Missionaries or Parents? How are religions growing?
For the last years, the demographic potentials of religiosity have been my primary focus of research. Repeatedly, people asked me whether proselytizing or high fertility would be more important for the success of a religious tradition.
Now, Christopher P. Scheitle, Jennifer B. Kane and Jennifer van Hook jointly addressed this topic in a compelling study, freshly published in the Journal for the Scientific Study of Religion.
Demographic Imperatives and Religious Markets: Considering the Individual and Interactive Roles of Fertility and Switching in Group Growth
by Christopher P. Scheitle, Jennifer B. Kane, Jennifer Van Hook
Journal for the Scientific Study of Religion
Volume 50, Issue 3, pages 470–482, September 2011
The authors explored the range of familial ("fertility") and proselytizing ("switching") outcomes of various religious traditions in the USA.
Concerning fertility, they discerned high-fertility religious groups such as the Old Order Amish (with a Total Fertility Rate (TFR) of 6 to 7) from low-fertility groups such as Liberal white Protestants (TFR 1.40 - 1.8). They could have added the Shakers with a TFR near zero, though.
Concerning switching, the scientists found highly proselytizing groups such as Jehova's Witnesses with a percentage gain of 83% (!) down to the Catholics with a gain of 8%. But interestingly, high readiness to bring in new believers also tended to go with higher rates of defection, especially among the younger adults. For example, Jehova's Witnesses also lost 67% (!) of those raised in the faith compared to a still striking 32% of Catholics. Therefore, the net effect of switching remained in a range of about +- 20% (+17% for Jehova's Witnesses, - 24% for Catholics).
Which effect is stronger?
By calculating various models, the team found out that fertility clearly beats switching in long-term-success: Groups with fertility high above replacement levels could grow exponentially while abstaining from any proselytizing and even taking high defection rates (again - cp. the Old Order Amish, orthodox Jews, Hutterites etc.). In contrast, switching lead to nowhere in the long run, if those joining the group did not raise enough children - any childless convert raising the membership by one for just some decades.
In fact, the winning strategy turned out to be a "preach-then-breed"-combination: High rates of missionary activity during the first generations in order to build a strong membership base, followed by high fertility rates and subsequent exponential growth. Mormonism is an US-example of this successfull strategy: It grew from 23 members in 1830 to 15.000 in 1846 and 30.000 in 1856 primarily by proselytizing - followed by generations of high fertility contributing to its contemporary multi-million membership (and a TFR of still about 3).
These findings seem to be very relevant for evolutionary studies of religion, too: They emphasize the two-tiered process of biocultural evolution: Religious messages have to appeal to human sociopsychological needs to survive cultural evolution, and then to offer on-average reproductive advantages in order to expand throughout the centuries (and contributing to the spread of genes related to religiosity, as predicted by Robert Rowthorn). Any religious tradition not overcoming these two obstacles won't survive for long in evolutionary history.
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So, from your diagram, it would appear that, taking the 'most religious' (worshipping twice a week or more)in Germany as a group, they are not reproducing at replacement level. Bearing in mind your previous results, this would appear to illustrate the fact that fertility is not just a function of religiosity; it is multivariate.
Do we know what the other variables are Michael? Also, do we know the relative importance of each of these variables on fertility? Have there been multivariate studies of fertility?
Yes, there are a whole range of multivariate analyses available and still more under way - but it is a daunting task! For example, the economic system has an enormous impact: While children are valuable as future allies and pension system in agrarian systems, they are becoming very costly in post-industrial societies. Add the fact that religious traditions and institutions evolve into very diverse forms as do those of "education" and all of these factors are interacting (i.e. the Old Order Amish relying on agrarian economy and avoiding modern education for religious reasons) - and it becomes obvious that there can be no single once-and-for-all-answer for every situation. It seems that evolutionary studies have again to proceed by numerous case studies, as presented here by Scheitle, Kane & van Hook.