behavioural economics: there are positive feedback loops (things get better =&gt; people&#x2F;companies earn more money =&gt; they can borrow more =&gt; they spend more =&gt; things get better) in both directions (things get worse =&gt; people&#x2F;companies earn less =&gt; they can borrow less or go bankrupt =&gt; they spend less =&gt; thing get worse), so even if the underlying events (the external input into the system) are independent and uniformly distributed, the characteristics of the system produce cyclical effects.