The ratio.

It is not mathematically possible for two sibling children to behave well at the same time. Tonight my nine-year-old son threw himself writhing onto the bed, weeping until he fake-choked, because I said we wouldn't order pizza and would be having mashed potatoes and Ikea meatballs instead. His brother, usually the one to throw fits, pumped his fist excitedly "I love Ikea meatballs! They taste great with gravy!" He checked on his brother who was banished to his room to see if he was feeling better. He chattered away talking and asking questions. The good behavior of one child is proportional to the bad behavior of the other.