Based on investment theories and guided by Mussel's (2013) intellect model, the present study investigated reciprocal relations over 1 year (2021-2022) between investment traits (need for cognition, achievement motives, epistemic curiosity) and fluid and crystallized cognitive abilities in 565 German elementary school children (298 girls; M[subscript age] = 8.40, SD = 0.59; 59.5% with immigration background). Children's fluid and crystallized abilities increased over time, whereas fear of failure and curiosity decreased. Investment traits barely predicted change in cognitive abilities. However, mathematical ability predicted change in most investment traits (0.14 [less than or equal to] |[beta]| [less than or equal to] 0.20), even after accounting for control variables. Results largely contradict investment theories but support the role of crystallized abilities for the development of investment traits in elementary school age.