The Misunderstood Study by Jay Giedd and How There Is No Such Thing as An Underdeveloped and Fully Developed Brain
Posted: Mon Nov 17, 2025 4:14 am
The popular study that was done by Jay Giedd about how the brain doesn't fully develop or level off at 25 years of age has been a study that has been misunderstood by the layman and others alike to mean that ages younger than that like teens and younger have an underdeveloped brain in key regions that makes it very hard to impossible to do certain things as well. However, this notion is far from the truth. It's not that the data in of itself is wrong, it's the interpretation of that data that is wrong.
The one thing you have to understand about that study is the markers of maturity that was used such as the grey and white matter ratios. Grey matter is the representation of the neurons, and the white matter is the representation of the myelinated axons. Now keep that in mine for later. Through this study, they show a decrease of grey matter due to synaptic pruning as people age, and an increase of white matter as people age. Age 25, which was the highest in their sample, showed greater levels of white matter which functionally makes connections faster between neurons or in how they put it, "more efficient or reinforced connections".
Now from face value, this seems pretty straight forward, right? Nope and this is where it gets juicy. White matter, which is the representation of myelination, is very plastic as with grey matter. Myelination is the fatty substance that wraps around the axon for higher action potential speed that is made by oligodendrocyte cells, which are in turn, made by oligodendrocyte precursor cells (OPC's). There are two distinct processes of myelination: independent experienced driven myelination and dependent experienced driven myelination.
Independent experienced myelination allows for the proliferation and differentiation of OPC's that mature into late OPC's, then immature oligodendrocyte cells, and finally mature oligodendrocyte cells that wraps myelin around the axon without little to no axonal activity. Independent driven myelination is the most critical in the prenatal phase of brain development and very early infancy, that is also critical in the increase of proliferation and differentiation of OPC's that in turn, increase the rate or how fast things myelinate that is a little after the onset of puberty.
Dependent experienced driven myelination myelinates axons based on axonal activity or in other words, stimulation of connections of the brain. This kind of process not only strengthens existing myelin sheath around axons but can also play a role in the proliferation and differentiation of OPC's. This process describes that the more you practice a particular skill or knowledge, the more you are stimulating that specific connection of the brain and thus, myelinating that connection.
Now, one may try to argue that the independent experienced process is the most critical and the dependent process plays a less critical role that has minimal effect that will show differences in later ages, but there are a few problems with this and for one, the independent process is the most critical during the fetal and early infancy stages, as well as puberty, and secondly, without the dependent process, we wouldn't have a lot of myelin at all.
So, experience is the key factor to white matter growth, and the independent factor plays in the background. Having this understanding in mind, we grow and mature based on the environment that allows us to have the experiences that we undergo that matures us, and not as a result of fixed linear stages of how the brain matures.
There is a reason young kids can be more proficient and skilled at a certain thing than an adult and vice-versa. A study done by Leah H. Summerville reinforces this notion by saying: "However, these data also illustrate the challenges of applying general patterns of neurodevelopment from group-based to individual inference, as there is substantial variance in brain network connectivity that is unrelated to age. For example, some 8-year-old brains exhibited a greater ‘‘maturation index’’ than some 25-year-old brains" (https://sci-hub.se/10.1016/j.neuron.2016.10.059).
Her statement is further reinforced by this study here: https://pmc.ncbi.nlm.nih.gov/articles/PMC3135376/. In this study, it not only shows individuals as young as 8 or 7 exhibiting greater maturation indexes than people in the 20's, but it shows the same thing with pre-teens and teens having greater maturation indexes than someone much older and vice-versa. All of this reinforces my point. That is what the data in the well-known study is trying to show is. It's not that it's saying there are fixed biological stages of the brain where it makes it very hard for a particular age to group to grasps certain things or skills well, but it's a result of environmental differences among varying age groups due to how the system is designed. For example, take for instance a 7-year-old who is dependent on her parents, or a 30-year-old who is living on his own, has a career, and etc. The former is an environment that doesn't demand executive functioning skills to such a frequent and intense manner, whereas the latter is forced into an environment where he has to use a lot of executive functioning skills starting when he was 18 living on his own where he has to plan and problem solve.
Another example is a 20-year-old living out on his own where he has a job and a 30-year-old living and depending on his parents that can't take care of himself. The former is going to have superior executive functioning skills than the latter, and in turn, their brains are going to reflect that notion.
As a final note, the prefrontal cortex (PFC) being the last part to develop is a bit inaccurate. While it's true that it is the last part to develop during the fetal stages of brain development, it doesn't necessarily develop that way due to the plastic nature of grey and white matter. The reason why it's referred to as the last part to develop and even shows in the data, is because of brain processing order that goes bottom to top and back to front. That means that the areas that get stimulated first are the areas in the back and bottom due to brain processing order and not as fixed developmental sequences other than the fetal stages of development. With the nature of this post, should I rename this post "Critical Age Theory" or what?
