How do we know human brain development stops around age 25?

Question

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).

Answer 1

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.

2. 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.

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.}

Answer 2

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