2.1 Relevant theories on the mass-count distinction

The distinction between mass nouns, such as dough, and count nouns, such as ball, is syntactically expressed in many languages, including English and Dutch. Such languages are referred to as number-marking languages. In these languages the mass-count distinction affects the distribution of singular vs. plural morphology. Following Chierchia (1998) and Krifka (2009), we assume that mass nouns in number marking languages have the syntactic properties listed and illustrated for Dutch and English in (1):

Besides typical mass nouns such as dough and typical count nouns such as ball, there are so-called flexible nouns, which sometimes behave as mass nouns, and sometimes as count nouns, as illustrated in (2) for Dutch:

The noun touw (‘rope’) is mass if it is quantized by a measure phrase, as in (2a), while it is count if it is preceded by a numeral and followed by a plural morpheme, as in (2b).

There is also a semantic side to mass and count. Quine (1960) introduced the term cumulativity and argued that mass nouns refer cumulatively. The same mass noun can refer to either one (small) amount of substance, or to a combination of several amounts of the same substance. For instance, two amounts of dough put together still results in dough. Obviously, this does not apply to singular count nouns: the combination of two cars does not result in car. A second important notion regarding the semantics of mass and count is divisivity (Cheng 1973). Mass nouns are divisive, but count nouns are not: parts of dough are also referred to as dough, but parts of a car are not referred to as car. These observations led to the suggestion that the distinction between mass and count is based on individuation: count nouns quantify over individuals, mass nouns do not (Quine 1960; Gordon 1985; Bloom 1994 a.o.).

Nevertheless, such an account does not cover all the facts. There is no one-to-one mapping between, on the one hand, cumulative, divisive (non-individuated) objects/substances and mass nouns, and on the other hand, non-cumulative, non-divisive (individuated) objects and count nouns. For example, mass nouns and plural count nouns are both interpreted as cumulative, and can thus not be distinguished by means of cumulativity (Pelletier 1979; Gillon 1996). Furthermore, nouns such as furniture, cutlery, and silverware quantify over individual items, but are syntactically mass.

Gillon (1996) makes an attempt to solve this problem by proposing that mass nouns are lexically unspecified with respect to individuation. From this perspective, count nouns refer to individual items and are specified as such in the lexicon, whereas world knowledge determines the reference of mass nouns. For example, whereas both furniture and dough are syntactically mass nouns, and therefore lexically unspecified, our experience tells us that furniture refers to distinguishable, individual items, but dough does not. Chierchia (1998) elaborates on this lexical account by claiming that all mass nouns actually refer to individuals but that it is the plurality value of the noun that distinguishes between count and mass.

Although lexical accounts such as the ones described above may be empirically adequate, we believe they are not the most elegant, as the relevant distinctions are merely stipulated in the lexicon. Furthermore, it is not always the case that a typical count noun such as table gets a count interpretation (see (3a) below), or that a typical mass noun such as water receives a mass interpretation (see (3b) below). Making use of the observed syntactic differences between mass and count, Borer (2005) proposes that the interpretation of a noun as mass or as count follows from its syntactic environment: count syntax (such as an indefinite determiner or a plural morpheme) renders a count interpretation, while the absence of such syntax yields a mass interpretation. In other words, all nouns can be either count or mass, even nouns that are conventionally referred to as count (such as table), and nouns that are conventionally referred to as mass (such as water). This is illustrated in (3):

According to Borer (2005), the fact that some nouns are more often referred to as mass nouns and other nouns as count nouns is due to convention. In this sense, all nouns are flexible between mass and count, because each noun has a “bare” entry in the lexicon, unmarked for the mass-count distinction. This bare entry receives the correct mass or count value structurally. Nouns become count nouns when they are embedded in count syntax. In all other cases, nouns have a mass interpretation. Nonetheless, it is not clear in this account how nouns such as furniture, that are syntactically mass (This store has many different types of furniture), receive an interpretation based on individuation.

