Which researchers argued that fluid and crystallized intelligence are the critical aspects of intelligence?

The researchers who argued that fluid and crystallized intelligence are the critical aspects of intelligence are Cattell and Horn. Fluid intelligence refers to the capacity to reason and solve new problems independently of previously acquired knowledge, which tends to decline with age. In contrast, crystallized intelligence involves the ability to use learned knowledge and experience; it generally remains stable or even increases as individuals age. Cattell and Horn's model has greatly contributed to our understanding of the structure of intelligence, distinguishing between these two types of cognitive abilities and emphasizing their importance in psychological assessments and educational contexts. Their work highlights that intelligence is not a singular ability, but rather encompasses various forms that can evolve over the lifespan.