Polybius, History, and Human Nature
For Polybius, as for Thucydides, human nature was the key causative factor in history, apart from natural disasters. Several modern Polybius scholars have attached great importance to the role of tyche in Polybius, arguing that it represents an external, all-guiding force that controlled the rise and decline of states and guided all events towards the rise of Rome. However, I do not agree with this reading. What Polybius says in his preface is that the rise of Rome was so impressive and extensive that it 'seemed like' fortune was guiding all events to that end. Elsewhere, it rarely means more than the unexpected or the genuinely inexplicable. I think another part of the problem that has awarded too much emphasis to tyche is the excerpts that were made of Polybius by Byzantine scholars. These were thematic and one volume was on tyche. This has, therefore, meant that plenty of passages on tyche have survived and perhaps distorted the impression of its role in his work.
So, to Polybius and human nature. In his sixth book, Polybius gives an account of the cycle of politeiai (the different types of political community) and how each is likely to change from its good to its bad or degenerate type. He chooses that model for a particular reason - to illustrate the fragility of the good 'simple' types. Polybius divides the simple types into three good, and three bad:
GOOD: kingship, aristocracy, democracy
BAD: tyranny, oligarchy, ochlocracy (mob rule)
In the cycle (the Greek word is anacyclosis) kingship declines into tyranny, aristocracy into oligarchy, democracy into ochlocracy (mob rule). At each stage human nature is key. Its manifestation looks a little different at each stage, but similar traits lie behind each.