Word order is a linguistic property that may be difficult to establish for sign languages. This is due to the possibilities of simultaneous constructions in the visual–gestural modality, in which multiple articulators (e.g., the two hands) may be producing different signs at the same time (Vermeerbergen, Leeson & Crasborn 2007), which has an effect on temporal sequencing:

This is often discussed as one caveat for word order research on sign languages (Johnston et al. 2007; Leeson & Saeed 2012), which is complicated further by other types of complex structures (Emmorey 2003) – see also Section 2.2 below. Nonetheless, word order is one of the more well-studied parts of sign language structure. In a survey of 42 sign languages, Napoli & Sutton-Spence (2014) find that SVO and SOV are the most common word orders across their sample, a finding which reflects the most common word orders among spoken languages (Dryer 2013). However, Napoli & Sutton-Spence (2014) also conclude that there are a number of generalizations that can be made based on their sample of languages. For example, they find that SOV seems to be grammatical in all languages, at least for some constructions. Furthermore, they observe that SVO is often preferred in reversible constructions – that is, when the A and P argument referents are equivalent (e.g., if both are [+human]) and could switch roles without rendering the sentence semantically awkward. For instance, in Example (2) a and b are reversible, whereas c and d are not.

Later work has also showed that word order in some sign languages is affected by properties such as an intensional/extensional distinction in verbs (Napoli, Sutton-Spence & Quadros 2017) or the complexity of verb modification (Napoli & Sutton-Spence 2014; Bjerva & Börstell 2016). However, it has been argued that animacy has an effect on the emergence of word order patterns. Meir et al. (2017) investigated word order patterns in three young sign languages. They found that arguments are introduced according to a “humans first” principle, which means that they even find OVS ordering with [+human] objects, an order that is extremely rare among the spoken languages of the world. In sentences with an inanimate object, SOV was preferred. The authors conclude that animacy – more specifically [+human] vs. [–animate] – is a key cognitive factor in the emergence of word order structure. The animacy effect on word order preferences has also been shown in a number of experimental studies using silent gesture tasks. Such tasks involve asking non-signers to invent ad hoc silent gestures (i.e. without simultaneous speech) to describe events. In these studies, certain word order patterns emerge that are generally not attested in the participants’ native language and the patterns are affected by the animacy of the arguments (e.g., Goldin-Meadow et al. 2008; Hall, Mayberry & Ferreira 2013; Hall et al. 2015).

Thus, there seem to be animacy effects on word order preferences across sign languages, motivated by disambiguation and [+human]-based prominence. Though not necessarily an instance of (differential) object marking in its own right, it shows that some of the very same motivations are at play in shaping sign language structure.

Since the early days of sign language linguistics, it has been known that verbs may be modified to “incorporate” features of arguments (e.g., Friedman 1976; Padden 1988).2 This concerns two main types of modification: directionality and classifiers.

Directionality has been argued by some to be an agreement device, since verbs are directed in space to indicate who is doing what to whom, whereas others see it as a reference tracking device of gestural origin (cf. Liddell 2011; Lillo-Martin & Meier 2011; Dotter 2018; Fenlon, Schembri & Cormier 2018; Pfau, Salzmann & Steinbach 2018). In short, a verb like GIVE may have a lexical citation form resembling a hand gripping an object and reaching out from the signer’s own body outwards. In Figure 2, we see a citation form of a Swedish Sign Language sign for ‘give’. The movement in the citation form depicts the signer giving something outwards and the handshape in this form would be appropriate for a small, flat object (e.g., a sheet of paper). However, when modified, the start and end points of the verb may be modified to mirror the locations of the argument referents, e.g., moving from the signer towards the side to denote ‘I give him/her’ (Figure 3), or moving between two locations away from the signer’s body to mean ‘s/he gives him/her’ (Figure 4).3

The handshape selected for a verb like GIVE would constitute the classifier, chosen to match the object of transfer (i.e. the Theme), sometimes argued to carry meaning by itself. For example, a verb like EAT may use a closed fist to denote eating a carrot or an ice-cream, depicting how these objects are gripped, whereas eating an apple will require a cupped handshape, as if holding an apple. In the illustrated example figures, the giving action could not involve, e.g., a large box, as the handshape does not conform to the correct depiction of a large box being handled. This is why the handshapes have sometimes been argued to constitute classifiers, as they are selected based on semantic (physical shape) properties of the involved referent (Wilcox 1998; Emmorey 2003).

