There are studies on the effectiveness of unit tests (and other layers of tests) from the 1980s. Unit tests catch about 20% of bugs. Integration tests catch 40%

There are studies on the effectiveness of unit tests (and other layers of tests) from the 1980s. Unit tests catch about 20% of bugs. Integration tests catch 40% of bugs, but the overlap between the two is small, so you still benefit from having both. Which one came first is irrelevant, the question is if they help us write better software.