1 Introduction

This paper reports a comprehension study of the Bosnian/Croatian/Serbian (BCS) modal verb morati ‘must’ with 3-to-6-year-old children and adults. Modals like morati express two distinct ‘flavours’: deontic¹ (1a; obligation) and epistemic (1b; inference from evidence or knowledge) (Palmer 1986; Kratzer 1991; Hacquard 2011; i.a.). This flavour-variability poses a learning challenge, as the same modal verb maps to more than one non-actual meaning, closed to observation (Stephany 1979; Papafragou 1998; van Dooren et al. 2017; i.a.). They are quintessential “hard words” (Gleitman et al. 2005). How do children learn and navigate modal flavours for flavour-variable modal verbs of their input languages? How does modal verb interpretation develop over the preschool years? And, what factors most influence children’s modal verb interpretations as deontic vs. epistemic? Our results suggest that syntactic bootstrapping (Gleitman 1990) and pragmatic inferences specific to modal meanings (Traugott & Dasher 2002) play central roles in modal flavour development, the former earlier on than the latter.

We compare our BCS study to prior work on English modal verb must that used the same methodology (Cournane & Pérez-Leroux 2020). While English must and BCS morati are both flavour-variable necessity modals, they differ in several ways that may affect their learning paths. Firstly, they differ syntactically. Morati is a main verb that occurs in different syntactic constructions when deontic than when epistemic. For example, morati shows subject agreement only when used deontically (1a), and only occurs in default 3sg.prs form when epistemic (1b). English must, on the other hand, is a form-invariant auxiliary verb that shows only partial deontic versus epistemic syntactic differences. Speakers prefer deontic interpretations when must combines with eventive verbs (2a) (see Cournane & Pérez-Leroux 2020), but epistemic interpretations with the event as habitual are also possible. And, speakers readily get epistemic interpretations for must when it combines with a stative (be or grammatical aspect) (2b), but can still get a deontic interpretation if the state is future-shifted (i.e. “be wearing his boots” is obliged to hold after the utterance time) by context or temporal adverbials (e.g. by tonight).

Second, English must and BCS morati differ in how they are used in the input to children. Corpus work shows that must is used primarily as epistemic in the input to young children (about 80% Epistemic; van Dooren et al. 2017, based on the Manchester Corpus, Theakston et al. 2001). This epistemic usage bias is also apparent in sociolinguistic corpora of casual spoken English (Tagliamonte & D’Arcy 2007, i.a.). Morati, on the other hand, is overwhelmingly used as root (deontic and other root meanings) in the input. It occurs only 3.6% (18/494 uses) of the time as epistemic (Veselinović & Cournane, 2020, based on SCECL Corpus, Anđelković et al. 2001).

Cournane & Pérez-Leroux (2020) ran a picture preference task with English-speaking preschoolers and adults, testing preferred interpretations (deontic or epistemic) of must with eventive verbs (2a) and the same verbs marked with grammatical aspect (2b). Adults behaved as expected, choosing more deontic pictures than epistemic for sentences like (2a), and primarily epistemic pictures for those like (2b). However, children never distinguished by sentence type. Three-year-olds had an overall deontic bias, consistent with early modal verb production patterns (see Papafragou 1998; Hickmann & Bassano 2016; Cournane 2021). By age 5, children had a strong epistemic bias for both sentence types, even though adults significantly preferred deontic interpretations for (2a). What explains these non-adult behaviours? Properties of English modal verb syntax and usage patterns for must leave open several possibilities for why these differences obtained.

By age 5, English children’s epistemic bias may reflect the fact that must is used mostly epistemically in their input. Or, they may tune in to the fact that must is overtly above aspect marking in sentences like (2b) (related to the epistemic interpretation, see §2.1) and overextend the high epistemic interpretation to sentences that don’t reveal must’s position relative to aspect (like with bare verbs, 2a) (Cournane & Pérez-Leroux 2020). Finally, children may overcommit to pragmatic inferences from deontic to epistemic, of the type If you are obliged to p, you likely p (Traugott 1989; Traugott & Dasher 2002). For example, 5-year-olds may hear the sentence in (2a) and interpret it – like adults – as deontic, but then quickly move from the obligation to the inference that the obligation is carried out, causing them to select the epistemic pictures (depicting indirect evidence of the subject carrying out the VP).

BCS morati provides an ideal, natural counterpoint to English must to help adjudicate between these competing hypotheses. Unlike must, morati is primarily deontic in the input, so input frequency by flavour should not drive an epistemic bias in BCS children. And, unlike English, BCS morphosyntax provides unambiguous cues to deontic vs. epistemic interpretations in the majority of sentences. BCS children should thus have clearer input cues for using syntactic bootstrapping to learn when morati is deontic and when it is epistemic. Only the pragmatic story predicts the same epistemic picture-selection bias for BCS morati as for English must; this inference is available to adult speakers of English and BCS (for any deontic necessity modal).

Our results show that BCS children begin with a weak but significant distinction between morati sentence types (1a vs. 1b) at age 3, in the adult-like direction. This result differs from English, where 3-year-olds showed a significant overall deontic picture preference. BCS syntax plausibly supports this early distinction, as the BCS constructions show rich morphosyntactic cues to modal interpretations, in many ways more akin to attitude verbs (e.g. want, hope, think) than modal verbs of English (see Hacquard & Lidz 2016). But, by age 5, BCS children show the same non-adult pattern as age-matched English children: they over-select epistemic pictures for morati (compared to adults), including for sentences like (1a). We thus argue that our results support a pragmatic story: older preschoolers commit more to the likelihood inference from deontic morati/must than do adults. This behaviour may be related to findings in the developmental psychology literature showing preschool children endorse strong conformity to rules (see Kalish 1998, i.a.) and that the ability to treat adherence to social rules as violable, unlike adherence to physical laws, develops over the preschool years (see Shtulman & Phillips 2018).

We discuss our results in light of pragmatically-driven models of semantic change from root > epistemic (a commonplace, unidirectional change observed for modal verbs, e.g. Lightfoot 1979; Traugott 1989; Bybee et al. 1994), which argue that epistemic uses of modal verbs arise from conventionalization of pragmatic inferences like those we argue for here (Traugott & Dasher 2002; i.a.). We make one major alteration: historical linguists who support this theory of semantic change assume adults must be the drivers, arguing children are not pragmatically sophisticated enough to perform this kind of inference (Nerlich & Clarke 1992; Traugott & Dasher 2002). Our results show preschool children are more likely to commit to this inference than adults, and at a time when they are still learning the semantics of modal verbs.

