intelligence involves the ability to learn, survive in the environment, and adapt to changing environments. The studies involved in the past decades indicate that the involvement of the parietal lobe, frontal lobe, and cortex along with dopamine receptors play an important role in determining the intelligence level of the brain. How these parts of the brain communicate with each other and the mass they contain is an important deciding factor for the intelligence any species has.
The term “genius” has come a long way from classifying someone who is particularly innovative or adept, but neuroscientists are still far away from discovering what sets their brains apart from non-geniuses. Differences within the brain and, therefore, the accompanying thought processes, are only a part of the mysterious puzzle. The characteristics of the parietal lobe, frontal lobe, and cortex along with dopamine receptors differentiate a brain of a genius from that of an average human.