Independent Driven Myelination: https://pubmed.ncbi.nlm.nih.gov/36974372/
Dependent Experienced Myelination: https://pmc.ncbi.nlm.nih.gov/articles/PMC5667660/ and https://www.youtube.com/watch?v=etqNwdMqsco
Other sources: https://pmc.ncbi.nlm.nih.gov/articles/PMC3135376/ and https://sci-hub.se/10.1016/j.neuron.2016.10.059
The one thing you have to understand about that study is the markers of maturity that was used such as the grey and white matter ratios. Grey matter is the representation of the neurons, and the white matter is the representation of the myelinated axons. Now keep that in mine for later. Through this study, they show a decrease of grey matter due to synaptic pruning as people age, and an increase of white matter as people age. Age 25, which was the highest in their sample, showed greater levels of white matter which functionally makes connections faster between neurons or in how they put it, "more efficient or reinforced connections".
Now from face value, this seems pretty straight forward, right? Nope and this is where it gets juicy. White matter, which is the representation of myelination, is very plastic as with grey matter. Myelination is the fatty substance that wraps around the axon for higher action potential speed that is made by oligodendrocyte cells, which are in turn, made by oligodendrocyte precursor cells (OPC's). There are two distinct processes of myelination: independent experienced driven myelination and dependent experienced driven myelination.
Independent experienced myelination allows for the proliferation and differentiation of OPC's that mature into late OPC's, then immature oligodendrocyte cells, and finally mature oligodendrocyte cells that wraps myelin around the axon without little to no axonal activity. Independent driven myelination is the most critical in the prenatal phase of brain development and very early infancy, that is also critical in the increase of proliferation and differentiation of OPC's that in turn, increase the rate or how fast things myelinate that is a little after the onset of puberty.
Dependent experienced driven myelination myelinates axons based on axonal activity or in other words, stimulation of connections of the brain. This kind of process not only strengthens existing myelin sheath around axons but can also play a role in the proliferation and differentiation of OPC's. This process describes that the more you practice a particular skill or knowledge, the more you are stimulating that specific connection of the brain and thus, myelinating that connection.
Now, one may try to argue that the independent experienced process is the most critical and the dependent process plays a less critical role that has minimal effect that will show differences in later ages, but there are a few problems with this and for one, the independent process is the most critical during the fetal and early infancy stages, as well as puberty, and secondly, without the dependent process, we wouldn't have a lot of myelin at all.
So, experience is the key factor to white matter growth, and the independent factor plays in the background. Having this understanding in mind, we grow and mature based on the environment that allows us to have the experiences that we undergo that matures us, and not as a result of fixed linear stages of how the brain matures.
There is a reason young kids can be more proficient and skilled at a certain thing than an adult and vice-versa. A study done by Leah H. Summerville reinforces this notion by saying: "However, these data also illustrate the challenges of applying general patterns of neurodevelopment from group-based to individual inference, as there is substantial variance in brain network connectivity that is unrelated to age. For example, some 8-year-old brains exhibited a greater ‘‘maturation index’’ than some 25-year-old brains" (https://sci-hub.se/10.1016/j.neuron.2016.10.059).
Her statement is further reinforced by this study here: https://pmc.ncbi.nlm.nih.gov/articles/PMC3135376/. In this study, it not only shows individuals as young as 8 or 7 exhibiting greater maturation indexes than people in the 20's, but it shows the same thing with pre-teens and teens having greater maturation indexes than someone much older and vice-versa. All of this reinforces my point. That is what the data in the well-known study is trying to show is. It's not that it's saying there are fixed biological stages of the brain where it makes it very hard for a particular age to group to grasps certain things or skills well, but it's a result of environmental differences among varying age groups due to how the system is designed. For example, take for instance a 7-year-old who is dependent on her parents, or a 30-year-old who is living on his own, has a career, and etc. The former is an environment that doesn't demand executive functioning skills to such a frequent and intense manner, whereas the latter is forced into an environment where he has to use a lot of executive functioning skills starting when he was 18 living on his own where he has to plan and problem solve.
Another example is a 20-year-old living out on his own where he has a job and a 30-year-old living and depending on his parents that can't take care of himself. The former is going to have superior executive functioning skills than the latter, and in turn, their brains are going to reflect that notion.
As a final note, the prefrontal cortex (PFC) being the last part to develop is a bit inaccurate. While it's true that it is the last part to develop during the fetal stages of brain development, it doesn't necessarily develop that way due to the plastic nature of grey and white matter. The reason why it's referred to as the last part to develop and even shows in the data, is because of brain processing order that goes bottom to top and back to front. That means that the areas that get stimulated first are the areas in the back and bottom due to brain processing order and not as fixed developmental sequences other than the fetal stages of development. With the nature of this post, should I rename this post "Critical Age Theory" or what?
Independent Driven Myelination: https://pubmed.ncbi.nlm.nih.gov/36974372/
Dependent Experienced Myelination: https://pmc.ncbi.nlm.nih.gov/articles/PMC5667660/ and https://www.youtube.com/watch?v=etqNwdMqsco
Other sources: https://pmc.ncbi.nlm.nih.gov/articles/PMC3135376/ and https://sci-hub.se/10.1016/j.neuron.2016.10.059