Humans keep brains plastic and unmyelinated well into the third decade of life

Evidence

After this initial growth spurt, our brains continue adding more connections for holding and processing information (new glial cells, axons, and synapses) over the next three decades of life and even beyond, especially in the neocortex. Consider our white matter and, specifically, the process of myelination. As vertebrate brains mature, their white matter increases as the (axonal) connections among neurons are gradually “burned in” and wrapped in a performance-enhancing coating of fat called myelin. This process of myelination makes brain regions more efficient, but less plastic and thus less susceptible to learning. To see how human brains are different, we can compare our myelination with that of our closest relatives, chimpanzees. For the cerebral cortex, figure 5.2 3 shows the fraction of myelination (as a percentage of the adult level) during three different developmental periods: (1) infancy, (2) childhood (called the “juvenile period” in primates), and (3) adolescence and young adulthood. Infant chimpanzees arrive in the world with 15% of their cortex already myelinized, whereas humans start with only 1.6% myelinized. For the neocortex, which has evolved more recently and is massive in humans, the percentages are 20% and 0%, respectively. During adolescence and young adulthood, humans still have only 65% of their eventual myelination complete, whereas chimpanzees are almost done, at 96%. These data suggest that, unlike chimpanzees, we continue substantial “wiring-up” into our third decade of life. — Joseph Henrich, The Secret of Our Success, 214

Categorization

• QUESTION: Key question for the reader’s goal: does this extended myelination timeline—plasticity into the third decade—correlate with when conformist/prestige biases peak in influence, i.e., who is most coercion-vulnerable?
• XREF: Connects to Chudek & Henrich’s cultural evolutionary account that socially transmitted biases and brain plasticity coevolved; myelination timing bears on when calibrational learning windows close. — connects to: Human cognitive plasticity extends through enduringly plastic development shaped by culture — Joseph Henrich, The Secret of Our Success: How Culture…, 220; Brain growth continues fifteen years after birth through self-formation — Catherine Malabou, What Should We Do with Our Brain?, 71
• SHARE: The human-vs-chimp myelination percentages (1.6% vs 15% cortex at birth, 0% vs 20% neocortex; 65% vs 96% by young adulthood) would intrigue an evolutionary-developmental psychologist.

Broader Connections

• Brain growth continues fifteen years after birth through self-formation #corroborates (0.82) — Catherine Malabou, What Should We Do with Our Brain?, 71
• Human cognitive plasticity extends through enduringly plastic development shaped by culture #corroborates (0.78) — Joseph Henrich, The Secret of Our Success, 220