Börstell (2017) suggests that these two strategies – directionality and classifier handshapes – show different alignment, since they generally target different types of objects. Directionality often targets transitive P and ditransitive R arguments, whereas classifier handshapes target transitive P or ditransitive T arguments. The underlying motivation for this is one of semantics and iconicity, and this interacts with animacy. With ditransitive constructions, the transfer is normally made between people, whereas the object of transfer is generally inanimate.

What is noteworthy about verb directionality – across many sign languages – is that it appears to be strongly preferred with [+animate] arguments and it is rather the object that is indicated (i.e. directed towards), with subject indication being less frequent (cf. Rathmann & Mathur 2002; Mathur & Rathmann 2012; Pfau, Salzmann & Steinbach 2018). That is, if a verb is modified with respect to only one argument, it tends to be the object, suggesting that directionality is more of an object marking (or tracking) device than a subject marking device. Based on corpus data from British Sign Language, it was shown that object indication is more common than subject indication and also that directionality that indicates the object (or, rather, patient) appears to be preferred with 1^st and 2^nd person rather than 3rd person (Cormier, Fenlon & Schembri 2015; Fenlon, Schembri & Cormier 2018; Schembri, Cormier & Fenlon 2018). Again, this points to prominence of arguments affecting the patterns of marking. For American Sign Language, it has been shown that 1^st person forms of certain directional verbs are irregular, hence being treated differently from any non-1^st person form (Hou & Meier 2018). Fenlon, Schembri & Cormier (2018) argue that the preference for 1^st person verb modification is directly grounded in the use of the body as representing one of the argument referents:

Thus, although there might be modality-effects in the motivations for directional modification, it is again apparent that it is a device that targets arguments (particularly objects) with certain features, and that these features overlap with those associated with DOM – in this case, animacy and person. Pfau, Salzmann & Steinbach (2018) note the differential marking patterns with directional modification in that it “is sensitive to the person, animacy and specificity value, e.g. is triggered only by local person, animate or specific arguments, […] and represents an instance of differential argument encoding”.

Directionality concerns flagging on verbs rather than the arguments, which, with regard to differential marking patterns, has been found among spoken languages too – that is, properties of objects may affect which marking the associated verb takes. This is related to DOM and sometimes referred to as differential object indexing (DOI) (Iemmolo 2011). Although DOM and DOI share similarities and are often discussed as part of a similar phenomenon, it has been argued that there are diachronic and functional differences between them (cf. Witzlack-Makarevich & Seržant 2018).

Verb directionality is found in many – if not most – sign languages of the world, but has not fully emerged in very young sign languages (Aronoff, Meir & Sandler 2005; Padden et al. 2010). However, some sign languages have developed dedicated markers for indicating argument roles, usually termed agreement auxiliaries (see Sapountzaki 2012). Such auxiliaries are independent elements used as directional verbs, that is, moved in space to indicate subject and object referents. They are often used when the verb does not allow for spatial modification due to phonological restrictions, normally because of having a body-anchored (i.e. contact with the signer’s own body) articulation.

In a survey of this type of auxiliaries, Sapountzaki (2012) finds that they are found in sign languages across the globe (see Figure 5), but show some cross-linguistic similarities in their grammaticalization path. The auxiliaries tend to come from three main types of sources: pronominal pointing signs, directional verbs (e.g., GIVE or GO-TO), or the sign PERSON. The former two have in common that they are already used with spatial modification, that is, they point in signing space. Interestingly, several sign languages have grammaticalized the noun PERSON into this type of auxiliary. Thus, even though the sign is not originally a pointing sign, it acquires the directional element and moves in signing space to indicate argument roles by moving from subject location to object location (like directional verbs) (Pfau & Steinbach 2013). It has been argued that this auxiliary is used preferably with animate objects and in ditransitive constructions it is modified according to the indirect rather than direct object (see Pfau, Salzmann & Steinbach 2018). Example (3) shows the use of the agreement auxiliary in German Sign Language, which has grammaticalized from the sign PERSON.

Sapountzaki (2012) notes that several of these auxiliaries are restricted to constructions with [+human] arguments. Thus, they are not only more frequent with human referents – as seems to be the case with verb directionality in general – but are, in fact, exclusively used with human referents. This clearly suggests that animacy shapes sign language structure and more specifically so with regard to argument marking/indexing, mirroring the general DOM/DOI pattern.

For Spanish Sign Language, which also uses a PERSON-derived auxiliary, it is only the object referent that is indicated (Costello 2016). It has been claimed for German Sign Language, too, that it is only the object that is obligatorily marked by the PERSON auxiliary, but in both languages, the auxiliary may coincide with overt objects, thus suggesting that they are not object pronouns (Pfau & Steinbach 2013; Costello 2016). However, a more recent analysis of this sign in German Sign Language suggests that it only marks the object and, by virtue of being used solely with animate objects, it has thus been argued to constitute a DOM element (Bross 2018).

