³³See Killingsworth and Gilbert (2010) for the claim that “A wandering mind is an unhappy mind”. The problem is, however, that such studies don’t usually show that it is mind-wandering that makes people unhappy. It is also possible that the causal effect is the opposite: when we are unhappy, thoughts start wandering more (Smallwood et al., 2009). This could be because negative mood is related to unresolved goals or personal problems, which are then processed during mind-wandering. If you’re sad, it may be because you are experiencing problems, and those problems need extra processing by mind wandering (Poerio et al., 2013), in line with footnote 16 above.