2 Background

3 Flavour Preference Task

3.1 Methods

3.1.1 Participants

Participants were 56 typically-developing monolingual-BCS children and 9 dialect-matched adults from Sarajevo, Bosnia and Herzegovina. An additional four children were excluded (three ended participation early, one only chose the right-hand side of the screen). Children were divided into four age groups: 3-year-olds (n = 14, M = 42 months, SD = 3.7 months), 4-year-olds (n = 15, M = 54 months, SD = 3.4 months), 5-year-olds (n = 15, M = 66 months, SD = 3.9 months), and 6-year-olds (n = 12, M = 77.5 months, SD = 3.3 months). Children were recruited through one public pre-school in Sarajevo. Testing was done at the preschool, after obtaining parental consent and child assent. If parent and child consented to audio-recordings, the session was recorded. Adult controls (M = 34.6 years, range = 23–62, SD = 10.46) were recruited by word of mouth. Education ranged from vocational high school to post-graduate degrees.

3.1.2 Audio and visual stimuli and norming procedure

Based on Veselinović (2019)’s grammaticality task testing BCS modal sentence interpretations by syntactic constructions, we decided to make all of our test sentences fit two morphosyntactic formulae, one that clearly and only aligned with root interpretations (including Deontic), which we refer to as monoclausal (11a), and the other that clearly and only aligned with epistemic interpretations, which we refer to as biclausal (11b). These language-specific morphosyntactic considerations dictated the use of plural subjects to reveal lack of agreement on the modal verb in the epistemic contexts, as well as specific embedded verbs to be used (those with imperfective forms). Due to these considerations, the materials from Cournane & Pérez-Leroux (2020) could not be directly adopted for BCS and new stimuli were created.

1. (11)
1. a.

1. monoclausal: Subj Mod._SUBJφ DA V._PFV

1.  
1. b.

1. biclausal: Mod._3SG Subj DA V._IPF

The subject was always two bear cubs, referred to by the childlike term mede. We aimed to have the morphosyntactic frame as consistent across test sentences as possible, so the predicates were chosen based on the following criteria: (i) the aspectual pair of verbs had to exist, and (ii) the imperfective had to be the simple form, while the perfective had to be formed via prefixation from the imperfective. The full list of normed predicates is in (12).

1. (12)

1. a.
2. d.
3. g.
4. j.
1. (u)beru – ‘pick’
2. (po)jedu – ‘eat’
3. (o)kupaju se – ‘bathe’
4. (o)peru – ‘wash’
1. b.
2. e.
3. h.
4. k.
1. (o)brišu – ‘wipe’
2. (po)kose – ‘mow’
3. (sa)melju – ‘grind’
4. (na)prave – ‘make’
1. c.
2. f.
3. i.
4. l.
1. (na)crtaju – ‘draw’
2. (sa)kriju – ‘hide’
3. (is)peku – ‘bake’
4. (na)pune – ‘fill’

We then designed twelve stories, each consisting of an introductory context picture and sentence or two (as in Figure 4, Section A), three test pictures (actual, deontic, epistemic; as in Figure 4, Section C), and three test sentences with the predicates from (12): unmodalized, monoclausal (11a), biclausal (11b). The context was intended to suit any of the test sentences. Deontic pictures depicted a future-oriented scenario relative to the predicate of the test sentence (e.g. for the bathing scenario the bears are muddy and bathing has not yet occurred) and an authority figure (thought to help young children access deontic interpretations, e.g. Dack & Astington 2011). The epistemic pictures show obscured ongoing events (e.g. for the bathing scenario there are clues that the bears are in the bathtub, but we cannot see them). The epistemic pictures are essentially indirect visual evidence scenes (Papafragou et al. 2007). In what follows we focus on the deontic and epistemic pictures, and the monoclausal and biclausal sentences.¹²

We conducted an online norming study to ensure our stimulus pictures were good depictions of deontic and epistemic interpretations, and to test compatibility of our set-up and test sentences with the pictures we’d created (see Veselinović 2019, for further details on the norming process). The goal was to use the results as guidance for omitting stories (we designed two more than we intended to use), and improving the pictures where necessary before conducting the main study. Participants were adult native speakers of BCS, primarily from Sarajevo, Bosnia & Herzegovina (n = 108 included in the final analysis), who were recruited through social media.

Participants completed a 15-minute Qualtrics survey, where they rated sentence-picture matches on a 1–5 Likert-like scale for how well the sentence served as accompanying text for the picture. They rated all three sentence types for each picture. Each participant saw only one of the pictures for each story, rating sentences for 12 pictures total (4 of each of the 3 types). Participant exclusions meant that each picture was rated variable amounts, by between 27 and 38 participants. We counterbalanced order of presentation of sentences and randomized order of pictures.

In order for our story pictures to be considered successful for use in the main study, the Deontic pictures should receive a higher rating for the monoclausal sentence than for the biclausal sentence, and the Epistemic pictures should show the reverse pattern. We z-transformed all results to account for variation in how participants use the scale (Schütze & Sprouse 2014), and created plots like Figure 1 for each predicate. Predicates varied in the magnitude of the difference between sentence ratings by pictures across and within stories.

Figure 1

Sample results of average z-transformed ratings for sentences from the Hiding Story, with the predicate kriti se ‘hide’, plotted by picture type (Epistemic left, Root (Deontic) right) and sentence type (biclausal dark grey, monoclausal light grey). Number of judgements per sentence-picture pair is reported below plot.

We used the result plots for each story to make decisions and changes related to the story items. Most importantly, we completely omitted the two stories where the pictures did not show the required pattern of differences between ratings for sentence types (as in Figure 1), these were (po)jedu, ‘eat’ (12d) and (po)kose, ‘mow’, (12e). We also made small modifications to some pictures in an effort to strengthen the magnitude of difference for deontic or epistemic interpretations (n = 5/35 pictures; 2 context pictures, to remove possible biasing to the deontic follow-up; 2 deontic pictures to reinforce the obligation/imperative feeling; 1 epistemic picture). For example, for the Washing Story (12j), about washing up after dinner, the epistemic picture did not show a strong contrast, with a weaker preference for the biclausal sentence than desired. In an effort to increase the epistemic reading of the picture, we removed a few visible dirty dishes, as they likely constituted direct evidence for some speakers that the bears were washing them). The full list of stories used in the main study, following post-norming adjustments, and including filler stories, is available in the folder “BCS Experiment” at www.osf.io/v9ure/.

