American church members in California and studied their activities in trying to promote their beliefs and win converts to their church. Lofland noted that most of their efforts were ineffective and that most of the people who joined did so because of personal relationships – often family relationships – with existing members. Lofland summarised his findings in 1964 in a doctoral thesis entitled "The World Savers: A Field Study of Cult Processes", and in 1966 in book form (published by Prentice-Hall) as Doomsday Cult: A Study of Conversion, Proselytization, and Maintenance of Faith. It is considered to be one of the most important and widely cited studies of the process of religious conversion, and one of the first modern sociological studies of a new religious movement. The Church of Scientology attempts to gain converts by offering "free stress tests". It has also used the celebrity status of some of its members (most notably that of the American actor Tom Cruise) to attract converts. The Church of Scientology requires that all converts sign a legal waiver which covers their relationship with the Church of Scientology before engaging in Scientology services. Research in the United States and in the Netherlands has shown a positive correlation between areas lacking mainstream churches and the percentage of people who are members of a new religious movement. This applies also for the presence of New Age centres. On the other end of the proselytising scale are religions that do not accept any converts, or do so very rarely. Often these are relatively small, close-knit minority religions that are ethnically based such as the Yazidis, Druze, and Mandaeans. The Parsis, a Zoroastrianism group based in India, classically does not accept converts, but this issue became controversial in the 20th century due to a rapid decline in membership. Chinese traditional religion lacks clear criteria for membership, and hence for conversion. However, Taoism does have its own religious conversion ceremony which seems to be adopted and modified from Chinese Buddhist refuge-taking ceremonies. The Shakers and some Indian eunuch brotherhoods do not allow procreation, so that every member is a convert. Fostering conversion Different factors and circumstances may operate and interact to persuade individuals of groups to convert and adopt a new set of religious doctrines and habits. Religious enthusiasm for proselytism can play a role. For example, the New Testament chronicles the personal activities of the Apostles and their