A virus spread around the world. Turning 9/10ths of the population into littles. 4 feet tall immature adults who require the same caregiving and careful looking after as child. They're as intelligent as adults their age but as mature as toddlers, unable to focus, prone to mood swings.

The remaining 1/10th of humanity became bigs. Extremely intelligent and competent 10 feet tall people who have a strong parental instinct that littles cause to go into overdrive.