Cross-linguistic differences in gesture

Gestures accompanying speech (i.e., co-speech gestures) can vary in form and function. For example, they may present images of physically present or absent concrete entities and/ or actions (i.e., iconic gestures), locate physically non-present entities in gesture space (i.e., abstract deictic gestures), point at physically present objects (i.e., concrete deictic gestures) or be simple and rapid hand movements which direct attention to the rhythmical peak of speech (i.e., beat gestures) (McNeill, Reference McNeill1992, Reference McNeill and Brown2006). Irrespective of their form, gestures are tightly linked to speech (Kita & Özyürek, Reference Kita and Özyürek2003; McNeill, Reference McNeill1992; So, Kita & Goldin-Meadow, Reference So, Kita and Goldin-Meadow2009); they convey relevant information to what is expressed in the parts of speech they overlap with (see Özyürek, Reference Özyürek, Church, Alibali and Kelly2017 for a review). Even though all types of gestures can be found in different languages, there are also systematic cross-linguistic differences in patterns of gestures (cf. Gullberg, Reference Gullberg, Bhatia and Ritchie2012; Kita, Reference Kita2009).

One of the most studied domains in relation to cross-linguistic differences in gesture patterns is expression of motion events due to cross-linguistic variation in the linguistic encoding of path and manner of those events (Talmy, Reference Talmy2000). For example, native speakers of English tend to conflate manner and path components of an event into a single clause in their speech (e.g., ‘The boy ran into the house’), while native speakers of Turkish tend to encode path information in the verb in the main clause and optionally express manner outside the verb in another subordinate clause e.g., Oğlan (koşarak) eve girdi ‘The boy (by running) entered the house’ (cf. Özçalışkan, Reference Özçalışkan2016). Following speech patterns, native speakers of English tend to conflate manner and path components into a single gesture (e.g., moving fingers in rapid movements while moving them forward as if running) while native speakers of Turkish tend to produce separate gestures for manner (i.e., ran) and path (i.e., entered) (Kita & Özyürek, Reference Kita and Özyürek2003; Özçalışkan, Reference Özçalışkan2016; Özçalışkan & Slobin, Reference Özçalışkan, Slobin, Greenhill, Littlefield and Tano1999). Therefore, speakers show cross-linguistic variation with regard to the shape and form of gestures that accompany speech (see for further evidence: Brown & Gullberg, Reference Brown and Gullberg2008; Kita & Özyürek, Reference Kita and Özyürek2003; McNeill & Duncan, Reference McNeill, Duncan and McNeill2000; Özçalışkan, Reference Özçalışkan2016; Özyürek, Kita, Allen, Brown, Furman & Ishizuka, Reference Özyürek, Kita, Allen, Brown, Furman and Ishizuka2008). Recently, similar gesture patterns have been found in a comparison of blind English and Turkish speakers’ motion event descriptions, showing that these gesture patterns are shaped by language specific ways of encoding and packing semantic information rather than seeing or adopting to others’ gesture patterns in the culture (Özçalışkan et al., Reference Özçalışkan, Lucero and Goldin-Meadow2016). These findings have been explained by the Interface Theory (Kita & Özyürek, Reference Kita and Özyürek2003) which postulates interactions between gesture and spoken language production where language-specific encoding and packaging of semantic information influence the form of gestures.

Cross-linguistic differences in gestures have also been found for spatial frames of reference, e.g., absolute frame of reference (e.g., north, south) versus relative frame of reference (e.g., right, left) (Kita & Özyürek, Reference Kita and Özyürek2003; Levinson, Reference Levinson2003), spatial expression of time (Kita, Reference Kita2009) and time metaphors (Bostan, Börütecene, Özcan & Göksun, Reference Bostan, Börütecene, Özcan, Göksun, Papafragou, Grodner, Mirman and Trueswell2016; Casasanto & Jasmin, Reference Casasanto and Jasmin2012; Gu, Mol, Hoetjes & Swerts, Reference Gu, Mol, Hoetjes and Swerts2017; Núñez & Sweetser, Reference Núñez and Sweetser2006).

