All languages employ some morphosyntactic means of distinguishing the core noun phrase (NP) arguments within a clause. The two basic predicate types are intransitive and transitive verbs, giving three core grammatical functions: S indicates intransitive subjects (The girl slept); A, “agent” of a transitive verb (The girl saw a pig); and P, “patient” (The girl saw a pig). Some languages (e.g., Chinese, English) distinguish A and P using word order: thus, we know which mammal saw which, because A always precedes the verb and P follows.
However, many languages employ case-marking to distinguish A and P, as in Latin:
- 1a.
Puella venit.
girl.(nom) come.pres.3sg
“The girl comes.”
- 1b.
Puella puer-um audit.
girl.(nom) boy-acc hears.pres.3sg
“The girl hears the boy.”
- 1c.
Puella-m puer audit.
girl-acc boy.(nom) hear.pres.3sg
“The boy hears the girl.”
Since S (intransitive subject) never co-occurs in a clause with either A or P, it needs no unique marking. Conversely, A and P always co-occur, and therefore must be marked differently to avoid confusion. Assuming a resolution of the tension between speaker effort (only produce essential morphemes) and listener comprehension (keep potentially ambiguous forms distinct), there are two major solutions. To distinguish A and P morphologically, it is most economical to either group S and A together, using the same case-marking for both, or else group S and P together, again using the same case for both.
These groupings, maximizing economy and comprehensibility, are exactly what we find: only two major morphosyntactic systems occur in the world's languages. The “accusative” system groups all subjects together (nominative), as opposed to all objects (accusative), as in Latin, Turkish, and Japanese, giving an [SA][P] pattern. Conversely, the “ergative” system groups intransitive subjects and objects together (absolutive case), as opposed to transitive subjects (ergative), giving an [SP][A] pattern. Here is an illustration of this from the Australian language Yalarnnga (taken from Blake Reference Blake1977):
- 2a.
ngia wakamu
me.(abs) fell
“I fell.”
- 2b.
nga-tu kupi walamu
me-erg fish.(abs) killed
“I killed a fish.”
- 2c.
kupi-ngku ngia tacamu
fish-erg me.(abs) bit
“A fish bit me.”
Strikingly, languages rarely mark each of the three core functions; clearly, only one member of the opposition needs overt marking. We therefore usually find a morphologically unmarked case: in the accusative system, the SA grouping (i.e., nominative), and in the ergative system, the SP grouping (i.e., absolutive). Thus, in the Latin examples, only accusative P has an overt suffix, while nominative SA is unmarked; and in Yalarnnga, only ergative A has a case suffix, while absolutive SP is unmarked (the parentheses in the [1] and [2] examples shown earlier indicate this null case morphology on SA and SP, respectively). In both systems, the unambiguous argument S is typically unmarked, again maximizing economy and clarity.
Both the accusative and the ergative systems are widespread, among languages which are really and genealogically diverse. Clearly, humans are not genetically adapted for one or the other system; moreover, since these are the major, but not the only systems that occur cross-linguistically, it would be incoherent to suggest that they are parametrized. It seems reasonable to conclude, then, that languages have generally adapted to maximize learnability and economy by utilizing the major systems.
Logically, other possible alignments of A, S, and P exist. For instance, [AP][S] marks A and P in the same way, but S differently; this, however, would set up exactly the confusion between A and P, which the major attested systems neatly avoid. This system occurs in a restricted set of pronominals in some Iranian languages; however, Comrie (Reference Comrie1989, p. 118) notes that it is not stable, instead representing the change from an earlier ergative system to an accusative system. Such marking is unattested for core NPs. Since it does not occur, it is most likely unlearnable – hardly surprising, since it is dysfunctional.
Three broad possibilities remain. First, a tripartite system consistently uses a distinct form to mark each of A, S, and P. This lacks the economy of the two major attested systems, and is vanishingly rare. One or two Australian languages are reported as having a tripartite system for all NPs: Warrungu (Comrie Reference Comrie, Haspelmath, Dryer, Gil and Comrie2005) and Wangkumara (Breen Reference Breen and Dixon1976). Clearly, this system is learnable, but is strongly dispreferred by human learners; as predicted, then, languages have generally not adopted this system. Second, a neutral system would not differentiate between A, S, and P at all, either by position within the clause, case-marking, or head-marking (i.e., verbal morphology indicating the person/number of the core arguments). Although this occasionally occurs, in a very restricted manner, for pronominals (Comrie Reference Comrie, Haspelmath, Dryer, Gil and Comrie2005), it is again unattested as a system for marking core NPs, and is thus, we can speculate, unlearnable. The third possibility is the split-S, or active system, which case-marks S (intransitive subjects) differently according to whether they are semantically agents or patients. This case system does occur, but is cross-linguistically rare (Blake Reference Blake2001, p. 124), arguably, again, because it lacks the economy of the two major systems.
Mixed systems, however, occur frequently. Both major systems exhibit differential case-marking (see Jäger Reference Jäger2007), meaning that NPs receive different – or zero – marking according to their position on a hierarchy of animacy and/or definiteness, sketched in Example (3) below (Blake Reference Blake2001, p. 137):
- 3.
1st person pronoun>2nd person pronoun>3rd person pronoun>proper noun>full NP
Accusative languages typically mark P overtly only towards the top of the hierarchy; English has case distinctions for a subset of pronouns (I/me, etc.), but none for full NPs. In fact, accusative languages nearly always have differential object-marking (Blake Reference Blake2001, p. 119). P arguments lower on the hierarchy are zero-marked, while higher ones are overtly accusative. Conversely, ergative systems work upwards, typically confining overt ergative marking to full NPs, or a subset thereof: Blake (Reference Blake2001, p. 192) notes that the Australian language Mangarayi marks ergative only on inanimate nouns, lowest on the hierarchy. In both systems, restricting overt marking to a subset of arguments achieves greater economies. Interestingly, most ergative languages are actually “split ergative,” often marking NPs high on the hierarchy (pronouns) by the accusative system, but lower NPs as ergative. This alignment may appear difficult to learn, but Jäger (Reference Jäger2007) demonstrates, using evolutionary game theory, that split ergative is actually highly efficient and stable, and fully functionally motivated.
Finally, case-marking and head-marking often co-occur within a language as grammatical function-signalling strategies for core NPs. Crucially, though, these strategies typically conspire, ensuring that no function is marked twice: a case/agreement hierarchy has subjects at the top (generally signalled by verb agreement) and indirect objects and other non-core NPs at the bottom (often signalled by case). This is another highly efficient system, and again illustrates the way languages apparently evolve to be learnable.
