This post is being written hastily and therefore may be more incomprehensible than usual.
Greenberg’s Universal 38 says:
Where there is a case system, the only case which ever has only zero allomorphs is the one which includes among its meanings that of the subject of the intransitive verb. (Greenberg, 1963, 59)
A slightly better way to put this (more clearly clarifying what the universal says about languages that code case by means of non-concatenative morphology) might be: if a language makes a distinction between nominative/absolutive and ergative/accusative case by means of concatenative morphology, then there is always at least one ergative/accusative suffix form with nonzero phonological substance. Roughly, there’s a preference for there to be an ergative/accusative affix rather than a nominative/absolutive affix (but it’s OK if there are phonologically substantive affixes for both cases, or if ergative/accusative is zero-coded in some but not all environments).
On the other hand, Greenberg’s statement of the universal makes clear a rather interesting property of it: if you’re thinking about which argument can be zero-coded in a transitive sentence, Universal 38 actually says that it depends on what happens in an intransitive sentence: the one which can be zero-coded is the one which takes the same case that arguments in intransitive sentences take. If the language is accusative, then the nominative, the agenty argument, can be zero-coded, and the accusative, the patienty argument, can’t. If the language is ergative, then the absolutive, the patienty argument, can be zero-coded, and the ergative argument, can’t. (I mean can’t as in can’t be zero-coded in all environments.)
This is a problem, perhaps, for those who think of overt coding preferences and other phenomena related to “markedness” (see Haspelmath, 2006, for a good discussion of the meaning of markedness in linguistics) as related to the semantics of the category values in question. Agenty vs. patienty is the semantic classification of the arguments, but depending on the morphosyntactic alignment of the language, it can be either the agenty or patienty arguments which are allowed to be zero-coded. This seems like a case where Haspelmath’s preferred explanation of all phenomena related to markedness—differences in usage frequency—is much more preferable, although I don’t think he mentions it in his paper (but I might have missed it—I’m not going to check, because I’m trying to not spend too long writing this post).
Anyway, one thing I wonder about this universal (and a thing it’s generally interesting to wonder about with respect to any universal) is how it’s diachronically preserved. For it’s quite easy to imagine ways in which a language could end up in a situation where it has a zero-coded nominative/absolutive due to natural changes. Let’s say it has both cases overtly coded to start with; let’s say the nominative suffix is -ak and the accusative suffix is -an. Now final -n gets lost, with compensatory nasalization, and then vowels in absolute word-final position get elided. (That’s a perfectly natural sequence of sound changes; it happened in the history of English, cf. Proto-Indo-European *yugóm > Proto-Germanic *juką > English yoke.) The language would then end up with nominative -ak and a zero-coded accusative, thus violating Universal 38. So… well, I don’t actually know how absolute Universal 38 is, perhaps it has some exceptions (though I don’t know of any), and if there are enough exceptions we might be able to just say that it’s these kinds of developments that are responsible for the exceptions. But if the exceptions are very few, then there’s probably some way in which languages which end up with zero-coded accusatives like this are hastily “corrected” to keep them in line with the universal. Otherwise we’d expect to see more exceptions. Here’s one interesting question: how would that correction happen? It could just be that a postposition gets re-accreted or something and the accusative ends up being overtly coded once again. But it could also be that subjects of intransitive sentences start not getting the -ak suffix added to them, so that you get a shift from accusative to ergative morphosyntactic alignment, with the zero-coded accusative becoming a perfectly Universal 38-condoned zero-coded absolutive. That’d be pretty cool: a shift in morphosyntactic alignment triggered simply by a coincidence of sound change. Is any such development attested? Somebody should have it happen in a conlang family.
According to Wichmann (2009), morphosyntactic alignment is a “stable” feature which might be a problem if alignment shifts can occur in the manner described above. But then again, I wonder how common overt coding of both nominative/absolutive and ergative/accusative is, actually—most Indo-European languages that mark cases have it, but I did a quick survey of some non-IE languages with case marking, both accusative (Finnish, Hungarian, Turkish, Tamil, Quechua) and ergative (Basque, Dyirbal) and they all seem to code nominative/absolutive by zero (well, Basque codes absolutive overtly in one of its declensions, but not in the other two). If it’s pretty rare for both to be overtly coded, then this correction doesn’t have to happen very often, but it would surely need to happen sometimes if Universal 38 is absolute or close to it.
Greenberg, J. H., 1963. Some universals of grammar with particular reference to the order of meaningful elements. In Greenberg, J. H. (ed.), Universals of Language, 73–113. MIT Press.
Haspelmath, M. (2006). Against markedness (and what to replace it with). Journal of linguistics, 42(01), 25–70.
Wichmann, S. & Holman, E. W. (2009). Temporal stability of linguistic typological features. Retrieved from https://www.academia.edu/13245711/Temporal_Stability_of_Linguistic_Typological_Features.