The witch-cult hypothesis' most prominent and influential advocate was the English Egyptologist Margaret Murray, who promoted it in a series of books – most notably 1921's "The Witch-Cult in Western Europe" and 1933's "The God of the Witches". Murray's books were the sources of many well-known motifs which have often been incorporated into Wicca. The idea that covens should have 13 members was developed by Murray, based on a single witness statement from one of the witch trials, as was her assertion that covens met on the cross-quarter days four times per year.