Frequent and recurrent speech and gesture pairings, at different levels of semantic and syntactic encodings within and across languages, have also been used to argue for the existence of multimodal construction units. Such an argument postulates the existence of language-specific lexical and syntactic multimodal constructions that are entrenched symbolic units in line with a usage-based approach to language. For example, in Turkish, expressions that encode manner and path would exist as separate symbolic units co-occurring with verbs and corresponding manner and path gestures; whereas, in English, both the manner and the path particle would constitute a symbolic unit together with conflated manner and path gestures (Zima, Reference Zima2014).

Previous studies also point to differences in the amount of gestures per speech units across languages. For example, Italian culture has been suggested to be a high gesture culture (Efron, Reference Efron1941; Kendon, Reference Kendon1992) while (British) English has been described as low gesture culture (Graham & Argyle, Reference Graham and Argyle1975). Direct comparisons of gesture rate, on the other hand, are rare in the literature. So (Reference So2010) for example showed that Mandarin speakers in mainland China gestured less than American English speakers, suggesting English is a relatively higher gesture culture than Mandarin-Chinese. In another study, Cavicchio and Kita (Reference Cavicchio and Kita2013) found that Italian is a relatively higher gesture culture than British English.

Building upon previous research on this topic, here we focus on gesture rate as a measure to investigate to what extent gestures change as a result of language contact between speakers of two different languages, and whether contact can influence gesture rates in bilinguals. We should note that what gives rise to gesture rate differences across languages is not well-understood as the differences might be linked to the specificity of the language at different levels (e.g., lexical, syntactic, information packaging, prosody or simply to the speech rate itself). It is beyond the scope and the ambition of this paper to account for the differences in gesture rate in the languages we study. We do, however, present some speculations in the discussion section about the link between gesture rate and type of languages we study.

Gestures and bilingualism

Few previous studies have investigated what happens to different types of gestures when speakers regularly use more than one language – especially when the two languages differ in their gesture rates. Even though such a question has not been asked for bilinguals growing in language contact situations before, most of the earlier work on bilingual gestures focused on second language learning and the amount of gestures in relation to language proficiency and dominance in L2 speakers (e.g., Gullberg, Reference Gullberg1998, Reference Gullberg2006; Pika, Nicoladis & Marentette, Reference Pika, Nicoladis and Marentette2006; Sherman & Nicoladis, Reference Sherman and Nicoladis2004) and has produced some mixed results.

Some of the previous studies investigated whether bilinguals use more gestures in their weaker second language (L2) than in their stronger first language (L1) as a possible learner's strategy, comparing gesture rate in the L1 to the gesture rate in the L2. It has been found that bilingual adults use abstract deictic gestures more often with their L2 than with their L1 (e.g., Gullberg, Reference Gullberg1998; Marcos, Reference Marcos1979; Sherman & Nicoladis, Reference Sherman and Nicoladis2004). As for iconic gestures, some studies found no difference across L1 and L2 with regard to gesture rate (Sherman & Nicoladis, Reference Sherman and Nicoladis2004) while some found more iconic gestures in the L1 (Gullberg, Reference Gullberg1998). In the light of these findings, it has been suggested that iconic and abstract deictic gestures (henceforth, we refer to abstract deictics when we mention deictics) might be related to speech in different ways (Gullberg, 2013; Nicoladis, Mayberry & Genesee, Reference Nicoladis, Mayberry and Genesee1999; Sherman & Nicoladis, Reference Sherman and Nicoladis2004). Deictic gestures have been suggested to co-occur with grammatical or discourse organizational difficulties. Gullberg (Reference Gullberg1998) for example suggest that speakers may use deictic gestures when they have problems with expressing tense, using deictic gestures to help indicate the sequence of events by mapping them out spatially (Gullberg, Reference Gullberg1998). Deictic gestures can also help with discourse organization by allocating a specific gesture space to referents, for example, and by referring back to those spaces the next time the same referent is mentioned (Gullberg, Reference Gullberg1998, Reference Gullberg2006; Yoshioka, 2008). Iconic gestures, on the other hand, may emerge when speakers are trying to be particularly detailed or imagistic (Alibali, Kita & Young, Reference Alibali, Kita and Young2000) and may be used to mediate difficult speech for the listener (Beattie & Shovelton, Reference Beattie and Shovelton2000; Sherman & Nicoladis, Reference Sherman and Nicoladis2004).

