Caroline Criado Perez opens with a deceptively simple observation: most of the data we use to understand and design the world was collected from men, about men, and assumed to apply to everyone. This is the "gender data gap" — a silence so pervasive we barely notice it. From crash-test dummies modelled on the male body to medical research that excluded female subjects, the default human in modern society is implicitly male.

This default isn't usually the result of deliberate malice. It emerges from a world where men have historically been the ones making decisions, writing the research, and setting the standards. Women become an afterthought — or worse, invisible. Criado Perez argues that this invisibility has real, measurable, sometimes deadly consequences for half the world's population.