Limited Population Size
Balance of Forces
Other Feedback Loops
Second Order Feedback
Figure 2: Membership of the Methodist Church of Great Britain, the Pentecostal & New Churches, Free Church of Scotland, & United Reformed Church, as percentages of the population of their countries, where data is known.
|Figure 4: Annual change in Methodist Church membership, smoothed over a 10 year period, compared with the annual smoothed population change. See note  for explanation of periodicity in the numbers from 1900.|
|Figure 5: Annual change in Church of Scotland membership, smoothed over a 10 year period, compared with the annual smoothed population change.|
d. To double check I tried a variety of smoothing periods and this time averaged all the data points in the period, not just the end points. Figure 7 shows once such graph with a 7 year period, deliberately chosen to be out of phase with the 20 year inter-war period.
The periodicity is still there, so short of doing a statistical test, we can assume it is real.
The first negative low point is 1914-18 - the effects of world war 1, the inevitable losses and records not fully kept up to date. The rise to 1926 is post war membership readjustment, with the real state of affairs following up to 1939. The same happens again with a low point early 1940s and it peaking again 1954 as records are updated in the post war period. Remember these are not the membership figures but change in membership.
The next trough is 1978, a longer period than before and a peak in the early 1980s. This is likely an effect of the baby boom now becoming adults and entering membership and thus temporarily slowing the decline. It has only been down since.
Thus the best explanation of the periodicity is change is demographics, due to the two world wars - a 25 year effect, and the effect of world war 2 on birth rates, a roughly 30 year effect.
e. There is a peak in 1906, 20 years before the periods looked at above; and another peak at 1882, 24 years before that. These times may be coincidence. The 1880s saw membership fall behind population as the period of revivals came to an end. Early 1900s saw rapidly falling birth rates - but this could be a generational effect from the 1880s drop in conversion.
Sudden changes in populations, where the whole population, or a church sub population, can often see a knock on demographic effect 20-30 years later, so these effects are not surprising.