Some other studies have focused on the effects of high vs. low rate of gesturing on bilingual gesture rate. Pika et al. (Reference Pika, Nicoladis and Marentette2006) found that English(L1) –Spanish and French(L1) –English bilinguals living in the English-speaking part of Canada produced more iconic gestures while speaking English compared to English monolingual speakers. This difference was not present for deictic gestures. The authors interpreted these findings as evidence for gesture rate transfer from higher-gesture language (Spanish and French) to lower-gesture language (English), assuming Spanish and French are both higher-gesture languages than English. Note, however, that the study did not have monolingual baselines for gesture rate in Spanish and French. Therefore, it is also possible that bilinguals might have gestured more than monolinguals overall rather than transferring gesture rate. Such a trend was indeed shown by Nicoladis et al. (Reference Nicoladis, Pika and Marentette2009) who found no evidence for gesture rate transfer for English–French bilingual children in Canada even though bilingual children tended to use more iconic gestures than monolingual comparison groups while speaking in both English and French. The authors suggested that bilinguals have more “choices” for how to package verbal messages compared to monolinguals, and bilinguals may gesture more than monolinguals which will help them hold information in memory while they search for how to package their message.

Smithson, Nicoladis and Marentette (Reference Smithson, Nicoladis and Marentette2011) on the other hand did not find differences for iconic gestures in English between monolingual and bilingual children living in Canada (Chinese–English and French–English bilinguals) which they interpreted as “bilingualism alone does not lead to a higher gesture rate” (p. 342). The study, however, again did not have monolingual baselines either for French or Chinese, which makes it difficult to evaluate whether the authors’ proposition is generalizable to the other language of bilinguals.

Collapsing iconic and deictic gestures into one category, representational gestures, So (Reference So2010) compared the gesture rate of Chinese–English bilingual speakers in Singapore, where English is taught in schools from early on, to the gesture rate in monolingual Mandarin-Chinese and in monolingual US English speakers. So found higher gesture rate in monolingual English than in monolingual Chinese. Even though bilingual speakers’ gesture rate was not different from the monolingual baseline while speaking in English, they produced more gestures than the monolingual baseline while speaking in Chinese. Based on those findings, So argued that gesture rate for representational gestures was more likely to be transferred from the relatively higher gesture language (i.e., English) to the relatively lower gesture language (i.e., Chinese) than the other way around. No transfer effect was found for non-representational gestures, i.e., gestures that do not bear semantic relations to their referent (for example beat gestures that direct attention to the rhythmical peak of speech, McNeill, Reference McNeill1992, Reference McNeill and Brown2006). So, therefore, concluded that representational gestures were more likely to be transferred than non-representational gestures.

In a more recent study, Cavicchio and Kita (Reference Cavicchio and Kita2013) investigated the gesture rate of Italian–English bilinguals, some of whom were living in Italy and some in the UK. They found higher gesture rate in monolingual Italian than in monolingual English speakers. Unlike the findings from Pika et al. (Reference Pika, Nicoladis and Marentette2006) and So (Reference So2010), Cavicchio and Kita did not find evidence for gesture rate transfer. Instead, bilingual speakers maintained the cross-cultural differences in gesture rate. Note that Cavicchio and Kita did not differentiate between different types of gestures (e.g., differentiating between iconic versus deictic gestures as in Pika et al. or representational versus non-representational gestures as in So). If representational gestures are indeed more likely to be transferred than non-representational gestures (So, Reference So2010), the lack of gesture rate transfer in Cavicchio and Kita might have been conflated by collapsing all types of gestures together in the analysis.