The above-described auxiliaries can be seen as dedicated argument marking/indexing devices, as their sole purpose is to indicate the syntactic relations between argument and verb. However, it is not marking on the arguments themselves and, as described previously (e.g., Cormier 2012; Gil 2014), sign languages seem to lack argument flagging, even when pronominal. The one exception is Israeli Sign Language, which has a dedicated object pronoun. That is, a pronominal sign used exclusively with syntactic objects, unlike the unmarked index point pronoun (which can be used for any argument role). This pronoun has grammaticalized out of the sign PERSON and is restricted to negative value verbs or psych-verbs with [+human] objects in the singular (Meir 2003). However, in a survey of 28 sign languages, based on grammar descriptions, dictionaries, and consultation with language experts, Börstell (2017) found that there are several candidates for dedicated object markers in other languages, some of which are identical in form and similar in function to the object pronoun in Israeli Sign Language. For example, in most of the sign languages of the Nordic countries – Danish Sign Language, Finland-Swedish Sign Language, Finnish Sign Language, Norwegian Sign Language, and Swedish Sign Language – there is an object pronoun derived from the sign PERSON, just as in Israeli Sign Language. Figure 6 illustrates the form of PERSON in Swedish Sign Language, a form which is shared across all the sign languages that use PERSON as an object pronoun, including Israeli Sign Language. Though the sign is derived from PERSON, Börstell (2017) shows how the form is reduced when used as an object pronoun in Swedish Sign Language, supporting the idea of having gone through a grammaticalization process. In all sign languages with PERSON as an object pronoun, the sign, which in its citation form is articulated forward in neutral signing space, is oriented towards the location of the referent. This means that the sign is indeed used as any pointing sign, equal to a simple index point, but also sharing properties with the auxiliary forms in other sign languages. As a pointing sign, using it as a 1^st person object pronoun entails articulating on the signer’s own body (Figure 7).

In the six languages for which PERSON has grammaticalized into an object pronoun, its function is clear. The sign replaces a lexical NP, normally a known discourse referent, and always [+human], which points to its pronoun status as well as prominence being the important factor in its distribution. Though there are several indications that the pronoun function has indeed grammaticalized from PERSON (e.g., by being reduced), it is restricted to human referents, thus retaining some semantics of ‘person’ by denoting human referents. Apart from Israeli Sign Language, the sign languages with the object pronoun function of PERSON are all in Scandinavia, which may suggest a shared origin or language contact. There are unfortunately not yet any deeper analyses of the object pronoun in any of these Scandinavian sign languages apart from Swedish Sign Language (Börstell 2017), thus this specific case is described further in Section 2.4 below.

However, while the exact distribution and function of the sign in the different languages may differ, such as being used with a more or less diverse set of verbs or being restricted to singular reference (cf. Meir 2003; Börstell 2017), it does occur in the place of a lexical NP in the object position. In this respect, it behaves like a pronoun would and by being used exclusively with [+human] objects, it is an object pronoun that exhibits DOM distribution. In other cases, the unmarked index point would be used.4

It is noteworthy that the sign PERSON is so prevalent in this domain. In fact, Börstell (2017) notes that there are – at least – four categories of grammaticalization of PERSON (with the same form across languages) which all relate to argument marking, summarized in Table 1. These sign languages are mostly found in Europe (Figure 8), although this may reflect a bias in the sample, which is skewed towards sign languages that have been researched.

Apart from the object pronoun grammaticalization, the sign PERSON has in Lithuanian Sign Language and Russian Sign Language (possibly related languages) developed into a reflexive pronoun (Kimmelman 2009; LGKŽ 2012).

As previously described in this paper, PERSON is also the source for a so-called agreement auxiliary in some sign languages. Börstell (2017) lists four sign languages – Austrian Sign Language, Catalan Sign Language, German Sign Language, and Spanish Sign Language – in which PERSON has grammaticalized into an argument indicating auxiliary (Pfau & Steinbach 2013; Costello 2016; Krebs, Wilbur & Roehm 2017). Again, these signs tend to be restricted to [+human] or at least [+animate] (prominent) arguments.

Finally, in Georgian Sign Language, PERSON has developed into an ergative case marker. Like in the other languages, the sign is only ever used with [+human] referents, but it is never directed in space towards the location of its referent. Unlike the other languages, Georgian Sign Language uses a form of PERSON that is cliticized to an NP and marks subject rather than object arguments (Tamar Makharoblidze, p.c.). Thus, it could be seen as an instance of differential subject marking (DSM) rather than DOM.

