“…Such flexibility in control mode engagement is observed from age 8 on, but not in younger children, who appear to rely on reactive control even in situations where proactive control would seem to be more efficient (Blackwell & Munakata, 2013; Chatham, Frank, & Munakata, 2009; Chevalier et al, 2014; Vallesi & Shallice, 2007). For instance, Chatham et al (2009) used pupil dilation, a well-established index of cognitive effort (e.g., Beatty, 1982), to track early and late mental effort engagement associated with proactive and reactive control, respectively.…”