Overall, the mixed findings in the literature make it hard to draw strong conclusion regarding whether gesture rate is transferred or not. Also, due to the absence of monolingual comparison groups in most of the published studies, it is hard to tease apart the effects of being bilingual in general versus proficiency on gesture rate. Previous studies have not provided an explanation for a mechanism for adaptations of gesture rates from one language pattern to another, either (e.g., from high to low levels). Finally, while some studies report general gesture rate, others focus on the rates of different gesture types such as iconics and deictics, which makes the comparison of findings across different populations difficult.

Present Study

The purpose of this study is to explore what happens to gesture rate when one relatively higher-gesture language (as minority language) comes into contact with a relatively lower-gesture language (as majority language) and whether gesture rate is more likely to be transferred for some gesture categories than others (i.e., iconic versus deictic). To answer those questions, we study gestures of Turkish–Dutch bilingual speakers in the Netherlands (while speaking in both Turkish and Dutch) as well as gestures of monolingually raised speakers of Turkish in Turkey and monolingually raised speakers of Dutch in the Netherlands.

We also test if Turkish is indeed a higher gesture language than Dutch as Mediterranean cultures are usually found to be relatively higher gesture cultures (Barzini, Reference Barzini1964; Cavicchio & Kita, Reference Cavicchio and Kita2013; Kendon, Reference Kendon1992; Scheflen, Reference Scheflen1972) even though the reasons for this are not clear. We expected Turkish monolinguals to produce more gestures than Dutch monolingual speakers, as previous studies usually found gesture rate to be higher in Mediterranean area than that in North Europe (Barzini, Reference Barzini1964; Cavicchio & Kita, Reference Cavicchio and Kita2013; Kendon, Reference Kendon1992, Scheflen, Reference Scheflen1972). However, we acknowledge that the reasons for cross-linguistic differences in gesture rate are not completely understood and therefore we do not have clear predictions about the status of iconic versus deictic gesture rate.

As for the bilingual gesture rate, one possibility is that bilingual speakers will transfer gesture rate due to daily contact between the Turkish and Dutch speaking communities, as speakers are known to adjust their gestures according to their interlocutors: for example, due to a mimicking strategy (Holler & Wilkin, Reference Holler and Wilkin2011). Bilinguals may reduce gesture rate in their higher gesture language, Turkish, as an adaptation to the lower gesture rate of the majority language, Dutch, due to everyday contact with Dutch speakers. These predictions would derive from an account that considers social factors influencing gesture production.

An alternative prediction, based on usage-based accounts, is that gesture rates should not differ between monolinguals and bilinguals considering that the speakers in this study have been frequently exposed to each language from very early on in their lives and are proficient in each language.

A third possibility is that bilinguals will have higher gesture rates than either monolingual group. This prediction is based on previous literature that has suggested that bilinguals activate their two languages simultaneously (Broersma, Carter & Acheson, Reference Broersma, Carter and Acheson2016; Grosjean, Reference Grosjean and Nicol2001) and inhibiting the task-irrelevant language might induce cognitive cost (Sorace & Serratrice, Reference Sorace and Serratrice2009). In that case, bilinguals may exploit iconic and/or deictic gestures to help organize their speech and to reduce cognitive load, a pattern that has been suggested to be in place for monolingual speakers (Alibali & DiRusso, Reference Alibali and DiRusso1999; Goldin-Meadow, Reference Goldin-Meadow, McClelland and Siegler2001; Goldin-Meadow, Nusbaum, Kelly & Wagner, Reference Goldin-Meadow, Nusbaum, Kelly and Wagner2001; Wagner, Nusbaum & Goldin-Meadow, Reference Wagner, Nusbaum and Goldin-Meadow2004) (see Nicoladis, Reference Nicoladis2006, Reference Nicoladis2007; Nicoladis et al., Reference Nicoladis, Pika and Marentette2009 for a similar discussion about bilingual gestures).

