Jensen (1969) proposed two level-theories of mental abilities, which sparked a debate on the association of heredity versus environment with intelligence after his studies, got published in the "Harvard Educational Review" in 1969. He demonstrated clear-cut genetic differences in the average intelligence of races and social classes. Jensen suggested that two genetically based levels of intelligence exist.
In other word intelligence consists of two levels of abilities. The Level I ability is associative learning, which consists of Short-Term Memory (STM), rote learning, attention and simple associative skills. The Level II is called cognitive learning, which consists of abstract thinking, symbolic thought, conceptual learning, and use of language in problem solving.
He argued that Level I ability, i.e. associative learning is equally distributed across all racial and national groups but on the contrary, Level II, i.e., cognitive learning is concentrated more the middle class Anglo-American populations than in the lower class black populations.
According to him, genetic differences in intelligence exist among people coming from different races, nationalities, and social classes.
Jensen conducted studies of intelligence comparing identical fraternal twins. Identical twins have identical genetic dispositions for that their IQs are assumed to be similar.
Fraternal twins like ordinary siblings genetically less similar, which results in the assumption that their IQs less similar. Jensen found support for his views in these findings. Studies with identical twins produced an average correlation of .86, whereas studies of ordinary siblings produced an average correlation of .47.
Jensen on basis of his studies believed that genetic factors are more important than environmental factors for one's intelligence.