The government in Ender's world plays a huge role in reproductive decisions, imposing financial penalties and social stigma on families who show great genetic potential to have a "third" like Ender. What does it mean to be a third?

Ender is a "Third," and although the designation is not fully explained in the first chapter, it becomes easy to conclude that being a "Third" means being the third child in a family. (It also reflects the idea of being a "third-class" citizen.) Since Ender's family has three children, his parents are breaking the population code (with permission) by having more than the allotted two children. The government authorized his parents to have a third child, but the other children, including his brother and the kids in his class at school, mock him anyway for being a Third. Thus, they have established a pattern of isolation of Ender, giving him an independence and a loneliness that pervades his character throughout the rest of the novel.