US states collect sex and gender data on official government forms to understand, identify, classify, and surveil populations. These forms’ gender boxes—sets of questions about sex, gender, and gender identity paired with a wide variety of answer options—can mean the difference between legibility and erasure or between surveillance and privacy. They also create classic disclosure and legibility dilemmas that disproportionately burden transgender, nonbinary, gender-nonconforming, and intersex individuals. And yet, the socio-legal forces determining the design of these gender boxes have been insufficiently studied. Documents obtained through public records requests and interviews with civil servants responsible for form design demonstrate that gender box design stems from the competing yet mostly inertial pressures that define the socio-legal contexts of street-level bureaucracy. In other words, gender boxes are products of the institutional, technological, political, and social contexts in which they are designed. Specifically, gender boxes look the way they do because they are subject to the effects of bureaucratic processes, social networks, expertise, intergovernmental dependence, norms, path dependencies, and technologies, with implications for research and advocacy.