The Census publishes a module on disability reaching back to 1850. During the 1800’s, the categories of disability included deaf and dumb, idiotic, and insane–labels that have since given way to contemporary categories such as cognitive difficulty and independent living difficulty. According to the Census description, insanity means:
The term Insanity applies to individuals who once possessed mental faculties which have become impaired,” not to persons suffering a mental disability from birth.
https://usa.ipums.org/usa-action/variables/INSANE#description_section
People suffering from a mental disability since birth were categorized as idiotic:
The 1860 instructions to enumerators [Census administrators] reserved the term “idiocy” for “persons who have never possessed vigorous mental facilities, but from their birth have manifested aberration.” People who once had mental abilities but who had lost them were considered INSANE rather than IDIOTIC.
https://usa.ipums.org/usa-action/variables/IDIOTIC#description_section
Deaf and dumb is another category that has since given way to labels such as persons with hearing difficulty. Deaf and dumb, in the 1800s, were described as:
DEAF identifies persons who were both “deaf and dumb,” the contemporary terminology for persons with severe hearing or speech disabilities.
https://usa.ipums.org/usa-action/variables/DEAF#description_section
Looking only at the decades between 1850 and 1880, it appears that the percent of insane people increased markedly, while the percent of idiots and the deaf and dumb increased more moderately. Figure 1, below, shows the percentage of the dumb, idiotic, and insane between 1850-1880. The percentage of respondents that reported being idiotic increased by .020% between 1850 and 1880. The percentage of the dumb and deaf increased by .025%–in the same neighborhood as the idiotic. However, the percentage of the insane grew by .106%–about five times the increase of the idiotic and the deaf and dumb.
What is responsible for the climb in the percentage of population who are insane? It is unclear. One suggestion is that diagnoses improved. Another idea is that more people were willing to report having mental health issues. Nevertheless, cognitive disabilities were on the rise even in the 1800s.