How did Christianity grow over 2000 years from a tiny religious sect to the largest family of religions in the world - and what can this tell us about social change in the future?

Kiwi and Australian researchers have turned to a new modelling approach to unravel the influences of political hierarchies, social inequality, and population size on the spread of Christianity across 70 island cultures stretching from Madagascar to New Zealand.

After assessing Christian missionary efforts from 1668 to 1950, the researchers found the spread was driven more by the influence of political leaders than grass roots empowerment.

They found that the common missionary approach — to focus on winning over political leaders — might just have paid off as Christianity spread faster in smaller societies and in those with stronger political leadership.

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The study's leader, Dr Joseph Watts, who undertook the research while at the University of Auckland's School of Psychology, said his findings offer a new and fascinating window into large-scale human behaviour and the process of cultural change.

"If you look at our contemporary world, some things spread incredibly quickly while others take a very long time so here we provide evidence of why that might be."

The Austronesian societies they investigated shared a common ancestral language and are located across Southeast Asia, East Africa and the South Pacific.

Historically, they ranged from very small egalitarian family-based communities to large politically complex societies such as Hawaii.

Conversion typically happened in the 18th and 19th centuries, and while some societies took less than a year to convert, others took up to 205 years.

The range of social structures and conversion histories makes Austronesian societies ideal for theories about how cultural change happens.

The results of the study showed cultures with political leadership structures were often the fastest to convert to Christianity.

This supported a "top-down" process of conversion whereby chiefs and elite leaders, themselves converted by missionaries, were highly influential in spreading Christian doctrine among their people.

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In contrast, social inequality was not related to conversion times.

This challenged one of the most widely-cited reasons for Christianity's popularity, that it spread from the "bottom-up" by empowering lower classes and promising to improve the lives of the less privileged in the afterlife.

The research, published today in the international journal Nature Human Behaviour, also found that Christianity spread most quickly among small populations.

This helped clarify the importance of population size in processes of cultural change.

"While people often think of big societies as sources of innovation, our findings show that bigger societies can also be slow to pick up on new ideas," Watts said.

"In a small population, it becomes more likely that beliefs will be transmitted relatively quickly, particularly if they are being driven by leaders and other powerful figures."

Fellow study author Professor Quentin Atkinson, also of the University of Auckland, said finding answers about how beliefs have spread in the past gives us insight into how they might spread in the future.

"This research can help us understand how both the size and the structure of populations influence the diffusion and adoption of new institutions, ideologies or technologies."