Which fact about brain development is commonly cited for two-year-olds?

• A. The brain is about 70% of adult size.
• B. The brain is about 55% of adult size.
• C. The brain is about 40% of adult size.
• D. The brain is about 90% of adult size.

Answer: (B)

A key point here is that a toddler’s brain has already grown a lot, reaching a substantial portion of its adult size but not close to full size yet. The estimate around just over half of the adult brain is what’s commonly cited for this age, and 55% fits that idea well. It signals rapid early growth and high neural plasticity without suggesting the brain is already near adult weight. The higher figure would imply more advanced development than is typical for a two-year-old, while the lower figures would underestimate the brain’s size at this age, and the near-adult percentage would be too advanced for a toddler.