Marking hierarchies and language change

I am currently travelling, so this blogpost will only very briefly discuss the topic of my current research in grammar reconstruction: the role of marking hierarchies in language change.
The notion of marking hierarchies has it roots in the markedness theory by Roman Jakobsen and implies that grammatical categories (e.g., singular - plural) typically are in a mutual, hierarchical relation, where one of the categories are morphologically unmarked, whereas the other is morphologically marked. The unmarked category thus has a higher position within a hierarchy of grammatical properties (singular < plural). These grammatical relations are, according to some authors, general, or "universal", anchored in our in-born grammatical system. However, we know that this is a problematic notion: there are a substantial amount of languages where the actual morphological marking contradict the proposed markedness hierarchies. Further, not all languages have morphology. Morphological marking alone cannot be the identifyer of marking hierarchies.
On the other hand, there is an obvious connection between the observed marking hierarchies and frequency. Superior categories, "unmarked" in the traditional markedness theory, are more frequently used in speech and in text. Again, the definion may be problematic, since not all languages have corpora that enable a detailed study of category frequency. Also, marking hierarchies based on frequency may contradict marking hierachies based on general morphological marking observations.
My current study on grammar reconstruction, which I have been writing about in several blogposts, indicate a clear correlation between change rates and marking hierarchies: superior categories, which are more frequent in grammar and most likely to be unmarked grammatically, have substantially lower change rates (and slower pace of change) than inferior categories, which have higher change rates (and faster pace of change). I will continue and follow up this topic in a coming blogpost. 

References
Bickel, Balthasar (2008), 'On the scope of the referential hierarchy in the typology of grammatical relations', in G. Corbett Greville and Michael Noonan (eds.), Case and Grammatical Relations. Studies in honor of Bernard Comrie (Amsterdam - Philadelphia: John Benjamins), 191-210.
Croft, William (2003), Typology and universals (Cambridge textbooks in linguistics, 99-0104661-0; Cambridge: Cambridge Univ. Press).
Comrie, Bernard (1981), Language universals and linguistic typology : syntax and morphology (Oxford: Blackwell).
Dixon, Robert M V (1994), Ergativity [Elektronisk resurs] (Cambridge: Cambridge University Press).
--- (1997), The Rise and Fall of Languages [Elektronisk resurs].
--- (2010a), Basic linguistic theory. Vol. 2, Grammatical topics (Oxford: Oxford University Press).
--- (2010b), Basic linguistic theory [Elektronisk resurs]. Vol. 2, Grammatical topics (Oxford: Oxford University Press).