The phenomenon of reverse sexual imprinting is when two people live in close domestic proximity during the first few years in the life of either one, and both become desensitised to sexual attraction, now known as the Westermarck effect, was first formally described by him in his thesis "The History of Human Marriage" (1891). Westermarck was also a scholar of Morocco and offered a positivist view of how its folk religion was formed in his two-volume work "Ritual and Belief in Morocco" (1926). He had started his fieldwork in Morocco as early as 1898, and visited the country 21 times in the next thirty years, spending in total seven years in the country. He also studied his favorite subject, marriage, there, publishing in 1914 "Marriage Ceremonies of Morocco". A radical free thinker for his time, he critiqued Christian institutions and Christian ideas on the grounds that they lacked foundation. He was also a moral relativist and in his two-volume "The Origin and Development of Moral Ideas" (1906–1908) he argued that moral judgements are not rational but are based on emotions and on social approval or disapproval. As a consequence, he also denied the existence of general or universal moral truth. In the UK, his name is often spelled Edward. His sister, Helena Westermarck, was a writer and artist.