3.1.3 Main study procedure and materials

Participants were invited to play a picture preference game with a character, Penguin, presented on a laptop computer using MATLAB_R2014a with Psychtoolbox (Brainard 1997). Responses were recorded using the left and right arrow keys. Penguin’s pre-recorded speech has happy affect and natural prosody, spoken by a male actor who is a native speaker of BCS with experience in voice acting for children’s TV.

Penguin is first shown holding a book (Figure 2). He lets participants know he will tell them stories. For the first two training items, involving identifying nominals referents, the book is turned so the pictures are presented at the same time as Penguin says the training sentences (Figure 3). In the next two training items, the prompt is heard before the inside of the book is turned to face the participant, and the sentences involve identifying events (more similar to test items). Participants are taught to point to the page that “Penguin was looking at” when the inside of the book was only visible to Penguin. During this phase, participants’ wrong answers are corrected.

Figure 2

Introduction: Penguin and his book.

Figure 3

Sample training item.

A sample experimental trial is provided in Figure 4. The experimental trials first display an initial picture (Figure 4-A), with Penguin’s voice providing a short context. Then Penguin turns the book back to face himself, flips the page (an audible page turn noise plays), and says the target sentence, either monoclausal or biclausal (Figure 4-B). The book is reoriented towards the participant, as Penguin says Pogledaj! ‘Look!’ and reveals the two pictures (Figure 4-C). The participant is prompted to point to which picture Penguin was looking at. One page of the book presents a picture compatible with a deontic interpretation (Figure 4-C, left page), the other a picture compatible with an epistemic interpretation (Figure 4-C, right page). After choosing the picture, the child participants were sometimes asked Kako znaš? ‘How do you know?’ to make the task more interactive and gain qualitative insights. Not all children responded, and for some that did, this yielded feedback like, Ja sve znam! ‘I know everything!’ or Naučio me tata ‘Daddy taught me.’ On following trials, those children were instead asked to say what Penguin said and/or what they saw in the picture that made them choose it.

Figure 4

Sample trial with stimuli for set-up (A), test (B), and selection (C) presentations.

For the sample item, if participants interpret the test sentence as deontic (‘according to the household rules or normative behaviour’) they will choose the picture in which the bears are covered in mud. This represents a future-oriented interpretation consistent with deontic morati (bathing has not yet occurred, but needs to), the target response for the monoclausal sentences. The alternative picture is not readily compatible with the deontic reading – there is evidence the bears are already in the bath. If the participant interprets the test sentence as epistemic (‘according to what can be seen and what I know about bathing’), they will choose the picture in which the bathing bears are not directly visible, but all clues point towards the bears taking a bath. This illustrates a present-oriented interpretation, consistent with epistemic morati. The alternative picture is not readily compatible with the epistemic reading – bathing is not depicted.

3.1.4 Design

Each participant saw 4 training items, followed by 10 test items and 5 fillers, in random order (the first post-training item was always a filler). Fillers used the verbs voljeti ‘love’, mrziti ‘hate’ and htjeti ‘want’, which, unlike morati, are not meaning-variable. They were similar to test items in terms of syntactic complexity, but only required attending to depicted emotional states (happy vs. crying face). We used filler accuracy as an exclusion criterion, as a way to filter out participants who may not have been doing the task as intended.¹³ Since chance performance would be 2.5, we set the limit to 4/5 as unambiguously above chance. Every participant saw the same 10 test stories. Which sentence type appeared with which story was randomized and balanced for each participant (5 monoclausal, 5 biclausal). Onscreen position of the test pictures was also counterbalanced (left vs. right).

3.2 Results

3.2.1 Quantitative

All analyses were conducted with R (R Core Team 2013). Anonymized data and R scripts are available in the folder “BCS Experiment” at www.osf.io/v9ure/. Participants with filler accuracy at or below 3/5 correct were excluded from analysis. No adults or 6-year-olds were excluded, and only one 5-year-old was (leaving n = 14). Four 4-year-olds were excluded (leaving n = 11). Three-year-olds fared more poorly, with half of participants excluded (leaving n = 7), with only slight change to age distribution (M = 41 months, SD = 4.5 months). Impressionistically, the younger children in general were more likely to struggle with the task, with more of them acting confused (i.e. hesitating, looking for help responding), which is in line with these exclusion patterns.

Table 1 reports aggregate group means for epistemic picture choices, by sentence type. As expected, adults chose overwhelmingly epistemic pictures for biclausal sentences (no adult chose fewer than 4/5), and mostly deontic pictures for monoclausal sentences, differentiating interpretations by sentence type. However, for monoclausal sentences adults chose the unexpected epistemic picture an average of 1.67/5 times. For children, there is less difference between average epistemic picture choice rates between sentence types. That said, 3-year-olds chose epistemic pictures on average 2.29/5 trials for monoclausal sentences, the closest to the adult rate, while six-year-olds were the least adult-like on these trials, averaging 3.5/5 epistemic picture choices, with some children at ceiling. For biclausal sentences, older child groups chose somewhat more epistemic pictures for biclausal sentences than younger groups.

Table 1

Mean choice of epistemic pictures by age group and sentence type.

Maximum = 5. Standard Deviation in parentheses.

	Monoclausal	Biclausal
3-year-olds	2.29 (1.11)	3.29 (0.49)
4-year-olds	3.09 (1.14)	2.64 (0.92)
5-year-olds	3.00 (1.11)	3.64 (0.84)
6-year-olds	3.50 (0.91)	3.58 (0.79)
adults	1.67 (1.00)	4.78 (0.44)

We first assess the Input Frequency Hypothesis that children will select fewer epistemic pictures for morati sentences relative to adult controls and English-learning peers because, unlike must, morati is used primarily as deontic in the BCS input. We used the glmer function in the statistical package lme4 (Bates et al. 2014) in R (R Core Team 2013). The data were fitted into a generalized linear-mixed (logit) model using the maximum likelihood method (Laplace Approximation) (Baayen 2008; Dixon 2008; Matuschek et al. 2017). The model had selection of the epistemic picture as the reference for the dependent variable. We first compared children to adults for choice of epistemic pictures, including participant and item as random effects, giving the model: (epistemic ~ group + (1|participant) +(1|item)). We found no significant difference between groups in overall likelihood of epistemic picture selection (GroupChild, ß = –0.20, Z = –0.772, p = 0.44). We ran another model looking only at the child group, to test the effect of age in months (continuous) on choice of epistemic pictures (epistemic ~ agemos + (1|participant) + (1|item)). We found a significant positive effect of child’s age on choice of epistemic pictures, such that the older the child the more likely they were to select epistemic pictures (AgeMos, ß = 0.027, Z = 3.334, p = 0.0009***). Percentages of epistemic picture choices by age group descriptively illustrate this effect: 3-year-olds: 51%; 4-year-olds: 54%; 5-year-olds: 65%; 6-year-olds: 72%), compared to the adult 64%. We can therefore reject the input frequency hypothesis, as BCS children do not show a significant preference for deontic picture selections relative to adults, and increasingly prefer to select epistemic pictures with age.

