Suppose that the threshold IQ for effective leadership is about 105 (roughly the top one third of the population of England, when IQ is normed at 100 average and with a standard deviation of 15).
In other words, by IQ 105 I mean a level of general intelligence (as measurable by capacity for abstract and systematic thinking, ease of memorising, swiftness of calculation etc.) which is somewhat but not much above average for European natives.
(This is roughly the IQ of an effective foreman, police or Army Sergeant, or a master craftsman - somebody with potentially high technical skills, good practical reason and tactical nous.)
I am suggesting that IQ 105 is the threshold for 'intellectuals' in a global and historical context - for those who take leadership roles in social domains requiring cognitive expertise (most obviously priests, sorcerers, sacred kings, law makers and judges, in military societies those roles where 'generalship' is required and so on).
In a population with an average IQ of 70 or less, there would be (functionally) zero individuals capable of intellectual leadership. Therefore it would be intellectually egalitarian.
In a population with an average IQ of about 80, there would either be just a few percent or less of the population as an intellectual leadership - therefore at most specific individual leaders (and not an intellectual leadership class).
In a population with an IQ of about 90, there would be an intellectual elite of leaders (more than just a few percent, something around 10 percent of the population). It would probably be ruled by an upper class or caste.
In a population with an IQ of about 100 plus, there would be around a third of the population above 105 IQ, and therefore potential leaders - this would be a middle class society, tending toward some version of 'democracy' or majority (skilled class) rule (eg rule by organised skilled workers, crafts, guilds, unions etc).
Average IQ 70 or less (two and one third SD below IQ 105) - zero/ very small proportion of individuals are intellectuals, therefore no significant social role for cognitive specialists.
Average IQ around 80 (one and two thirds SD below 105) - a few percent of intellectuals, therefore individual intellectual leadership.
Average IQ around 90 (one SD below 105) - an elite of intellectuals.
Average IQ around 100 (one third of an SD below 105) - a mass-minority middle class of intellectuals.
The general point is that the proportion of 'intellectuals' in a society (when 'intellectual is defined as above) is likely to be a strong influence on the general type of society, and the diffusion and nature of power in that society.
There are things which a society with a given IQ distribution can and cannot do - higher average IQ brings new capabilities and powers, but also new pitfalls and problems - since being an abstracting and systemising intellectual is a disposition which entails that life will be framed in abstract and systematic fashion.
Hence the societies with an average IQ around 100 must expend tremendous resources on internal propaganda, to maintain cohesion of the abstract world of the large minority of intellectuals - such that much intellectual activity is merely keeping intellectuals in order, and not contributing to social well being...
This is NOT a prediction relating to monarchy, oligarchy and democracy - rather relating to whether power is individual (e.g. Big Man), minority elite (Priesthood), or minority class (a large minority of multi-specialized intellectual/ skill specialists).
This set of assumptions is consistent with my impression that the societies which have the most cohesive and obvious intellectual elites are mid-range IQ societies - despite that these intellectual elites are less intellectual than the intelligentsia of high IQ societies.
Indeed the intelligentsia of high IQ societies (those c. ten percent of the population with an IQ greater than 120) are a different thing altogether from the intellectual elites of the mid-range societies which constitute much of the current and recent world populations; and quite possibly are not a sustainable leadership class.
It is possible that things work better over the long term when there are only a handful percent of >120 IQ intelligentsia - not enough to form an elite, but enough to provide specialist expertise and advice, operating as one-off gifted individuals not as an interest group.
The 'smart fraction' theory suggests that there is a threshold IQ around 108
The argument here and in the above paper is different' but 108 is anyway so close to 105 that the two values cannot reliably be distinguished in individuals and small groups.)