The more complex a society the greater the need for high IQ. As the number of humans living in social proximity increases more sophisticated social structures are required. A settled way of life amplifies this need further. The variety of occupations increases and, most importantly, the amount of stored knowledge becomes both larger and, once writing is available, more stable. Social organisation becomes looser and informal social support lessens. In place of a single world view competing ideologies vie for supremacy. Change and innovation become much more probable. There is so much more to potentially think about and learn, although any individual may actually have to know less than the hunter-gatherer to survive because of division of labour.
The individual in such a society is required to both learn more complex and less immediately obviously practical skills and knowledge and to deal with a greater range of human personalities and ideas. A man’s life contains less physical activity. As he works with his brain rather than his hands, his focus of attention changes. Knowledge becomes obsolete through innovation and consequently the need to learn throughout life increases. There is less certainty and fewer simple cultural mooring posts. The individual has to make more intellectually demanding decisions.
To live in a more complex society requires a qualitative change in mental abilities. There is an ever increasing shift from learning that which is concrete to that which is abstract, both in terms of understanding the whys and hows of the natural world at a level beyond mere surface observation, for example, the extraction of metal from ores, and in contemplating the organisational problems posed by larger associations of human beings. Much of what is to be learnt has no connection with the natural world and consequently no innate interest for Man who has to persuade himself intellectually that such things should be learnt because they lead to useful outcomes.
The existence of writing enhances such behaviours but it does more than that. The storing of information in a stable form means that information can be disseminated more widely and more certainly. Oral traditions inevitably result in variation. So of course do written records but they are far less prone to change, especially where moveable type printing exists. Moreover, a written record is a permanent statement of what was thought or claimed at one time. It can be compared with later written or oral accounts of the same subject in a way that a society with a purely oral tradition can never compare past and present accounts. In addition, written documentation allows not only a vast increase in what can be handed down from generation to generation but also much more complex information. It also greatly extends the time over which information may be transmitted. According to Plato, Socrates lamented the use of written records because he believed they stifled the intellect, but what would we know of Socrates today if no written records had been made of his thought? The answer is nothing.
As societies become more complex the way in which people learn changes. Instead of invariably learning by personal instruction and example, human beings often have to learn without direct human assistance, for example by reading, or by listening to the spoken words of others without any practical demonstration. This is because in modern industrialised societies the number of people who really understand the technology which is in general use is seriously inadequate. This means that people are routinely expected to use technology without a proper understanding of it because there is no one to instruct them in its use.