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 8 or 9 year olds, 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.