It seems there are several factors that contribute to the cycles. The common theme throughout is human behaviour. For the shorter business cycles, the US Presidential cycle(4 years) is one of the influencing factors, since it impacts US fiscal and monetary policy which effects US consumption and therefore world GDP expansion and contraction. The longer secular cycles seem born of the basic tendency of each generation to grow complacent during times of falling inflation and falling interest rates and then learn the hard way that things are not always so favourable. Once we learn hard lessons, then we tend to be more careful and humble again for a few years until excess takes over again. My grandparents were farmers and lived through the depression. They wanted education and white collar jobs for their children and grandchildren and now their grandchildren and great-grandchildren are trying to remember what they told us about farming, sustainable practices and living without credit. Full circle