We fit further similar models to assess the Syntactic Bootstrapping Hypothesis that BCS children will be adult-like for structure-to-meaning mapping for morati earlier than English-learning peers because morati has categorical syntactic cues for flavour interpretation. We expect adults to pick significantly fewer epistemic pictures for monoclausal constructions than for biclausal; if child groups distinguish interpretation by structure, they should show this same contrast. To test this hypothesis, we ran a model testing the likelihood of epistemic picture choice by sentence type (biclausal vs. monoclausal) and age group (with child groups binned by year), and their interactions. We included random slopes for participant by sentence and random effects for item, giving the model: (epistemic ~ sentence * agegroup + (1+participant|sentence) + (1|item)). We set adults as the reference level. Adults show the expected highly significant effect of sentence type, with a significantly lower likelihood of epistemic picture selection for monoclausal sentences (Monoclausal, ß = –4.00, Z = –4.95, p < 0.001***). Compared to adults, all child groups were significantly less likely to pick epistemic pictures for the reference biclausal sentences (this effect weakens a little from ages 3 and 4 to ages 5 and 6, which is the adultlike direction for development) (Table 2). Child groups were all significantly more likely than adults to pick epistemic pictures for monoclausal sentences, showing significant interactions (Table 2). These overall effects show adults distinguish by sentence type, and child groups show both more unexpected epistemic picture selections for monoclausal sentences and fewer expected epistemic picture selections for biclausal sentences than adults.

Table 2

Model results for the generalized linear-mixed (logit) model using the maximum likelihood method (Laplace Approximation): epistemic ~ sentence * agegroup + (1+participant|sentence) + (1|item). Biclausal sentences are the reference level for sentence and Adults are the reference level for agegroup.

	ß	Z	p-value
Monoclausal	–4.00	–4.95	<0.001***
3-year-olds	–2.79	–3.60	<0.001***
4-year-olds	–3.17	–4.11	<0.001***
5-year-olds	–2.25	–2.88	<0.004**
6-year-olds	–2.28	–2.88	<0.004**
Monoclausal:3yos	3.22	3.64	<0.001***
Monoclausal:4yos	4.21	4.79	<0.001***
Monoclausal:5yos	3.44	3.86	<0.001***
Monoclausal:6yos	4.13	4.51	<0.001***

However, this model does not show whether each child group differentiates picture selections by sentence type, as adults do. To see that effect, we re-leveled the same model for each age group, so that the main effect of sentence type would be reported for the reference level. Three-year-olds weakly significantly differ by sentence type, in the adult-like direction (Monoclausal, ß = –0.78, Z = –2.15, p < 0.03*).¹⁴ No other child group significantly differs by sentence type. We thus find weak support for the syntactic bootstrapping hypothesis for 3-year-olds only (note English 3-year-olds had a significant overall deontic preference, and no child group differentiated by sentence type). While 3-year-olds show a distinction in the adult-like direction, their accuracy and the strength of the distinction is by no means adult-like: 3-year-olds also show significantly fewer epistemic picture selections for biclausal sentences than do adults and significantly more epistemic picture selections for monoclausal sentences than do adults.

We fit further similar models to assess the Pragmatic Inference Hypothesis that children will over-select epistemic pictures for monoclausal constructions relative to adult controls, but in line with English-learning peers, because both must and morati invite the same inferences in their deontic uses. To assess this hypothesis, we limit the analysis to monoclausal sentences. Adults were set as the reference level for age group (epistemic ~ agegroup + (1|participant) + (1|item)). For Monoclausal sentences, all child groups except 3-year-olds differed from the adult baseline, showing the predicted increasing likelihood of epistemic responses with increasing strength of effect by age group (Table 3).

Table 3

Model results for the generalized linear-mixed (logit) model using the maximum likelihood method (Laplace Approximation) on the Monoclausal sentence data: epistemic ~ agegroup + (1|participant) + (1|item). Adults are the reference level for agegroup.

	ß	Z	p-value
3-year-olds	0.42	0.99	0.32
4-year-olds	1.01	2.40	0.02*
5-year-olds	1.19	2.78	<0.01**
6-year-olds	1.86	4.03	<0.001***

Looking only within the child group, we looked at whether age in months (continuous) and type of sentence affected likelihood of epistemic picture selection over developmental time (epistemic ~ agemos + (1|participant)), first fitting only monocausal sentences and then only biclausal sentences. Children choose significantly more epistemic pictures as they age for both biclausal (agemos, ß = 0.02, Z = 2.32, p = 0.02*)¹⁵ and monoclausal sentences (agemos, ß = 0.03, Z = 3.22, p = 0.001**) sentence types, with a more significant effect for monoclausal sentences.¹⁶

These results support the pragmatic inference hypothesis, as children choose increasingly more epistemic pictures, away from the adult rate for monoclausal sentences and towards the adult rate for biclausal (where adults are at ceiling for choosing epistemic pictures). This pattern is fully parallel to the English results.

To show individual data and developmental shifts by age groupings we used a series of scatterplots, with each age group represented in a separate panel, shown in Figure 5 (as in Cournane & Pérez-Leroux 2020¹⁷). Each participant was plotted according to how many epistemic pictures they chose for biclausal sentences (x-axis, range = 0 – 5), and how many epistemic pictures they chose for monoclausal sentences (y-axis, range = 0 – 5). Participants were classified as ‘epistemic avoidant’ if they gave less than half of their total responses to both conditions as epistemic (Quadrant c), ‘epistemic dominant’ if they gave more than half epistemic responses overall (Quadrant b), ‘target-oriented’ if they chose the epistemic picture more often to the biclausal condition, than to the monoclausal condition (Quadrant d), and those who had the opposite pattern were labelled ‘contrarian’ (Quadrant a).

Figure 5

Individuals plotted by number of epistemic picture selections for biclausal (x-axis) and monoclausal (y-axis) sentences, faceted by age group.