Experimental evidence for the hypothesis that nouns become mass or count only by virtue of their syntactic environment comes from a study by Barner & Snedeker (2005), who show that for flexible nouns (such as rope and chocolate), it is not world knowledge, but the syntactic context of nouns (mass: a piece of rope or count syntax: three ropes) that yields a mass or a count interpretation. The results of Barner & Snedeker’s (2005) study leads them to propose that the distinction between mass and count is due to the individuation entailments of the noun phrase. A feature [+individual] licenses this individuation. The [+individual] feature is invoked either syntactically, by means of articles, plural morphology, numerals, etc. (i.e. count syntax), or lexically, as in nouns such as furniture, for which the [+individual] feature is part of their lexical denotation. From this perspective, regular mass nouns cannot individuate because the [+individual] feature is available neither syntactically, nor lexically. Barner & Snedeker’s analysis regards the mass-count distinction as a phenomenon mainly determined by syntax, except for object-mass nouns such as furniture, which are lexically specified for individuation. The suggestion that mass is distinguished from count through syntax was also developed in Bale & Barner (2004), who, similar to Borer (2005), argue that root nouns are underspecified with respect to mass-count (or “countability”). Root nouns then receive their mass or count specification when they combine with a functional head via a syntactic operation.

In this study we follow Borer (2005), Barner & Snedeker (2005) and Bale & Barner (2004) in assuming that nouns become mass or count via morphosyntactic means. The syntactic switch from mass to count and vice versa is easiest for flexible nouns such as rope, or chocolate, because our world knowledge or convention does not push them into a particular direction. Classical count nouns such as ball, and classical mass nouns such as dough “suffer” from convention and/or world knowledge, as ball is usually used with count syntax and refers to an individual item, whereas this is not the case for dough. Finally, for object-mass nouns such as furniture, we adopt Barner & Snedeker’s (2005) and Bale & Barner’s (2004) account that such nouns carry a lexical [+individual] feature. Our approach is schematized in Table 1.

In the next section we discuss some previous studies on the acquisition of the mass-count distinction in other languages than Dutch.

2.2 Previous acquisition studies on the mass-count distinction – children with typical development

English-acquiring TD children have been shown to be sensitive to the mass-count distinction relatively early. For instance, Gordon (1985; 1988) reports that 2–3-year-old children acquiring English apply the plural morpheme only to count nouns, thus obeying pluralization restrictions. Gathercole, Cramer, Somerville & Jansen Op De Haar (1995) showed 3 and 4-year-olds children unfamiliar objects and gave these objects novel names. They found that if the object was used in count syntax, the children also used the novel noun for new items of the same shape (but different material) (a blicket → blickets). In contrast, if the item appeared in mass syntax, the children extended the noun to a new item consisting of similar material (but a different shape), i.e. some blicket → blicket. Soja (1992), Soja, Carey & Spelke (1991; 1992) report similar results for even younger children acquiring English, namely between the ages of 2;0–2;6. However, these studies did not test the full range of mass and count nouns as presented in Table 1.

A more recent study by Barner & Snedeker (2005) fills this gap by experimentally investigating classical count and mass nouns as well as flexible and object-mass nouns. Basing themselves on Gathercole (1985), Barner & Snedeker developed a Quantity Judgment Task (QJT) to evaluate 4-year-old children’s knowledge regarding the mass-count distinction in English.

In their first experiment, Barner & Snedeker (2005) tested object-mass nouns (furniture, N = 4), and compared these to classical mass nouns (toothpaste, N = 4) and classical count nouns (shoes, N = 4). The experimenter showed the participant two pictures of two characters each having a certain amount of items or substance, and then asked which character had more. It is important to note here that one character had more of the relevant noun in terms of number of individual items, whereas the other character had more of the noun when regarding overall volume. For instance, in the experimental item testing the object-mass noun silverware, one character had two small forks and two small knives while the other character was portrayed with one large fork and one large knife. Moreover, in all experimental items the overall volume of the two large objects together was larger than the overall volume of the higher number of small items of the same kind. The object-mass, the classical count and the classical mass conditions are illustrated in Figure 1.

Figure 1

Illustration of Barner & Snedeker’s (2005) first experiment on object-mass, classical count and classical mass conditions, respectively (Figure 1 in Barner & Snedeker 2005: 50).

