The investment theory of Cattell supposes an influence of fluid on crystallized intelligence. The development of fluid intelligence largely depends on biological factors, of crystallized intelligence on fluid intelligence and environmental stimulation. To test this theory two contrasting samples representing a broad ability range were chosen, a Brazilian sample (ages 7 to 15, N = 833, mean IQ 92) and a sample with a higher ability level in Germany (ages 11 to 19, N = 722, IQ 118). Analyses of cross-lagged effects across two year intervals show similar effects of fluid intelligence on crystallized and vice versa (around beta = 0.17). Parental socioeconomic status and parental education have in both samples a slightly stronger effect on crystallized than on fluid intelligence. The first result refutes Cattell's theory, the second gives some support. The development of fluid intelligence also seems to be influenced by non-biological environmental factors resulting in a concept of intelligence as a malleable ability. (Contains 6 figures.)