Adam Kucharski (2020) The Rules of Contagion. Why Things Spread—and Why They Stop. 

Posted by celticman on Wed, 06 Aug 2025

According to William Kemarck’s 1924 model contagion follows an S-shaped curve. The assumption was that those susceptible would become infected.

Infect others, recover or die and go to heaven or hell, as in Christianity.

 In epidemiology, the reproduction number, or R, indicates how many new cases a single infected individual causes. If R is greater than 1, an outbreak grows. If it falls below 1, stops. Nobody goes to heaven or hell.

  • R > 1: It spreads.
  • If R < 1: It dies out.

Super-Spreaders, Paul/Saul and other apostles.

Diffusion, trade routes, Roman roads and shared language.   Kucharski means: ‘Contagion isn’t about possibility, it’s about actual impact’.

Many competing spiritual movements existed including Judaism. Christianity happened because of timing, networking, and social conditions aligned.

Comparative Chart: Religion Spread & Contagion Dynamics

Religion

Primary Diffusion Style

Early R-Number (Spread Rate)

Current Global Followers

Conversion Strategy

Modern Trend
Christianity Contagious + Hierarchical High (R ≈ 2–3) ~2.4 billion Evangelism, missions Declining in secular West
Islam Contagious + Hierarchical + Trade Very High (R > 3) ~1.9 billion Preaching, community, state adoption Growing globally
Buddhism Contagious + Relocation Moderate (R ≈ 1.5–2) ~500 million Monastic missions, cultural blending Stable or declining
Hinduism Relocation + Cultural Transmission Low (R < 1) ~1.2 billion Birth-based, diaspora Stable, low conversion
Judaism Ethnic + Relocation Very Low (R < 1) ~15 million Birth/community-based Preserving identity
Secularism / Non-religious Contagious (via education/media) High in secular societies ~1.2 billion+ Cultural shift, science, individualism Rapid growth in developed nations
  • Decreased Rt​ (Effective Reproduction Number): The "social reproduction number" for Christianity is often below 1 in modern societies. Each religious person is, on average, converting or influencing less than one  person (if such a person exists).
    • Lower Birth Rates: In some religious communities, birth rates are lower than in the past, reducing the ready-made number of new adherents.
    • Less Evangelism/Proselytization: There may be less active effort or effectiveness in sharing faith with non-believers.
    • Secularisation: Many children raised in religious households do not continue to practice as adults.
    • Disaffiliation and Nones: A growing number of people self-identify not as nuns, just as ‘nones’ (no religious affiliation), indicating a trend of disengagement from major religions

Britain: A Post-Christian Nation?

In the UK, those identifying as irreligious now rival the number of Christians. The decline is starkest among the youth, with only 1% of 18-24 year olds identifying with the Church of England.

An ‘autocatalytic model’ goes into reverse.

<46%

Of the U.S. population may identify as Christian by 2070, signalling a modern shift in current belief systems.

 

  • Weakening Social Networks: Religious participation is less embedded in schools, daily social life, reducing casual exposure and influence.

The factors and factionalism that once contributed to the rapid spread of religion (strong social ties, lack of alternative explanations, perceived necessity) have significantly diminished.  In many (or most) advanced secular societies, a shrinking base of adherents leads to levelling off of numbers and downward spiral.

In other words, a self-fulfilling prophecy. Read on.

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