Most adults appear in the target-oriented quadrant d. They were at ceiling for biclausal sentences, but ranged in their selections of epistemic pictures for the monoclausal sentences (range = 0 – 3). Overall, children move from being off-centre in the adultlike direction along the x-axis at age 3, to preferring epistemic pictures for both sentence types at age 6. Among all children, only 1 fell into quadrant c (‘epistemic avoidant’), and only 5 fell into quadrant a (‘contrarian’). The youngest children fall primarily in the target-oriented (d) and epistemic-dominant (b) quadrants. Four-year-olds show mixed patterns (as in prior studies: Fond 2003; Heizmann 2006; Cournane & Pérez-Leroux 2020). By 5, most children fall into the epistemic dominant quadrant, and by 6-years-old, almost all do. The 6-year-olds are the only group with no overlap with adults.

3.2.2 Qualitative Results

Children sometimes gave informative responses to our prompts for why they chose the pictures they did for individual trials. These results are intended to supplement and provide insights into our quantitative measures, bearing in mind that responses were inconsistent, with some children offering several informative responses and others none, variably over age groups.¹⁸ Of the post-exclusion children: 2/7 of the 3-year-olds gave informative elaborations, 11/11 of the 4-year-olds, 13/14 of the 5-year-olds, and 9/12 of the 6-year-olds. What follows references these children only.

First, we got evidence that the children understood our pictures as intended. Many children, in various contexts, pointed to the source of necessity when explaining their choice of deontic pictures. For example, when asked what they see in the picture they chose after Penguin said Mede moraju da se okupaju ‘The bear cubs must take a bath’, one 3-year-old (3;09) said Vidim kako su se isprljali ‘I see how/that they got dirty’. When Penguin said Mede moraju da samelju orahe ‘The bear cubs must grind walnuts’,¹⁹ one 4-year-old (4;11) chose the deontic picture and responded by saying Da im je mama donijela orahe. Moraju se orasi otvorit. ‘That Mama brought them walnuts. The walnuts must be opened’ (impersonal, monoclausal, with embedded infinitive).

For epistemic pictures, most 6-year-olds pointed to indirect evidence as support for choosing epistemic pictures, for both biclausal (13) and monoclausal (14) sentences, even when they accurately repeated monoclausal sentences (15). Some described the pictures in a way compatible with having direct evidence (14), where the child says they see the bears bathing (Figure 4, Epistemic). In (13)–(15), children first respond to Kako znaš? ‘How do you know?’.

1. (13)
1. Child:

1. Tako
2. thus
1. što
2. that
1. ide
2. goes
1. voda
2. water
1. i
2. and
1. unutra
2. inside
1. su.
2. be.3pl.
1. i
2. and
1. vidim
2. see.1sg
1. odjeću.
2. clothes
1. ‘Because the water is running and they’re inside. And I see clothes.’                                 (S56, 6;07)

1. (14)
1. Child:

1. Jer
2. because
1. je
2. is
1. Pingvin
2. penguin
1. rekao
2. said
1. mede
2. bear.cubs
1. se
2. se
1. moraju
2. must.3pl.prs
1. okupat.
2. bathe.inf
1. ‘Because Penguin said the bear cubs must take a bath.’

1.  
1. Exp.:

1. I
2. and
1. šta
2. what
1. ti
2. you
1. tu
2. there
1. vidiš?
2. see.2sg.prs
1. ‘And what do you see there?’

1.  
1. Child:

1. Da
2. da
1. se
2. se
1. mede
2. bear.cubs
1. kupaju.
2. bathe.3pl.prs
1. ‘That the bear cubs are taking a bath.’                                                                                    (S19, 6;07)

1. (15)
1. Child:

1. Tako
2. thus
1. što
2. that
1. je
2. is
1. torta
2. cake
1. u
2. in
1. pećnici.
2. oven
1. ‘Because the cake is in the oven.’

1.  
1. Exp.:

1. A
2. and
1. šta
2. what
1. je
2. is
1. rekao
2. said
1. Pingvin?
2. penguin
1. ‘And what did Penguin say?’

1.  
1. Child:

1. Reko
2. said
1. je
2. is
1. mede
2. bear.cubs
1. moraju
2. must.3pl.prs
1. da
2. da
1. naprave
2. make.3pl.prs
1. tortu.
2. cake
1. ‘He said the bear cubs must make a cake.’                                                                             (S56, 6;07)

Children sometimes recast biclausal test sentences as monoclausal in their follow-ups. We saw 6 instances of this by four different 4-year-olds. And one of the 3-year-olds also recast biclausal sentences as monoclausal a couple times, as in (16). This type of recasting was less common among older children – 0/13 of the 5-year-olds did this, and 2/9 of the 6-year-olds. Children did this in follow-ups both when they had selected the target epistemic picture and the non-target deontic picture, though the latter was less common and not true of the 6-year-olds.

1. (16)
1. Exp.:

1. Kaže
2. says
1. Pingvin:
2. penguin
1. ‘Mora
2. must.3sg.prs
1. da
2. da
1. mede
2. bear.cubs
1. prave
2. make.ipf.3sg.prs
1. snješka.’
2. snowman.
1. Koja
2. which
1. je
2. is
1. to
2. that
1. slika?
2. picture
1. ‘Penguin says: “The bear cubs must be making a snowman.” Which picture is that?’

1.  
1. Child:

1. Pingvin
2. penguin
1. je
2. is
1. reko
2. said
1. da
2. da
1. moraju
2. must.3pl.prs
1. pravit
2. make.inf
1. snješka.
2. snowman
1. ‘Penguin said they must make a snowman.’                                                                (3;06)

Older children sometimes recast biclausal sentences as non-modal (17). Among the 6-year-olds, 4/9 recast biclausal sentences as non-modal, including both who also recast them as monoclausal on different trials. Two of the 5-year-olds, and one of the 4-year-olds also recast biclausal sentences as non-modal. Neither 3-year-old did this. The 4-year-old recast two monoclausal sentences as non-modal as well, after choosing the epistemic picture for both.

1. (17)
1. Peng.:

1. Mora
2. must.3sg
1. da
2. da
1. se
2. se
1. mede
2. bear.cubs
1. kupaju.
2. bathe.ipfv.3pl.prs
1. ‘The bear cubs must be taking a bath.’

1.  
1. Exp.:

1. Koju
2. which
1. je
2. is
1. sliku
2. picture
1. vidio
2. saw
1. Pingvin?
2. penguin
1. (…)
2. (…)
1. Kako
2. how
1. znaš?
2. know.2sg.prs
1. ‘Which picture did Penguin see? … How do you know?’

