 The trajectory from simplicity to complexity appears linear.  But Wright and Diamond argue that appearances are misleading.  Looking closely at human history, we see many instances of collapse. Understanding the cause of circumscribed collapses in the past will foster healthy skepticism about where the human race as a whole is headed.  Wright looks at failed societies such as the kingdom of Ur, Ancient Maya and Rome, and Easter Island.  Diamond's second book, Collapse (2005), considers some of the same failures. Failures occur when societies enter a progress trap (Wright 2004, 5). A progress trap is a practice that drives environmentally unsustainable growth and is difficult to halt once it is in motion. Although this definition of the term ‘progress trap’ is due to Wright, Diamond studies essentially the same phenomenon and traces the process in greater detail than Wright does.