The introduction of holes in a parent compound consisting of copper oxide layers results in high-temperature superconductivity. It is also possible to dope the cuprate parent compound with electrons1, 2, 3. The physical properties of these electron-doped materials bear some similarities to but also significant differences from those of their hole-doped counterparts. Here, we use a recently developed first-principles method4 to study the electron-doped cuprates and elucidate the deep physical reasons behind their behaviour being so different from that of the hole-doped materials. The crystal structure of the electron-doped compounds is characterized by a lack of apical oxygens, and we find that it results in a parent compound that is a Slater insulator—a material in which the insulating behaviour is the result of the presence of magnetic long-range order. This is in sharp contrast with the hole-doped materials, which are insulating owing to the strong electronic correlations but not owing to magnetism.