We contribute to existing literature on bilingualism as well as multimodal language production in following ways. First, we provide gesture rate data from a novel language pair in the domain of bilingual gestures, i.e., Turkish and Dutch. Second, we study a different population of bilinguals from those studied earlier. Bilingual participants in this study were early learners and highly proficient in the L2. Therefore, they are different from previously studied speakers who started learning their L2 at a later stage and mostly had weaker proficiency in their L2 than in their L1. The advantage of studying such a population is that the gesture rate in each language is less likely to be modulated by language dominance. Furthermore, speakers growing up with both languages and cultures have had enough exposure to each culture to test whether some accommodation of gesture rate can take place from minority to majority languages or vice-versa.

Finally, going beyond previous research on gesture rate transfer, we situate our predictions on cognitive and social mechanisms that might modulate speech and gesture production.

Participants

Twenty heritage speakers of Turkish studying in Nijmegen, the Netherlands (14 females; M _age = 23.3, SD = 2.95), twenty monolingually raised Turkish speakers studying in Istanbul, Turkey (17 females; M _age = 22.2, SD = 1.75) and twenty monolingually raised Dutch speakers studying in Nijmegen, the Netherlands (14 females; M _age = 21.5, SD = 2.73) participated in the study in return for payment or course credit. All heritage speakers were second-generation immigrants who were born and were raised in the Netherlands by first-generation parents, who themselves were first-generation immigrants who moved to the Netherlands from Turkey (Mean immigration age was M _age = 15.9, SD = 5.12 for the mothers and M _age = 19, SD = 7.24 for the fathers). When the participants in this study were born, the mothers on average had already lived in the Netherlands for 9.2 years (SD = 6.66) and fathers for 11.15 years (SD = 7.46).

The bilingual speakers acquired Turkish as their first language (L1) at home during early years and Dutch as their second language (L2) to which they have had increasing exposure after the age of 4. They also had an early exposure to some Dutch from their parents, who were themselves late learners of Dutch. The speakers are highly proficient in both languages – within and beyond their home situations – and use each language regularly on a daily basis. On a 5-point Likert scale, bilinguals rated the frequency of their current language use in various environments and with various interlocutors (1 = never; 2 = rarely; 3 = sometimes; 4 = most of the time; 5 = all the time) as well as their proficiency in both Turkish and Dutch (1 = native; 2 = native-like; 3 = advanced; 4 = intermediate 5 = beginner). The analysis on the ratings showed that bilinguals’ self-rated frequency of language use for Turkish (M = 2.43, SD = 0.92) and Dutch (M = 2.91, SD = 1.31) was not significantly different, ß = −0.484, SE = 0.330, t-value = −1.465. Bilinguals rated their overall proficiency in Turkish to be somewhere between native-like and advanced (M = 2.40), although the rating scores were even higher for Dutch (M = 1.50), ß = 0.900, SE = 0.15, t-value = 2.853 (see Table C1 in Appendix C for the random effect structure of the analyses). Bilinguals also reported speaking mostly in Dutch at school and in Turkish at home with their parents while mostly mixing the two languages among Turkish speaking friends.

We used speech analysis software Praat (Boersma, Reference Boersma2001) to measure participants' articulation rate across both languages (number of syllables/time) for a 10 second speech sample from the elicited narratives (cf. De Jong & Wempe, Reference De Jong and Wempe2009 for the script). The articulation rate of bilinguals in Dutch (M = 4.42, SD = 0.57) did not significantly differ from that of the monolingual baseline in Dutch (M = 4.62, SD = 0.71) (ß = 0.191, SE = 0.204, t-value = 0.934). The comparison of articulation rate in Turkish did not show a significant difference between bilingual speakers (M = 4.44, SD = 0.63) and the monolingual baseline, either (M = 4.81, SD = 0.55) (ß = 0.375, SE = 0.188, t-value = 1.994, p = .053Footnote ¹).

Stimuli

We used two short silent videos (Azar, Backus & Özyürek, Reference Azar, Backus, Özyürek, Papafragou, Grodner, Mirman and Trueswell2016, Reference Azar, Backus, Özyürek, Gunzelmann, Howes, Tenbrink and Davelaar2017) to elicit narratives. In one video, three women engaged in cooking activities (kitchen video, Perniss & Özyürek, Reference Perniss and Özyürek2015) and, in the other video, two women and a man engaged in office activities (office video). Figure 1 illustrates stills depicting different segments from each video. See Appendix A for a detailed list of events taking place in each video stimulus.