1.  
1. Child:

1. Zato što
2. because
1. je
2. is
1. reko
2. said
1. da
2. da
1. se
2. se
1. mede
2. bear.cubs
1. kupaju.
2. bathe.ipfv.3pl.prs
1. ‘Because he said that the bear cubs are taking a bath.’                         (6;03)

Summarizing our results, younger children chose fewer epistemic pictures than older children, and 3-year-olds are more adult-like than older children when hearing monoclausal sentences, showing weak discrimination by sentence type. Younger children sometimes recast biclausal sentences as monoclausal, but unlike the older child groups they rarely recast either biclausal or monoclausal sentences as non-modal. Older children chose mostly epistemic pictures when hearing both biclausal and monoclausal sentences, showing particularly non-adult patterns for picture selections for monoclausal sentences. When asked why they chose the epistemic picture, they refer to indirect evidence in the scene, and sometimes claim they see things not depicted in epistemic pictures, by inferring from indirect evidence of an event to the event itself. When asked what they heard, they rarely recast biclausal sentences as monoclausal, but sometimes recast both biclausal and monoclausal sentences as their non-modal counterparts. Recasting results patterns are summarized in Table 4.

Table 4

Counts of individuals by age group (with number of individuals who contributed) and type of sentence recasting produced (sentence heard > sentence produced).

Recast Type	3-year-olds n = 2	4-year-olds n = 11	5-year-olds n = 13	6-year-olds n = 9
BiCl > MonoCl	1	4	0	2
Modal > Non-Modal	0	1	2	4

4 Discussion

This study has explored how preschool children learn and navigate interpretations of flavour-variable modal verbs. In particular, we have tested what factors influence child modal verb interpretation as deontic or epistemic: input biases towards one flavour, syntactic constraints on flavour, and pragmatic inferences in young children. We explored these questions using a picture preference task with BCS morati ‘must’ sentences with preschool children and adults. This BCS study builds on prior work using the same task with English must with similar age groups. BCS morati and English must are both flavour-variable necessity modals, but syntactic distributions by flavour and input flavour biases differ between the two languages. As such, BCS helps us better understand relative influences on modal flavour development, for BCS itself and more broadly in comparison to prior results for English (Cournane & Pérez-Leroux 2020).

First, our results allow us to reject the input frequency hypothesis, that BCS children will undergenerate epistemic interpretations of morati relative to adult controls and English-learning peers because, unlike must, morati is used primarily as deontic (and other root sub-flavours) in their input. At no age in our 3-to-6-year-old sample do children prefer deontic interpretations over epistemic for morati, and children show an increasing likelihood to select epistemic pictures as they age. The failure of the input frequency hypothesis to explain non-adult child biases is underlined by the comparison of BCS and English results (Cournane & Pérez-Leroux 2020): English children hear must primarily with epistemic uses in their input, but there was a significant overall deontic preference in the English 3-year-old group. The older children in both our BCS study and in the English study showed a preference for epistemic pictures, especially notable for sentence types for which adults prefer deontic pictures (monoclausal in BCS, must + bare verb in English). For English, the older children’s epistemic bias was possibly explained by input frequency bias, as they may gain increased awareness that must is used primarily as epistemic in their speech community (Tagliamonte & D’Arcy 2007). For BCS, this explanation for the non-adult epistemic bias found in older preschool children does not hold, casting doubt on this explanation for English.

Second, we found weak support in the youngest child group for the syntactic bootstrapping hypothesis, that children will be adult-like for structure-to-meaning mapping for morati: they will choose fewer epistemic pictures for monoclausal constructions than for biclausal, because, unlike must, morati has categorical syntactic cues for flavour interpretation. Our 3-year-old group, though small after exclusions, showed discrimination by sentence type in the adult-like direction. The 4-, 5- and 6-year-old children showed no discrimination by sentence type. In the English study, no child group showed any distinction by sentence type (unlike adults). Thus, with respect to syntactic bootstrapping we see noteworthy differences among the youngest children: BCS 3-year-olds show some sensitivity to syntax, English 3-year-olds do not (deontic bias). Our effect with 3-year-olds is small and further work should aim to replicate or corroborate.

Finally, we find support in the older child groups for the pragmatic inference hypothesis, that children will over-select epistemic pictures for monoclausal constructions relative to adult controls. Our 5- and 6-year-old groups chose somewhat more epistemic pictures for biclausal sentences than younger children, thus becoming more adult-like. But, at the same time, older children also chose more epistemic pictures for monoclausal sentences than younger children, becoming less adult-like, and this growth appears to be at a somewhat faster and more significant rate. Given the results for the syntactic bootstrapping hypothesis, that greater change for epistemic selections for monoclausal sentences is expected – as children show some distinction by sentence type at age 3 (fewer epistemic selections for monoclausal sentence), then the trajectory to becoming epistemic-dominant regardless of sentence type involves a greater rate of change towards epistemic selections for the monoclausal sentences. To become adult-like, after age 6 we expect children to gradually fall less for the inference and show distinction by sentence-type.

Considering the qualitative results, older children often pointed to the indirect evidence in the pictures (e.g. clothes on the ground by the bathtub) to support their picture choices, including for when the sentence stimuli was monoclausal. And, older children sometimes recast both sentence types as non-modal equivalents.²⁰ This may relate to the felicity of both in inferential situations: when one infers It must be raining on the basis of indirect evidence, one can mark that inference with must, or just say It is raining (assuming clear evidence) (von Fintel & Gillies 2010).

Most notably, the BCS results suggest that falling for pragmatic likelihood inferences is the most generalizable explanation for the older preschooler (age 5 and 6) epistemic picture-selection bias, observed previously in English. We see the same pattern in both languages: older preschoolers are more adult-like than younger children for the sentence-type that keys epistemic interpretations (English: must + aspect-marking, BCS: biclausal), but over-select epistemic pictures compared to adults and younger children for the sentence type that keys deontic interpretations (English: must + bare verb, BCS: monoclausal). This similarity obtains despite syntactic and input differences for must and morati. Unlike in English, where input biases, syntactic ambiguity, and pragmatic inferences all could contribute to increased epistemic picture selection, BCS allows us to tease these factors apart. We find the pragmatic story satisfying for the prior English results, as English-learning children command both flavours for modal verbs in production by age 3 for most children, much earlier than we find the epistemic bias (age 5 and 6).

