They also note that the majority of smells (as defined by their static analysis suite) are introduced when adding new features or enhancing existing ones, which

They also note that the majority of smells (as defined by their static analysis suite) are introduced when adding new features or enhancing existing ones, which is a bit different from the usual "maintenance creates smells" adage. Less than 20% of what they defined as smells were introduced by bug fixes, something I think of more when I think of "maintenance." I'm still a bit skeptical of the methods, but an interesting read.