Compulsive describes behaviour driven by an urge someone struggles to control, as in a compulsive liar. Compulsory describes something required by a rule or authority, as in wearing a seatbelt is compulsory. If a person cannot help doing it, it is compulsive; if a rule forces it, it is compulsory.
The core difference
Compulsive describes an inner urge that is hard to resist, while compulsory describes an outer requirement imposed by rule or law.
- compulsive — resulting from an urge that is difficult to resist: his compulsive checking of email became a real problem at work.
- compulsory — required by a rule, law, or authority: wearing a seatbelt is compulsory in most countries.
How to tell them apart
Both trace back to the same Latin root, compellere, meaning to compel, which is exactly why they get tangled. The difference is the source of the pressure: compulsive pressure comes from within a person, while compulsory pressure comes from an outside rule.
The mix-up often appears in workplace or school writing, where a required course is meant to be described as compulsory but gets written as compulsive, which wrongly suggests an uncontrollable urge rather than a rule.