Barner & Snedeker (2005) tested 12 English-acquiring children (age 4;0–4;6, M = 4;3) and 16 English-speaking adults. Their results show that in the classical count (shoes) and object-mass conditions (furniture), adults virtually always correctly base their judgments on number, namely 94% and 98%, respectively. In contrast, number-based judgments were never encountered in the classical mass condition (i.e. 100% of judgments were correctly based on overall volume). The children performed adultlike on the classical count and object-mass conditions: they correctly gave number-based judgments at 98% and 92%, respectively. In the classical mass condition they (incorrectly) judged classical mass nouns based on number at a rate of 40%, resulting in an accuracy rate of 60%. Although this is a substantial amount of non-adultlike responses, Barner & Snedeker do not elaborate on this. Instead, they focus on the significantly higher proportions of number-based judgments in the object mass and classical count conditions than in the classical mass conditions, and conclude that English-acquiring 4-year-olds are sensitive to the mass-count distinction.

Barner & Snedeker’s (2005) second experiment assessed quantity interpretations of flexible nouns (e.g. string(s), stone(s)) in both count and mass syntax. In this experiment, one character had one large item while the other character had three small objects of the same kind. This is illustrated in Figure 2.

Figure 2

Illustration of Barner & Snedeker’s (2005) second experiment, on flexible nouns (Figure 4 in Barner & Snedeker 2005: 54).

Two new groups of participants were tested, comparable in size and age to the participants of the first experiment: 12 English speaking children (age 4;0–4;5, M = 4;2) and 16 adults. The adult and child participant groups were each split into two, such that one subgroup received all flexible nouns in mass syntax (i.e. stones, strings, papers) and the other subgroup was presented with all items in count syntax (i.e. stone, string, paper).

The results of the second experiment show that in the count syntax condition, the adults virtually always based their judgments on number of individual items (97%), but rarely if the noun was presented in mass syntax (3%). The child results show a similar distinction: when flexible nouns appear in count syntax (i.e. with a plural morpheme), the children correctly base their judgments on number 95% of the time, vs. 25% of judgments incorrectly based on number when the flexible noun appears in mass syntax (i.e. without a plural morpheme). Thus, the children reach only an accuracy rate of 75% versus 97% of judgments accurately based on overall volume in adults. Again, Barner & Snedeker do not comment on this substantial (non-adultlike) rate of number-based judgments of flexible nouns in mass syntax by the children.

Overall, Barner & Snedeker’s (2005) study shows that English-acquiring 4-year-old children are sensitive to the mass-count distinction. However, note that Barner & Snedeker did not test children older than age 4, and that the 4-year-olds did not perform adultlike yet in all conditions. As noted above, they incorrectly base almost 40% of their quantity judgments on number in the classical mass condition (vs. 0% for the adults), and 25% in the flexible mass condition (vs. 3% for the adults). These results suggest that the acquisition of mass noun syntax takes more time than the acquisition of count noun syntax, leaving open the developmental question as to when exactly English-acquiring children become adultlike in their mass-count interpretations. In contrast, the 4;0–4;6-year-old English-acquiring children do perform adultlike on the object mass nouns. Barner & Snedeker explain this by assuming that object mass nouns are already specified with a [+individual] feature in the lexicon. As for Barner & Snedeker’s methods, we would like to point out that the number of participants was low, and that the participant groups for the two experiments were not the same. Moreover, the participants were split across conditions in their second experiment (on the flexible nouns), making the participant groups even smaller, and rendering it impossible to compare the interpretation of flexible nouns in count and mass syntax within subjects, and to the classical conditions. In addition, Barner & Snedeker’s visual materials are potentially ambiguous: the items that are intended to be smaller are identical to the large objects, and may therefore be interpreted as just being farther away, causing the participant to think that the picture with the higher number of items is always more. We should therefore interpret their results with caution.

