By 1913–20 the term "dementia praecox" was introduced to suggest the development of senile-type dementia at a younger age. Eventually the two terms fused, so that until 1952 physicians used the terms "dementia praecox" (precocious dementia) and "schizophrenia" interchangeably. Since then, science has determined that dementia and schizophrenia are two different disorders, though they share some similarities. The term "precocious dementia" for a mental illness suggested that a type of mental illness like schizophrenia (including paranoia and decreased cognitive capacity) could be expected to arrive normally in all persons with greater age (see paraphrenia). After about 1920, the beginning use of "dementia" for what is now understood as schizophrenia and senile dementia helped limit the word's meaning to "permanent, irreversible mental deterioration". This began the change to the later use of the term. In recent studies, researchers have seen a connection between those diagnosed with schizophrenia and patients who are diagnosed with dementia, finding a positive correlation between the two diseases.