“…Nonetheless, concrete-operational thinking represents an important prerequisite of formal thinking (Inhelder and Piaget, 1958;Powell and Kalina, 2009). It is associated with a range of learning outcomes and academic achievement (Jordan and Brownlee, 1981;Hattie, 2009), such as maths fluency and maths achievement (conservation ability; Arlin, 1981;Cooper and Schleser, 2006;Ramos-Christian et al, 2008;Krajewski and Schneider, 2009;Wubbena, 2013;Lambert and Spinath, 2018), as well as reading comprehension and reading achievement (conservation and classification ability; Arlin, 1981;Cartwright, 2002;Colé et al, 2014;Cartwright et al, 2017). At the same time, differences in the ability of concrete-operational thinking become clear, as e.g., students with learning disabilities show lower levels of concrete-operational thinking than their peers (Wember, 1986;Riley, 1989;Fakouri, 1991).…”