Figure 1. Stills form the two video stimuli, kitchen video at the top and office video at the bottom

Procedure

Prior to the data collection session, participants were informed that the study was about language production without any mention of gestures and then they signed consent forms. Participants watched the two stimulus videos one by one on a computer screen and narrated what they had watched to an addressee after each video. The computer screen turned white after each video and stayed white during the narrations. The addressees were not confederates; there was a different addressee in each session; and they did not see the videos before or during the narrations. Addressees were instructed that they could ask clarification questions once the narrative was complete and they were going to answer two short written questions about each narrative. Once the instructions were given, the experimenter left the room and came back after each narrative with questions for the addressee. Speakers repeated the task once in Turkish with a Turkish monolingual addressee and once in Dutch with a Dutch monolingual addressee, with at least a two-week interval between the two sessions. Turkish monolingual addresses were recruited from exchange university students from Turkey who were visiting the Netherlands for a semester abroad. The order of the two videos and the two language sessions was counterbalanced. All sessions were videotaped. Monolingual participants performed the task once.

Data coding

Data were transcribed and annotated using ELAN video annotation software (available online: https://tla.mpi.nl/tools/tla-tools/elan/) (Lausberg & Sloetjes, Reference Lausberg and Sloetjes2009). The data we present in this study were collected and annotated for a corpus of multimodal reference tracking by Turkish–Dutch bilinguals (in preparation). Meta-narratives such as commentaries about the characters were excluded from the corpus and the same exclusion criteria were used for all language and speaker groups.

First, the narratives were divided into clauses, units with a single subject argument and a single predicate (Berman & Slobin, Reference Berman and Slobin1994). Coordinated clauses were coded as separate clauses (e.g., ‘the man stood up and he walked to the bookshelf’ was coded as two clauses). Relative clauses that modified nouns (e.g., ‘the woman who was helping the man’) were not coded as separate clauses but as the modifier of the noun (in this case ‘who was helping the man’ was not coded as a separate clause). This was to make sure that the coding scheme was comparable across Turkish and Dutch (relative clauses are finite in Dutch but non-finite in Turkish).

Next, gesture strokes that co-occurred with any part of the speech clauses were identified. Stroke is the meaningful part of the gestural movement (Kendon, Reference Kendon2004; McNeill, Reference McNeill1992) as the expressive segments of the stream of manual production (Kita, van der Hulst & van Gijn, Reference Kita, van der Hulst, van Gijn, Wachsmuth and Fröhlich1998). We categorized strokes into iconic, deictic and non-representational gestures (gestures that do not depict information about their referent). However, we analyze only iconic and deictic gestures as these two types of gestures show up most frequently in adult storytelling (McNeill, Reference McNeill1992) and they are more likely to be transferred by bilingual speakers (So, Reference So2010). In total, 743 non-representational gestures were excluded from the analyses (185 in bilingual Turkish, 270 in monolingual Turkish, 155 in bilingual Dutch and 133 monolingual Dutch). The proportion of excluded gestures was similar across all speaker groups (15% for bilingual Turkish and monolingual Dutch and 16% for bilingual Dutch and monolingual Turkish).

Iconic gestures represent images of actions and entities (McNeill, Reference McNeill1992). Figure 2 illustrates an example of an iconic gesture. Deictic gestures are pointing motions that use spatial location to indicate discourse entities (e.g., Azar et al., Reference Azar, Backus, Özyürek, Gunzelmann, Howes, Tenbrink and Davelaar2017; Sherman & Nicoladis, Reference Sherman and Nicoladis2004). They can be executed with an extended index finger, thumb or with all fingers extended. Deictic gestures in our data set are abstract pointing gestures that co-occurred with referents that were physically absent in the environment and they could refer to persons or the objects in the narratives. Figure 3 illustrates an example of a deictic gesture.

