This study examines whether beliefs about the malleability of intelligence manifest in growth mindset behaviour and improved math achievement among Finnish and Estonian 4th graders. The sample consists of 368 students, 184 from both countries. Results show that the two mindset-instruments being compared--one capturing the generalised implicit beliefs about the malleability of intelligence and the other, more specific mindset-related behaviours--do not assess the same latent phenomenon. In both countries, the general idea of malleability of intelligence seems to have spread among the students. However, mindset profiles show that most students in both countries demonstrate a "mixed mindset" in their behavioural preferences, indicating that widespread notions about the malleability of intelligence do not necessarily manifest in growth mindset behaviour, therefore limiting realisation of students' true potential. In line with theory, students reporting an "authentic growth mindset," manifesting both in their words as well as behavioural preferences, demonstrated better academic achievement in math. The differences are discussed in the context of growth mindset pedagogy.