Put every school district's test scores on one scale and split them by sex. In reading, girls lead in 12,346 of 12,360 districts — 99.9 percent of the map. In math, the country splits nearly down the middle — and which half a district lands in tracks its affluence with eerie regularity.
Every gap in this series so far has been a gap between places. This one lives inside every classroom in America — and the geography decides only half of it.
SEDA reports average test scores separately for girls and boys in each district, on the same grade-level scale used in story № 4. Subtract one from the other and you get two national experiments: one whose result is the same everywhere, and one whose result depends on where you are.
The reading distribution doesn't straddle zero — it doesn't even approach it. The average district's girls read three-quarters of a grade level ahead of its boys, and the most boy-favorable district in America is barely above even. Math is a different object entirely: a distribution centered almost exactly on zero, with thousands of districts on each side.
Sort districts into deciles by overall achievement — a close proxy for affluence — and the math gap marches monotonically from girl-favoring in the poorest decile (−0.22 grade levels) to boy-favoring in the richest (+0.13). In the bottom decile, boys lead math in one district in five; in the top decile, in nearly three of four. The reading gap, meanwhile, is a flat line: roughly −0.8 everywhere, indifferent to money, region, or school quality.
This is the pattern Sean Reardon and colleagues documented when they first built these data: male math advantages appear in affluent, white-collar communities and vanish — or reverse — in poorer ones, while the female reading advantage is universal. Whatever produces the math gap, it is not fixed biology; fixed things do not vary this smoothly with the local tax base.
County by county, the math gap draws a recognizable America: boy-leads blue across the affluent suburban Northeast, the Mormon corridor, and the upper-Midwest farm belt; girl-leads rose across the Black Belt, the Mississippi Delta, the border South, and the reservation counties — the same low-income geography where, as story № 3 showed, boys' outcomes falter on every other measure too.
For a book about place, this chapter is a control experiment. The reading gap shows what a placeless disparity looks like: the same sign, nearly the same size, in every kind of community — pointing to causes that travel with childhood itself (development timing, reading habits, how elementary school rewards behavior). The math gap shows the opposite: a disparity that flips sign along the income gradient, pointing to causes that live in communities — expectations, role models, the gendered sorting of opportunity that affluence appears to amplify for boys and poverty to reverse.
When someone tells you a gap "is just how boys and girls are," the test is now simple: does it survive the map? One of these does. The other doesn't.