The students in Mr. Seemy’s class were arguing about how ambiguous the status of adulthood is, and how varied the guidelines are for determining when someone is mature. They wondered whether there was any biological basis for defining adulthood. Raul, Lisa, and Aliyah found some articles about scientific research on brain development, and shared their findings with the class.

“The brain changes a lot as a person grows from childhood through adolescence and into adulthood,” said Raul. “One important change is in the amount of gray matter and white matter in the brain.”

Aliyah added, “The amount of gray matter peaks in the early teenage years, and then decreases as a person grows older. The amount of white matter increases. Gray matter in the brain is associated with learning.”

“So what causes the different appearance of gray matter and white matter?” Mr. Seemy asked. “How do they work differently?”

“Well, the brain is made of billions of nerve cells called neurons,” Lisa responded. “Gray matter is made up mainly of the cell bodies of neurons. White matter is made up mainly of the long tails of the neurons (called axons). These tails are coated with a fatty substance called myelin, which gives them a white appearance.”

The illustration below shows the inside of two brains, labeled A and B. Based on the information above, which brain is the adolescent brain, and which is the adult brain? How do you know?

Discussion Question:

If the ratio of white to gray matter is a measure of how far one has gotten into adulthood, should we be using brain scans instead of chronological age to decide when people are allowed to drink alcohol, drive, and vote?