Average intelligence constrains the complexity of societies.
(Note: The level of intelligence does not guarantee complexity - complexity can be suppressed. Also, complexity can be imported from more- to less-complex societies. Also intelligence is necessary but not sufficient - average personality/ or 'national character', in particular, is very important.)
Assuming current average intelligence among natives in England as IQ 100:
115 - (i.e. average intelligence in England 120 years ago and probably for several hundred years previously). Can sustain an extremely complexly differentiated and specialized modern society based on continual medium term economic growth and the expectation of such growth, and underpinned by a continual stream of major technological breakthroughs.
(Note: breakthroughs also require creativity, very high intelligence is necessary but not sufficient.)
100 - Can not sustain modern society; but can sustain a large scale and complex but static agriculture and trade-based society. Innovations and breakthroughs happen but are too infrequent, dispersed and insufficiently revolutionary to affect the basic nature of the society - they simply lead to a greater population density and per capita wealth reverts to pre-innovation levels.
In the long term a society with an average IQ of around about 100 will stabilize at a no-greater-complexity than that of a moderately-complex but essentially static agrarian society - with moderately large cities, and moderate societal specializations - something like the Roman Empire.
85 - Simple agriculture, pastoralism, limited specialization of social function. No cities - only villages and towns. Simple technology.
70 and below - Immediate return hunter gatherer lifeways. Very little technology, no long term food storage, very little social differentiation, highly egalitarian.
Given that The West has probably gone from 115 to 100 in about 150-200 years (actually, I think the change has been somewhat greater or faster than this); it is interesting to speculate how far and how fast the process could continue.
Of course, at some point, social breakdown will reimpose the extreme harshness of natural selection on those of lower intelligence, but in the short to medium term, warfare, starvation and disease may mean that intelligence may continue to be selected against for a long time.
It seems conceivable, therefore, that the extreme rapidity of intelligence decline may - with a time lag, as societies benefit temporarily from the residual technological legacy of their ancestors - lead to a considerable overshoot of intelligence decline; such that there may be a reversion of two steps, not one.
The West may go all the way from modernity to simple agrarian societies - passing only briefly through a phase of complex agrarian 'empires' - in the space of a single digit number of generations from now.