As a non-native speaker, I wonder what the rules are for preferring one of "mandatory", "obligatory" or "compulsory" over the others.
The Corpus of Contemporary American English yields examples such as these:
Mandatory: 4865 cases
- mandatory safety nets around trampolines
- mandatory health care reform
- mandatory sentence for first-degree murder
- Unemployment sent me a letter, said this was a mandatory class.
- To stave off matting, daily brushing with a paddle brush is mandatory.
- He believes he can fill the role of rangy wing scorer that seems almost mandatory for championship teams.
- "Self-deprecation is mandatory here", he said.
Compulsory: 1060 cases
- compulsory health insurance
- states that prohibit compulsory unionism
- compulsory celibacy for the clergy
- added to the school curriculum as a compulsory second language
- U.S. imperialism can be thought of as a system of compulsory heterosexuality
- My landlord and mother made my weekly visits to the center compulsory if I was to continue running my business rent free from her basement.
Obligatory: 808 cases
- the obligatory happy ending
- when the obligatory compliments were out of the way
- I would hang up on my father after the obligatory five minutes
- the laptop computer has become nearly obligatory on campus
"Mandatory" is presumably related to "mandate" and used when it's a matter of law or regulation.
"Obligatory" is presumably related to "obligation" and used when it's a matter of moral, social or other obligation rather than law. (Like "the obligatory happy ending".)
"Compulsory" seems to mean that you are forced to do something, but the forcing mechanism is unspecified?
Is this approximately correct, and are there other rules of thumb for when to use which term?
There seems to be some cases where the terms are interchangeable: "Mandatory health insurance" and "compulsory health insurance" would be equally acceptable, wouldn't it?
Would "school curriculum compulsory second language" read the same if we substitute "mandatory" or "obligatory"?