How does this inferencing work in our task? An obligation normatively implies that the obligatory action is realized. This inference, that an event which is “necessarily obliged to happen in the future is also obliged to happen in the present” (Invited Inference: Traugott & Dasher 2001: 130), is available to adults speaking both English and BCS. Our epistemic pictures do not directly show the modal prejacent φ (i.e. The bears take a bath) occurring in the present, but they do show strong indirect evidence for φ. Indirect evidence scenarios nonetheless communicate that φ is true (von Fintel & Gillies 2010, i.a.; see also Lassiter 2016; Mandelkern 2019 for further discussion about how and why these scenarios support both epistemic must φ and plain φ). Thus, from the deontic ‘X must φ’, the reasoning path to our epistemic pictures is available, even if the modality in ‘X must φ’ is unambiguously interpreted as only deontic (as in our BCS monoclausal sentences). This work teaches us more about how pragmatic meanings associated with modals play out in development, adding to existing knowledge about modal pragmatics (i.e. scalar implicatures, Noveck 2001, i.a.; speaker meanings, Hacquard & Lidz 2016, i.a.).

Cournane & Pérez-Leroux (2020) argued that their results for the older child group may mask preschoolers learning from the syntax, as English shows modal auxiliaries overtly above grammatical aspect in the surface order (must > progressive/perfect marking), potentially feeding a bias for the higher (epistemic) interpretation of must (as in (4b,c)): Learning that must is higher than aspect may carry over to sentences where the syntax provides no overt cue (as with 4a).^21,22 Our BCS results for 3-year-olds lend some weight to this idea. BCS children were more adult-like at 3 than in the older groups. By age 5, and more so by age 6, children prefer epistemic pictures for both sentence types. We suggest that a plausible explanation for this change is that children never lose the sensitivity to distributional differences for morati, but that pragmatic changes gradually mask their adult-like, deontic, interpretations for monoclausal morati sentences. This continuity for deontic interpretations is in line with the corpus evidence that shows root modal verb uses occur from age 2 (in BCS, Veselinović & Cournane 2020; cross-linguistically, see Papafragou 1998; Hickmann & Bassano 2016).

We suggest that in both languages, the pragmatic layer obscures adult-like syntactic comprehension, as children overcommit (relative to adults) to the inference of the type If they must p, they likely do p. What causes this pragmatic change? One possibility is development in child understanding of normative behaviours (as outlined in §2.4). These social-conceptual changes develop in parallel to language and show some differences in facets of normative reasoning between younger and older preschoolers.²³ This inference-based explanation for our results makes it unsurprising that adults also sometimes chose epistemic pictures for monoclausal sentences, despite these not having epistemic interpretations in BCS (on acceptability tasks, Veselinović 2019). The child inferences are consistent with adult inferences in type, what differs is that children seem to either generate these inferences more than adults (deontic obligations generate likelihood inferences more for children than adults), or fall for them more easily than adults (deontic obligations generate likelihood inferences similarly for children and adults, but children are more likely to assume these inferences aren’t cancellable).

If the above is true, why are the 3-year-old groups different in English and BCS? We suggest that English-learning children may take a little longer to appreciate the temporal-aspectual cues to modal verb flavour interpretation in comprehension than BCS-learning children. While BCS and English modal verbs are similarly flavour-variable, they differ in their syntactic types and the informativity of their complements for giving away modal flavor for input usage. The explicit, categorical, cues from syntax available to BCS children – agreement marking on the modal itself, structural word-order differences, verbal aspect class differences – may help them bootstrap into the syntactically-constrained interpretation differences for morati in comprehension. The syntactic signatures of root versus epistemic uses of morati are similar to those differentiating desire versus belief attitude verbs (i.e. want vs. think) (Progovac 1993 for BCS), which have also been shown to help young children differentiate these modal meanings (Hacquard & Lidz 2016; Harrigan et al. 2016). In English, working out modal flavour from syntactic distribution may be more difficult as modal auxiliary verbs are invariant in form, occur in the same position regardless of flavour, and are usually followed by a bare verb. van Dooren et al. (2017; 2019) assessed English and Dutch input for whether, in principle, the tense and aspect semantics (Condoravdi 2002) in the clause could give away a modal verb’s intended meaning, since root modals tend to be restricted to future-orientation (Werner 2006; Klecha 2016; Rullmann & Matthewson, 2018).

If BCS children have more robust cues to modal verb flavour, why then does the BCS corpus show no child epistemic uses of modal verbs before age 4 while English corpora do? There are a few viable reasons for this. One possibility is sampling. Epistemic uses of morati ‘must’ and moći ‘can’ are proportionally rarer in the input than for English modal verbs like must or have to (Veselinović & Cournane, 2020), suggesting that they are rarer in BCS speech in general (note, speakers have other grammatical means of expressing epistemic modality in both languages). Young children use fewer modals overall than adults (as seen in corpus studies), so the likelihood of sampling a child epistemic use of morati or moći in BCS may simply be exceedingly low (unlike English). Alternatively, the lack of epistemic morati or moći in the child corpus study may be due to production-specific demands. Considering the qualitative results here, younger children sometimes recast biclausal stimuli as monoclausal. Perhaps these young children have particular difficulty planning or producing these constructions. This may be linked to the broad observation that comprehension often precedes production, and to cross-linguistic patterns of increased productive ability with biclausal structures after age 4 (see de Villiers & Roeper 2016). Whatever the cause, the BCS 3-year-old results merit further exploration in future work, as prior work on modal verbs in corpus studies of children 3-years-old and younger (§2.2), and other experimental work on modal flavour comprehension looking at 3-year-olds all show root biases (§2.3).

We used filler accuracy as an exclusion criterion, which led to the exclusion of primarily younger participants, most notably half the 3-year-old group. This was not particularly surprising to us, as 3-year-olds are a difficult group to work with for explicit behavioural tasks like ours – the task is demanding for attention and working memory. We selected our fillers as predicates which were invariant in meaning (unlike modal verb morati) but syntactically similar, landing on the following verbs: voljeti ‘love’, mrziti ‘hate’ and htjeti ‘want’. These are predicates of personal taste and desire, and their syntax is similar to the deontic construction for morati (1a). To succeed on these items the participant only had to attend to the emotional state of the bears in the picture. We acknowledge that it’s possible that there is a confound here: the 3-year-olds excluded may have been precisely those who have difficulty comprehending complex clause constructions (A.S. White, pc). It would be more telling if the filler verbs patterned with the epistemic construction for morati, as that kind of biclausal construction is also rare in the input (for modals, not necessarily for verbs in general) and children’s speech (in general, with a complete absence for modal verbs) for 3-year-olds (Veselinović & Cournane 2020). However, even the root modal construction, like the voljeti ‘love’, mrziti ‘hate’ and htjeti ‘want’ constructions (e.g. Mede vole da jedu čorbu ‘The bears want to eat stew’), involve a kind of sentential embedding (not of a CP, like in BCS epistemic constructions). Our exclusion may have thus filtered out those children who have general difficulty comprehending sentential embedding (TP or larger) as yet, rather than simply those who had difficulty with the task (e.g. attention or interpreting our images). This possibility further underscores the merit of future work on BCS-learning 3-year-olds (or children of other languages with similar modal verb patterns).

