This paper interrogates Michel Foucault’s claim, that the spread of psychiatric power originated in concerns around the educatability of idiot children in the latter decades of the nineteenth century, before being applied to adult “defectives”. It is argued that Foucault, although partially correct, fails adequately to consider the extent to which the base concept, of “instinct”, was linked in particular ways to female idiot sexuality. The paper challenges Foucault’s view through an analysis of a series of nineteenth century cases involving (...) the rape of female idiots, arguing that their sexuality was understood in terms of a relation to instinct which manifested in terms of an opposition between dangerousness and vulnerability. It then traces that opposition into the Mental Deficiency Act 1913 where, it is argued, it functioned in a collapsed form—now, the vulnerable were dangerous and the dangerous were vulnerable—and in which form it underpinned a psychiatrised regime for the control of mentally defective women through the control of their sexuality. (shrink)