Like Civilisation, the model breaks the world into squares – or in this case, the Eurasian continent – and characterises them according to the type of land present, how mountainous it was, and if it was farmed. Every farmed square was inhabited by an independent group of humans, which could either be organised or not. Military technologies were seeded in squares close to the Eurasian steppe – where horses were first harnessed by humans – and gradually diffused outwards as the simulation ran.

The researchers ran the model between the years 1500 BC, when organised groups were few and far between, and AD 1500. When compared with historical records, it predicted where and when large empires would form and persist with 65 per cent accuracy. When they removed the influence of military technologies from the picture, this fell to 16 per cent.

Removing the effects of elevation from the model only reduced the accuracy to 48 per cent, suggesting that warfare played a more important role than geography in determining which groups stabilised into societies. Turchin says warfare exerted a selection pressure on early groups of people, forcing them to work together to form a society – or be wiped from the map.