Hacohen (2009) improved Barner & Snedeker’s materials for her study on the mass-count distinction in Hebrew-acquiring children. She manipulated the pictures such that all small objects not only differed from the large objects in size, but also in some other property, preventing the smaller objects from being interpreted as identical to the larger objects but farther away. Moreover, objects were presented on a plate, providing depth and perspective to the objects. In addition, the drawings were replaced by photos, making the objects clearer. Subsequently, Van Witteloostuijn (2013) and Van Witteloostuijn & Schaeffer (2014) further developed Hacohen’s QJT, by adjusting some problematic items from Hacohen’s task (as witnessed by a pilot study with Dutch adults) and adding more experimental items. This renewed QJT had the same conditions as Barner & Snedeker (2005), but now with 4 items per condition (except for the flexible mass and count nouns, which had 12 items each). All items were randomized in one experiment and administered to one and the same group of Dutch acquiring children of different ages (N = 88, aged 4;1–12;8), making it possible to examine performance across ages, determine age of acquisition, and compare the interpretation of the different types of nouns within the same participants. Table 2 presents an overview of Van Witteloostuijn’s (2013) and Van Witteloostuijn & Schaeffer’s (2014) results (columns 2 through 6) and compares them to Barner & Snedeker’s (2004) results for English-speaking children in the first column. Although Van Witteloostuijn (2013) and Van Witteloostuijn & Schaeffer (2014) originally grouped all kindergarten-level children together (4;1–5;10), we split this group into two, so as to obtain a better comparison to the English data. Note, however, that the youngest age-group (4;1–4;6) only contains three children, which is why we also added a column with the original data of all the Dutch kindergarten-level children collapsed (4;1–5;10).

Van Witteloostuijn’s (2013) and Van Witteloostuijn & Schaeffer’s (2014) results show that Dutch-speaking TD children from age 6 onwards make a clear distinction between judgments based on volume in the classical and flexible mass conditions (21% and 23% of judgments incorrectly based on number, respectively) and judgments based on number in the classical and flexible count conditions (93% and 82% of judgments correctly based on number, respectively). As for the younger children, examining first the data of the entire kindergarten group, the Dutch 4;1–5;10-year-olds perform randomly on the flexible count and mass and classical mass conditions (flexible count: 58%, flexible mass: 54%, and classical mass: 52% of judgments based on number), but well above chance on classical count and object mass nouns (81% of judgments correctly based on number in both conditions). A priori, the numbers in Table 2 may suggest that English-speaking children acquire the mass-count distinction earlier than Dutch-speaking children. However, some caution is in place here. If we compare the youngest Dutch-speaking children (4;1–4;6, but, N = 3!) to their English-speaking age-mates, we see that they perform more similarly to the English-speaking 4;0–4;6-year-olds than the 4;7–5;10-year-old Dutch-speaking children. This may indicate a U-shaped learning pattern which occurs when lexical and rule-based knowledge interact, something that may well be the case in the acquisition of the mass-count distinction. If this U-shaped pattern turns out to be real, older English-acquiring children may also perform slightly more poorly than the young English-acquiring group tested by Barner & Snedeker (2005), and not be so different from the Dutch-acquiring children after all. However, note that the youngest Dutch-speaking group (4;1–4;6) consist of only three children, implying that we really cannot draw any firm conclusions from their scores. Recall furthermore our reservations regarding Barner & Snedeker’s original materials, small numbers of participants, and limited age groups, and that the English-acquiring 4-year-olds in Barner & Snedeker’s study were not adultlike in their mass noun interpretations. More research on 4;1–4;6-year-old Dutch-acquiring children and on 4;6–6-year-old English-acquiring children is clearly needed. If such research showed that English-speaking children acquire the mass-count distinction earlier than Dutch-speaking children, an explanation could be that syntactic cues that potentially distinguish mass from count are more ambiguous and rarer in Dutch. For example, Dutch has fewer quantifiers than English that differentiate between mass and count nouns (e.g. veel (‘much/many’), weinig (‘little/few’), and minder (‘less/fewer’)). Moreover, it is not unusual to combine a Dutch mass noun in mass syntax (so without a plural morpheme) with a numeral in Dutch (e.g. twee melk (lit. ‘two milk’)). For a more detailed analysis of the cross-linguistic differences in (the acquisition of) the mass-count distinction see Van Witteloostuijn (2013).

