Discussing the relationship between the different Pagan religions, religious studies scholars Kaarina Aitamurto and Scott Simpson wrote that they were "like siblings who have taken different paths in life but still retain many visible similarities". But there has been much "cross-fertilization" between these different faiths: many groups have influenced, and been influenced by, other Pagan religions, making clear-cut distinctions among them more difficult for scholars to make. The various Pagan religions have been academically classified as new religious movements, with the anthropologist Kathryn Rountree describing Paganism as a whole as a "new religious phenomenon". A number of academics, particularly in North America, consider modern Paganism a form of nature religion.