The 4-year-olds in our study show no discernable pattern: except perhaps that none of them are deontic-biased, many appear at chance and no effects pertaining specifically to this group are significant. This is consistent with prior studies looking at this age group in Spanish (Fond 2003); English and German (Heizmann 2006) and English (Cournane & Pérez-Leroux 2020). Our best guess is they are heterogenous for the phenomenon (some are like 3-year-olds, some like 5-year olds; note that in English, this pattern is more discernable, Figure in fn.17), and on top of that they are also, like the 3-year-olds, not yet very good at the task, so there is general inaccuracy in both directions.

Finally, the English work by Cournane & Pérez-Leroux (2020) and our pragmatic inference hypothesis to explain both the English and BCS results, are both inspired from language change theory. Modal verbs are known to change from root flavours to epistemic in diachrony, as discussed in §2.4. To explain this repeated, unidirectional, modal change, Traugott and Dasher (2002)’s Invited Inferencing Theory of Semantic Change, argues that conventionalization of implicature (pragmatic inference becoming conventionalized, or semantically encoded) is a driving force which creates new meaning representations for modal verbs (for a similar idea with indefinites, see Aguilar-Guevara et al. 2010). This model aims to explain why and how root modals have gained additional epistemic meanings in diachrony.

Proponents of this approach argue that this Conventionalization of Implicature is enacted by adults (Traugott & Dasher 2002: 34). Children are not considered pragmatically sophisticated enough to be innovators (Traugott & Dasher 2002: 41), rather innovations are linked to “communicative-expressive need” (Nerlich & Clarke 1992: 127) and come from online adult usage via creative coinages.²⁴ However, our results and those from Cournane & Pérez-Leroux (2020) suggest that from age 5 children may be especially prone to generating or overcommitting to these inferences, though adults seem to do so too, to a lesser extent. An important way that children differ from adult native speakers is that children are still in the process of working out and refining the semantic representations for their input language modals. For modal words, semantic and pragmatic meaning are intertwined in the input situations in a way that may make learning what is semantically encoded, as opposed to only implied, especially challenging. In light of this, the semantic encoding process may well be imperfect, with implied meanings sometimes grammatically encoded as truth-conditional (contra the input grammar) as part of the normal learning process of abducting word meanings from usage in contexts.

Our child results are in line with theories of language change which argue that children learning their first language(s) are an intuitive place to look for the driving forces of regular language changes, since children must build their grammars on the basis of indirect evidence from their input (e.g. Lightfoot 1979, 1999; Roberts & Roussou 2003; van Gelderen 2004; Cournane 2017). Of note, Cournane (2017) argues that input-divergent representations that occur after age 4 are the best candidates for actually catching on as changes in a language, as this is when children begin to care more about how their peers talk (Labov 2001, i.a.) and may reinforce each other’s input divergent grammars. Moreover, as discussed in §2.4, older preschool children may be especially prone to “falling for” the inference from obligation to likelihood due to differences in how these children reason about rules and normative behaviours, compared to younger preschool children and adults: older preschool children appear to be more categorical and deterministic than adults when it comes to reasoning associated with deontic modality. Our analysis is also consistent with this conceptual developmental pattern.

BCS and English child data on extant flavour-variable modals give only proof of concept – this inference occurs online, and more so in 5- and 6-year-old children than adults. What matters for the child-driven change approach is that children differ from adults in the direction of known patterns of change for grammatical elements of the relevant type. BCS, however, does not show us clear evidence for change as morati already has both modal interpretations (and as long as morphological cues remain clear the situation is probably quite stable). Further studies should look for this likelihood inference with modal verbs with only root meanings (i.e. root attitude verbs, see Lightfoot 1979; Traugott 2006; Hacquard 2013), to see if children show the same pattern.

5 Conclusion

Learning to interpret and use flavour-variable modal verbs like morati ‘must’ presents children with a challenging mapping task: they must assign more than one non-actual meaning to the same word. How do children learn and navigate modal flavour for these modal verbs in their input languages? How does modal verb interpretation develop over the preschool years? And, what factors most influence children’s modal verb interpretations as deontic vs. epistemic? The BCS study we presented has helped address these questions, and adds to results known from the same task in English (Cournane & Pérez-Leroux 2020). While both languages have flavour-variable necessity modals, must is primarily epistemic in the input, while morati is primarily deontic, and English morphosyntax only partially constrains interpretation, while BCS provides unambiguous morphosyntactic cues to both modal flavours. Our results showed that the youngest BCS children (age 3) were the most adult-like for distinguishing between modal interpretations by syntactic constructions, suggesting BCS’s clear syntactic cues to modal flavour are helpful for early child comprehension. Older child groups in our BCS study showed a strong epistemic bias, including for morati sentences which are deontic for adults. The result for older children was the same as for the English study with must, despite differences in input-frequency and structural cues correlated with flavour (deontic vs. epistemic).

We have argued that pragmatic inferences from deontic to epistemic interpretations – if they must p, they likely p – play an essential role in children’s behaviour in modal verb interpretation in the older preschool years. While these children understand deontic modal sentences, they infer that obligations hold and can select epistemic depictions to illustrate deontic sentences. The existence of this inference from deontic to epistemic was first proposed as an explanation for the commonly observed historical change in the meaning of modal verbs. Our results provide proof of concept that this mechanism for semantic change could be driven primarily by older preschool children.

Data Accessibility Statement

The full list of stimuli (test items and fillers) in BCS with English translations, anonymized data spreadsheets, and R scripts are available in the folder “BCS Experiment” at www.osf.io/v9ure/.

Abbreviations

BCS: Bosnian/Croatian/Serbian

nom: nominative

prs: present

ipf: imperfective

inf: infinitive

pfv: perfective

pl: plural singular

sg: singular

subj: subject

Ethics and consent

All consenting and assenting processes adhered to university IRB protocol (NYU IRB-FY2017-594), and local daycare administrative approval in Sarajevo, Bosnia & Herzegovina.

Funding information

This work was supported in part by National Science Foundation (NSF) grant BCS#1551628, “Acquiring the language of possibility”.

Competing interests

The authors have no competing interests to declare.