The next few blog entries summarize the main research findings on gender differences in the functioning and structure of the brain, including biochemical processes.
The biochemistry of the human body in general, and the brain in particular, is highly complex, which means that the terminology will be technical. I am trying to simplify the points as much as possible for the benefit of most readers, and I will continue to add graphs and links to other resource, should you wish to better understand the different topics. I hope that you will have the patience to continue following this important series to the end and see that it was worth it, even though it may seem a little technical at times.
As with all other entries in this series, when I use “male” and “female” I am always referring to humans, unless otherwise specified.
The Nervous System
Currently medical theory states that nearly all the functions of the entire body are guided and coordinated by the nervous system. (I believe, based on Islamic references, that the heart plays a bigger role than is currently mentioned in medical textbooks, but that is for another entry…). The nervous system is made up of a central nervous system (CNS) and a peripheral nervous system (PNS). The CNS is made up of the brain and the spinal cord. The PNS consists of all nerve parts and process outside of the CNS.
Head Circumference and Cranial Capacity
All studies show that males have larger head circumferences and cranial capacities for all age groups than do females. This in turn provides a crude measure of the brain size. None of these studies adjust for average differences in body weight between sexes.
Brain Size / Brain Weight / Brain Volume
All studies have shown males to have larger brains than do females, except one study showing brains being larger in females in children born prematurely.
When studies adjust the findings for average body size differences between sexes, the differences in brain size were reduced by half, but male brains were still significantly larger.
Size of the Brain Ventricles
When studies have shown differences in the size of brain ventricles, these have been in males having somewhat larger ventricles than those found in females. Brain ventricles are four cavities connected together, located within the brain, that contain spinal fluid, and which are part of the ventricular system. The ventricular system allows the nervous system to function. It provides nutrients to the brain, ensures that it remains in chemical balance, and protects the brain by providing shock absorption and allowing it to float in fluid.
Gray Matter and White Matter in the Brain
Research has repeatedly shown that males have a higher ratio of gray-to-white matter. In terms of volume, some studies have shown males to have more white matter in the brain than do females.
Gray matter is a tissue found on the surface of the brain (cortical tissue) and contains the cell bodies of nerve cells (neurons). White matter is found in the deeper tissues of the brain (subcortical tissue) and contains nerve fibers (axons), which are extensions of nerve cells (neurons). Many of these nerve fibers are surrounded by a type of sheath or covering called myelin. Myelin gives the white matter its color. It also protects the nerve fibers from injury. Also, it improves the speed and transmission of electrical nerve signals.
So we can of grey matter as being for the computation of information, and white matter as being for the communication of information.
Size of the Neocortex (cerebrum)
Most studies indicate that the neocortex is larger among males.
There is one study that concluded that the average size of the neocortex in males and females is equivalent once brain size as a whole was adjusted for the average size difference.
When the brains of humans and other species are compared, the size and shape pare of the structures are nearly identical, except for the neocortex. For instance, the size of the human neocortex is nearly four-times greater than that of chimpanzees. The neocortex is the primary portion of the human brain that is most highly developed in humans.
This is a good simple explanation of the three layers of the brain that are part of every textbook nowadays…
While I have some reservations about the current division of the roles and functions of the different areas of the brain, here is a good video to watch about the parts of the brain and their functions before reading the next few bullets to better understand them.
Frontal Lobes
Most studies have concluded that males have larger frontal lobes than do females. If these findings were adjusted for the overall larger size of the male brain, this size difference in the frontal lobes would be eliminated.
Frontal lobes, which comprise nearly a third of the volume of the neocortex, are important for intellectual tasks requiring long-term planning, impulse control, and moral reasoning.
Some studies seem to indicate that some of the sub components of the frontal lobes are larger in females, such as the size of the orbital frontal cortex.
Parietal Lobes
After adjusting for overall brain size differences between sexes, some studies have found the lower part of the left parietal love in males to be larger than in females. Parietal lobes appear to be mainly involved in interpreting sensory information related to touch and physical pain, as well as eye-hand coordination.
The postcentral sulcus extension into the Sylvian fissure, which is part of the parietal loves is longer in the brains of males according to some studies. It seems to be involved in the control of quick eye movements, visual spatial attention, spatial memory and other functions.
The Occipital Lobe
Based on a single study on the topic, there does not seem to be an overall difference in the size of the occipital love (located at the back of the brain, are involved in vision). One study indicated that the size of one part of the the visual cortex in females is larger than in males.
In the next entry, we will continue exploring the sexual differences pertaining to the brain, specifically with regards to the hemispheres and connective tissue.
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