Figure 2. Bilingual speaker speaking in Dutch (left panel) is producing an iconic ‘stirring’ gesture, referring to the action performed by the woman who is standing in the stimulus video (right panel). Her gesture is temporally aligning with roeren ‘stirring’ in her speech.

Figure 3. Bilingual speaker speaking in Turkish (left panel) is producing a deictic gesture referring to the woman who is walking in the stimulus video (right panel). His gesture is temporally aligning with o bayan ‘that woman’ in his speech.

A second coder coded around 13% of the gestures for reliability. The two coders had a high initial agreement for the presence of a stroke (84% for bilingual Turkish, 85% for monolingual Turkish, 87% for bilingual Dutch and 91% for monolingual Dutch); also a high agreement for the gesture type (iconic gestures, deictic gestures or other category of gestures). The two coders reached 100% agreement for the presence/ absence of a gesture stroke and the gesture type for each speaker group in a meeting where the initial discrepancies were discussed and resolved. Table 1 summarizes the initial agreement values for gesture type coding.

Table 1. Inter-rater reliability scores for gesture type coding

Analyses

Table 2 summarizes the total and mean number of speech clauses that were produced by each speaker group.

Table 2. Total and average number of speech clauses in Turkish and Dutch per speaker group (Standard Deviation)

Gesture rate is usually calculated in relation to speech based on either the number of clauses (So, Reference So2010; Gullberg, Reference Gullberg1998) or the number of (100) words (Cavicchio & Kita, Reference Cavicchio and Kita2013; Pika et al., Reference Pika, Nicoladis and Marentette2006; Sherman & Nicoladis, Reference Sherman and Nicoladis2004). We calculated gesture rate in relation to the number of speech clauses, as clause is considered to be an important processing unit for speech as well as for gesture production (Levelt, Reference Levelt1989, Kita, Reference Kita2009; Kita & Özyürek, Reference Kita and Özyürek2003). Additionally, we aimed to account for the structural differences between Turkish and Dutch, Turkish being an agglutinative language while Dutch is not. In most cases, the same event unit is expressed with fewer words in Turkish than in Dutch because Turkish uses suffixes to mark some information such as case marking which is mostly expressed with separate words in Dutch. For example, the event unit showing a man walking towards the bookshelf can be expressed with three words in Turkish (e.g., Adam kitaplığa yürüdü ‘Man walked to the bookshelf’), but six words in Dutch (De man liep naar de boekenkast ‘The man walked towards the bookshelf’). The same event unit, however, is expressed with one clause in each language. Therefore, while gesture rate that is calculated based on the number of words in speech would yield a higher gesture rate for Turkish (0.33) than for Dutch (0.17), calculating gesture rate per speech clause yields the same rate for both speaker groups, accounting for the cross-linguistic differences in morpho-syntax. Finally, subject and/or object arguments are usually dropped in Turkish but not in Dutch, therefore a word-based rate count would disadvantage Dutch gesture rate.

We performed linear mixed-effect models on the mean number of gestures per clause per participant (hence each participant contributing one data point for gesture rate) using lmer function from the lme4 package (cf. Bates, Maechler, Bolker & Walker, Reference Bates, Maechler, Bolker and Walker2015) in the software R, version 3.4.3. We simultaneously entered Language Type (Dutch versus Turkish) and Language Status (monolingual versus bilingual) as well as the interaction term of Language Type and Language Status as fixed effects in each analysis. Random intercepts for participants were also included in the analyses (see Table C2 in Appendix C for the random effects structure of the gesture analyses). We first examined the overall gesture rate collapsing two types of gestures – as in Cavicchio and Kita (Reference Cavicchio and Kita2013) and in So (Reference So2010) – so that we can compare our findings to those studies. Later, we performed separate analyses on iconic and deictic gestures (following Sherman & Nicoladis, Reference Sherman and Nicoladis2004; Gullberg, Reference Gullberg1998; Pika et al., Reference Pika, Nicoladis and Marentette2006). Appendix B provides a detailed summary of the fixed effect structures for each gesture rate analysis.