 What is important to note about the research guided by these theoretical tenets above is that all behavior is best explained in terms of underlying psychological mechanisms that are adaptations for solving a particular set of problems that humans faced at one time in our ancestry. Also, evolutionary psychologists stress that the mechanisms they focus on are universally distributed in humans and are not susceptible to much, if any, variation. They maintain that the mechanisms are a product of adaptation but are no longer under selection (Tooby and Cosmides 2005, 39–40). Clark Barrett’s (2015) accessible and wide ranging introduction to evolutionary psychology sustains this emphasis on evolved mechanisms as the main focus of evolutionary psychology research. Barrett also expands the scope of evolutionary psychology and notes the addition of research methods developed since Cosmides and Tooby first set out the parameters for research in the field. Some of Barrett’s proposals are discussed in sections 6 and 7 below. Todd Shackleford and Viviana Weekes-Shackleford (2017) have just completed a huge compendium of work in the evolutionarily based psychological sciences. In this volume a vast array of different research methods are presented and defended and there are a number of entries comparing the merits of alternative approaches to evolutionary psychology.