I've heard this stated as a fact more times than I can keep track of, but I haven't been able to find papers that actually demonstrate this. This seems to be the most cited paper on the topic, but it just claims that maturity of the brain by age 25 is widely established, and cites a CDC paper (reference 5) that does not appear relevant.

To be more specific, I want to know

  1. What exactly do we mean when we say the brain is "finished developing"? Is that when there are no more changes in volume of gray/white matter? Is it when synaptic pruning levels off?
  2. What is the evidence for this? What imaging modalities are used? What is the variance in the end-of-development timepoint?

I'm hoping to find a good paper that contains answers to these kinds of questions. My motivation for asking this is that I'm wondering whether this end of development is reflective of a deterministic biological process, or if it is a function of the environment (e.g. most people stop learning and meeting new people as much around 25).


2 Answers 2

  1. Based on a review by Kolb et al, 2012, it seems that "the brain is finished developing by 25" refers to the point when synaptic pruning in the cerebral cortex levels off, on average. However, the prefrontal cortex, the region most unique to humans and involved in executive function, develops in this way well into the third decade of life.

Time line of brain development in humans and rodents. B, birth.

  1. The above review cites a study by Petanjek et al., 2011 that investigates synaptic pruning in the prefrontal cortex by looking at differences in dendritic spine density in tissue samples from autopsies of human subjects ranging in age from 1 week to 91 years. As seen in the figure below, significant pruning is still occurring even by age 40.

The dendritic spine density plotted at the linear scale to illustrate the dynamics of changes occurring during the 100-y human lifespan. Regression curves fit the distribution of data from the basal dendrites.

So, a critical region of the human brain is definitely not finished developing by 25, or even 35.

As for whether this process is biologically "set in stone", I think it's fair to say that is still an open question. Looking at the second figure, there is quite a bit of variance and a relatively small sample size. Moreover, this was necessarily a cross-sectional study since we can only get postmortem tissue samples in human subjects. It is entirely possible that older subjects had more pruning because they lived in a less dynamic environment. Even if we could do longitudinal studies, the question of societal trends influencing learning environment would still remain.

That said, there probably is some degree of biological determinism given that we observe similar trends in other mammals. However, as humans tend to have some of the most gene-environment interactions of all animals, and the prefrontal cortex is the most recently evolved brain region, we can only get limited information from studies in other animals.

Given the importance of the human brain development timeline to policy decisions (see paper cited in the question), I think it's fair to say these questions need more attention.

Kolb, B., Mychasiuk, R., Muhammad, A., Li, Y., Frost, D. O., & Gibb, R. (2012). Experience and the developing prefrontal cortex. Proceedings of the National Academy of Sciences, 201121251.
Petanjek, Z., Judaš, M., Šimić, G., Rašin, M. R., Uylings, H. B., Rakic, P., & Kostović, I. (2011). Extraordinary neoteny of synaptic spines in the human prefrontal cortex. Proceedings of the National Academy of Sciences, 108(32), 13281-13286.

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    $\begingroup$ At some point this is really just a matter of semantics, because once "development" stops we can say that "aging" begins: the brain is a dynamic organ throughout the lifespan. $\endgroup$
    – Bryan Krause
    Jan 2, 2019 at 17:21

In the book The Naked Ape Desmond Morris (a zoologist and perhaps evolutionary psychologist) says that human brain stops developing when the skull is fully developed, which is around age 25. The reason the skull cannot fully developed until this age (as compared to other animals) is because if it is developed in the uterus of the mother, then she cannot gives birth the child, and thus both will die. The reason for this (as compared to other animals) is because human standing up, so the human female pelvis cannot open enough room to give birth a full-developed infant skull.

Some evidences that he gives supporting this:

  • Birthing in human is long and like torture, as compared to other animals. I've watched YouTube clips of giraffe's birth and chimpanzee's birth, and the babies are delivered under 5 minutes, and the mothers don't seem to have pain at all. (The giraffe continues to eat; the chimpanzee rolls around)
  • There exists no single fossil for a full-developed infant skull, suggesting that any mutation would have definitely died

Some notes:

  • My memory may be distorted
  • The book is a pop science book, and written in the 60s
  • $\begingroup$ While this answer gives some info, to me it doesn't answer part 1 (the crux) of the question. "What exactly do we mean when we say the brain is "finished developing"? Is that when there are no more changes in volume of gray/white matter? Is it when synaptic pruning levels off?" $\endgroup$ Dec 31, 2018 at 11:20
  • $\begingroup$ I think the author would say that "finished developing" simply means the brain cannot grow anymore in volume. $\endgroup$
    – Ooker
    Dec 31, 2018 at 11:34
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    $\begingroup$ Chris is correct: I want to know how we define end of brain development when we say it ends around 25. My impression was that the brain stops changing volume close to age 18, and so for it to end at 25 I would assume there is a more microscale definition. My motivation for this question was the thought that perhaps this statistic reflects the time when WEIRD people typically stop learning new skills, people, etc., and is not a deterministic biological program. I will update my question to clarify. $\endgroup$
    – eyeExWhy
    Dec 31, 2018 at 21:58
  • $\begingroup$ @eyeExWhy I don't think there is no suspense in microscale in brain development during lifetime. If there is, I don't think it's around age 25. But I'm not a neuroscientist so take my words for granted. If you specify age 25, then this is the only relevant information I know. At 18 year old, the skull almost finishes its development, but it would take to 25 to complete $\endgroup$
    – Ooker
    Jan 1, 2019 at 3:37
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    $\begingroup$ @Ooker If your source is correct, then it actually could be the case that the brain and skull do indeed stop increasing in volume by 25. I think the reason for the emphasis on synaptic density (basically how connected with each the neurons are) is that, in principle, it correlates more directly with brain function than does brain volume, which could reflect, e.g. number of glial cells which mostly do not process information $\endgroup$
    – eyeExWhy
    Jan 2, 2019 at 5:30

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