[6] He has advanced the view that women in classical antiquity, the Middle Ages, and the Early Modern period deliberately used herbal abortifacients as a means of fertility regulation.
[7][8] Historian Etienne van de Walle has quoted Riddle[9] as stating that "most women" in the Middle Ages knew that certain herbs and herbal products could be taken to induce an abortion in the early stages of pregnancy and that this knowledge was primarily shared amongst women, thus affording them "more control over their lives than we thought possible".
Van de Walle described Riddle as the "strongest advocate" for the position that women in classical antiquity, the Middle Ages, and the Early Modern period deliberately used herbal abortifacients, and has criticized his suggestion that "these drugs were perfected over centuries in a female culture of which males—who were doing the writing—had only a partial and imperfect understanding.
"[10] Historian Gary B. Ferngren has also taken issue with these hypotheses, particularly because of what he called the circumstantial nature of Riddle's evidence, writing that the ideas remained "unproved and unlikely".
[11] Other critics include demographer Gigi Santow, who wrote that Riddle overemphasizes the role of herbs and seeks "not so much to persuade as to convert,"[12] and medical historian Helen King, who has written that Riddle makes claims about modern pharmacology that are not supported by his source materials.