In his excellent book ‘Through the Eye of a Needle: Wealth, the Fall of Rome, and the Making of Christianity in the West, 350-550 AD’, Peter Brown says:
Hence the double aspect of the Christianity that had emerged in the Latin West in the crucial period between 370 and 400 AD. A new institution had become prominent in a society that knew what it was to give. Its upper classes had always valued the exhilarating “rush” associated with giving to an esteemed public cause, of which civic euergetism was the most spectacular and the most certain of acclaim. Great opportunities for giving now opened up in the relatively new Christian churches . But how would these traditions of highly personalized display impinge on a group that had hitherto been notable for its capacity for collective action? This was a real dilemma. Ideally, giving was open to all Christians. But this was a myth. It was no more true in the fourth century than was the nineteenth-century myth that the great Catholic Cathedral of Saint Patrick’s in Manhattan was built “through the pennies of Irish chambermaids.” (In reality, the first building of Saint Patrick’s was made possible through a campaign by which the bishop approached a hundred leading figures for $ 1,000 each.) Furthermore, what sociologists of modern religion call “skewness” appears to be an iron law in religious giving: 20 percent of the congregation usually contribute 80 percent of the funds of the religious community that they support.
This looks far too pat. The 80/20 factoid is derived from the Pareto distribution, which reflects wealth distribution in ‘modern’ societies. It will have been far more skewed in pre-modern settings, such as the Late Roman Empire. In addition, skewness is skewness and needs no support from ‘sociologists of modern religion’.
Brown quotes a paper by Iannaccone [Skewness Explained: A Rational Choice Model of Religious Giving, Laurence R. Iannaccone Journal for the Scientific Study of Religion, Vol. 36, No. 2 (Jun., 1997), pp. 141-157.] which says:
Whereas the rest may apply to all aspects of religious participation, skewness is truly a distinctive feature of giving. Professional fund-raisers consider skewness “a bedrock rule of thumb” relevant to virtuallyevery setting, large or small, religious and nonreligious (Hoge 1994: 103). In practice, it means that 20% of a congregation’s members provide more than 80% of the giving. Inevitably, these people also exercise substantial power, for who can afford to alienate the few families that keep the church afloat?
Well, that’s much more sensible, though having correctly stated that this skewness is a straightforward consequence of the properties of the statistical distributions that might plausibly be involved, Iannaccone then drags the regrettable and moth-eaten Chicago School stuff out of the store-cupboard and proceeds to make up lots of parameters, just for fun probably…but I don’t think you can dispute that the giving is at least as skewed as 80/20.