 Recent work has produced new challenges about how to account for the removal of barriers (McMahan 1993, Rickless 2011, Hanser 1999, Hall 2008), past behaviour (Persson 2013, Hanna 2014, 2015) and the significance of empirical research (Kahneman and Tversky 1983, Horowitz 1998, Sinnott-Armstrong 2008, Cushman et al 2008, Barry et al 2014). 