Spurred on by the discovery of Google’s ngram utility whilst reading this post by Doc Searls, I tried a query using the words disabled and handicapped.
I am struck by a couple questions”
1/ What happened around 1920 when disability became overtaken by handicapped. Especially since handicapped was only starting to appear in 1880s, prior to which disability was prevalent.
2/ Was it the passing of the ADA in 1980 and the International Year of Disabled Persons in 1981 that sharply reversed the use of “handicapped”?
An observation. Handicapped, the adjective, seemed to have kept increasing while the noun handicap drops away.
life tools 
It’s interesting to compare the terms “Disabled” and “Disability” too. The term “disabled” makes a conclusive statement about a persons’s abilities, while the term “disability” implies the person has some additional obstacles. Just because someone has a disability doesn’t mean they are not able to achieve, it just means they might struggle more than someone who has no disability. Labelling someone with a disability as being “disabled” makes a negative and baseless conclusion about their abilities without any assessment.
The use of “Handicapped” was earlier discouraged as it is derogatory because it alludes to begging, being derived from “cap-in-hand”.