Physicists have a long history of applying statistical models to the study of human behaviour and have tackled problems as diverse as the performance of financial markets and the spread of languages. Now, Marcel Ausloos and Filippo Petroni at the University of Liege have turned their attention to the dynamics of religion by relating the emergence, growth and demise of religions to phase transitions that occur during crystallization and other physical processes.

Using methods borrowed from statistical physics, the researchers searched for patterns in data describing the numbers of adherents to major religions -- including Christianity, Islam and Buddhism – over the past century. These patterns suggested that the changes in the numbers of religious adherents can be modelled using an “agent-based” approach. Agent-based models assume that the collective behaviour of a group of individuals arises from a set of simple rules that define how an individual interacts with others in the group. This approach has already been used to study a wide range of sociological and physical processes including crystallization.

The researchers then created a model of the time evolution of religions using the agent-based concept of “preferential attachment”, which was first formulated in 1999 to explain self-organizing networks such as the World Wide Web. Preferential attachment is the tendency of an individual to form relationships with popular individuals – which are defined as individuals who themselves have relationships with a large number of other individuals.

According to Ausloos, the number of adherents to a specific religion appears to follow a “growth-death law” that also describes how the size of crystalline regions grow and shrink in some materials. One striking similarity to crystallization is that religions can appear almost spontaneously in a process that is similar to the nucleation of crystals – with a popular leader often fulfilling the role of a nucleation point. A recent example of the spontaneous nucleation of a religion is the Church of Jesus Christ of Latter-day Saints (the Mormons), which was founded about 175 years ago in the US by Joseph Smith and now has nearly 13 million members worldwide.

The growth and demise of a religion can also be affected by phenomena such as mass conversions or genocide that affect entire groups -- rather than the interactions between individuals. Ausloos describes these influences as “external fields”, in analogy to externally-applied electric fields or temperature gradients that can affect the crystallization process.