Others who held that missionary was the only permitted position included Alexander of Hales and the author of "De secretis mulierum", who suggested that nonstandard positions might result in birth defects. Ruth Mazo Karras states that William Peraldus' treatise "Summa de virtutibus et vitiis" distinguished between sins against nature that were "according to the substance" (intercourse other than vaginal) and "according to the manner, as when a woman mounts." Nicholas Venette's 1770s-era sex manual praised the missionary position as the "common posture...which is most allowable and most voluptuous." Numerous sources have reported that in the United States, some states had outlawed positions other than missionary between husband and wife, or will grant a divorce to a woman whose husband makes love to her in another position. While many states formerly outlawed oral sex, anal sex, buggery, or other "unnatural" acts, no US law has banned dorso-ventral heterosexual sex, or specified which partner needed to be on top.