Throughout the nineteenth century, German medical, scientific and legal scholars found themselves puzzled and engaged by the diverse forms of human sexuality. Psychiatrists like Richard von Krafft-Ebing who were interested in explaining deviance encountered scientifically trained advocates for emancipation like Magnus Hirschfeld, and the result was the new--if unstable--discipline of sexual science. Because they based arguments for social intervention on knowledge of nature and the body, the field's proponents--like the advocates of eugenics and racial hygiene--argued that they were biologists. After 1900, this mutual biological engagement of sexual science and eugenics revealed itself in overlapping debates between the proponents of both fields.