Apart from the PERSON derivations, Börstell (2017) also mentions the existence of indirect object markers in some sign languages. Fragmentary dictionary data and consultation with language experts suggest that such markers are found in at least Israeli Sign Language and Russian Sign Language (Börstell 2017: 162), for which the markers seem to function mostly as indirect/benefactive pronouns, expressing ‘to/for someone’. In Georgian Sign Language, which has been influenced by Russian Sign Language, there are two indirect object markers (Makharoblidze 2015), one of which is similar in form to the one used in Russian Sign Language. Thus, object marking in some sign languages seems to target indirect objects, too. Since indirect objects often have animate referents, this is also relevant with regard to the question of DOM and differential argument marking (DAM) more generally.

The structure of Swedish Sign Language has been studied since the 1970’s, making it one of the sign languages with the longest traditions of linguistic research (cf. McBurney 2012). Nonetheless, the fact that the language uses a dedicated object pronoun had not generated any extensive research until Börstell (2017), which explains why only Israeli Sign Language had been used as a counterexample to the general lack of case-like marking in the pronominal paradigm. However, in the earliest dictionary of Swedish Sign Language (Österberg 1916), the author notes that there – apart from index pointing – exists a “pronoun”, which according to the photo illustration is identical to the 1^st person object form used today (see Figure 7). Unfortunately, no details are provided about the specific use of this sign at the time of publication of that early dictionary, but it is noteworthy that it was labeled a pronoun already a century ago.

Going deeper into the modern use of this sign in Swedish Sign Language, Börstell (2017) finds that the sign is used across signers in the Swedish Sign Language Corpus (Mesch et al. 2012), showing that it is not restricted to a certain register or lectal variation.5 Although it is used alongside the lexical noun PERSON, and a noun classifier with similar form, also derived from PERSON (cf. Bergman & Wallin 2001), the sign when used as a pronoun has a distinct form and distribution. For example, whereas the noun has a straight wrist when articulating the sign, the object pronoun often has a flicked wrist movement, which leads to a smaller, reduced articulation, pointing to grammaticalization. Furthermore, the mouthing – that is, the mouth articulation that may go along with a sign (Boyes Braem & Sutton-Spence 2001) – is most often formed by borrowing the mouthing pattern of the equivalent Swedish object pronoun, which includes a natural gender distinction (Börstell 2017: 138–140).

When it comes to function, the sign is used exclusively with [+human] objects (both P and R roles), like the corresponding sign in Israeli Sign Language.6 However, unlike the Israeli Sign Language pronoun, which restricts its object pronoun to specific verb domains, it is used with many different types of transitive verbs (see Example (4) and Figure 9).

Besides being used with verbs, the sign also appears frequently directly following a manual preposition, mirroring the use of object pronouns triggered by prepositions in, e.g., Swedish. Like most other sign languages, lexical NPs are not marked in any way to signal argument roles. Thus, the object marking is restricted to pronominal arguments.

Another difference compared to Israeli Sign Language, and possibly different from the other Scandinavian sign languages using the PERSON-derived object pronoun as well, is that the object pronoun in Swedish Sign Language can be used with plural referents by adding a horizontal movement, known as “plural sweep” (cf. Pfau & Steinbach 2006). However, the plural form is restricted to 1^st and 2^nd persons, again pointing in the direction of prominence as a key factor in the distribution of object marking. For 3^rd person plural, the index point with a plural sweep is used. Börstell (2017: 149–148) notes that, based on corpus data and native signer consultation, the index point may be used for the object function with [+human] referents. However, the reading of such constructions tends to be demonstrative, that is, referring to ‘that one’ rather than ‘him/her’ – compare Example (5) to Example (4) above.

Though there are certainly similarities between Swedish and Swedish Sign Language that may be an effect of contact, there are also clear differences, such as the restriction to [+human] referents and lack of a 3^rd person plural form in the object pronoun in Swedish Sign Language. In short, whereas Swedish has distinct object pronoun forms, the object pronoun in Swedish Sign Language exhibits a DOM distribution. The main points for arguing that this sign is a manifestation of DOM in Swedish Sign Language are:

Thus, Swedish Sign Language with its dedicated object pronoun should be the clearest example of the DOM phenomenon in any sign language, seeing as its distribution is affected by prominence in terms of both animacy and person.7 However, as this paper has argued, there are a number of properties found in sign language structure cross-linguistically that point to differential marking/indexing phenomena being at play, showing that they are indeed modality-independent.