One clear conclusion that can be drawn from the data in Table 2 is that both English and Dutch-speaking children acquire count nouns before mass nouns. Van Witteloostuijn (2013) and Van Witteloostuijn & Schaeffer (2014) argue that it is the overt (plural) morphology that accelerates the acquisition of count nouns, as opposed to mass nouns, which have no such overt morphosyntax. The more overt triggers there are in the input, the earlier the type of noun is acquired. Although the acquisition of number morphology (which is acquired around age 3, see Schaerlaekens 1980; van Wijk 2007) is a necessary requirement for the acquisition of the mass-count distinction, it is not sufficient. Children also need to learn that count nouns can have an indefinite determiner, while mass nouns cannot, and that count nouns can be directly preceded by a number, while mass nouns need a measure phrase to quantify over them. Furthermore, as Barner & Snedeker (2005) point out, they need to know what specificity is: whereas the use of a noun in count syntax (i.e., in the presence of a plural marker) clearly entails a reference to individuals, the use of a noun in mass syntax (i.e., in the absence of a plural marker) is unspecified regarding individuation. Thus, to correctly differentiate mass from count nouns, children need different pieces of knowledge. All these properties of count and mass nouns need to be properly clustered and mapped to the correct interpretation, which is why it may take some time to fully acquire the mass-count distinction. The fact that count nouns have many more overt cues in the input (plural morphology, indefinite determiners, numbers) than mass nouns may explain why, overall, count nouns are acquired before mass nouns.

For the Dutch-acquiring children, the distinction between classical count and mass nouns kicks in earlier than the distinction between the flexible count and mass nouns. For the English-acquiring children this is hard to tell, as the children participating in Barner & Snedeker’s (2005) classical mass-count experiment were not the same as the ones participating in the flexible mass-count experiment. Van Witteloostuijn (2013) and Van Witteloostuijn & Schaeffer (2014) argue that the earlier acquisition of the classical mass-count distinction is due to the additional, converging evidence from convention (Borer 2005) in the input, which distinguishes classical count from mass nouns (dog is usually and most frequently used in count syntax, whereas dough is usually and most frequently used in mass syntax). Crucially, the notion of convention relates to frequency of use and world knowledge, but not to lexical meaning. If nouns such as dog were lexically marked for a number-based meaning, they should always be used as a count noun, which is clearly not the case (see Borer’s 2005 example There is dog in the soup).

Finally, object-mass nouns are acquired early by both English and Dutch-speaking children. Van Witteloostuijn (2013) and Van Witteloostuijn & Schaeffer (2014) adopt Bale & Barner’s (2004) claim that object-mass nouns have a lexical [+individual] feature. They propose that as soon as the relevant noun is acquired, this lexical feature is in place and gives rise to a correct number-based interpretation from early on.

Overall, we propose the following acquisition pattern for TD children in English as well as in Dutch. Initially, all nouns can be interpreted as either referring to individual items or to unbounded volumes or substances. By attending to the input, children collect evidence from syntax, lexicon and convention/world knowledge to learn that some nouns are more likely to be interpreted as referring to individual items with boundaries (count) or as referring to unbounded volumes or substances (mass). Classical count nouns are acquired first, because overt syntax and convention/world knowledge provide strong, converging evidence. Object mass nouns are acquired early, too, because the input is non-ambiguous: they virtually never come with overt count syntax, and always refer to a collection of individual items, reflecting their lexical [+individual] feature. The input provides less direct evidence for (classical) mass nouns, because of the lack of overt morphosyntax, causing a delay in acquisition. Finally, the flexible mass-count distinction is acquired after the classical mass-count distinction because the morphosyntactic evidence in the input is not strengthened by evidence from convention.

Most important to the present discussion, the studies described above demonstrate that by the age of 6, TD children are sensitive to the mass-count distinction and are able to interpret the morphosyntax determining this distinction, at least in English and Dutch. Nevertheless, performance on the QJT, and thus sensitivity to the mass-count distinction continues to develop in later childhood. The question is what sort of knowledge children age 6 and up with SLI have of the mass-count distinction. Before we continue to our hypotheses and predictions regarding the mass-count distinction in children with SLI, let us briefly describe the relevant details of SLI.

2.3 Specific language impairment

Specific language impairment is a developmental language disorder that affects around 5–7% of children (Tomblin et al. 1997). Children with SLI do not form a homogeneous group: the disorder can affect different subdomains of language and these effects can be of varying severity (Leonard 2014; Novogrodsky 2015). One area that is typically influenced by the impairment is morphosyntax and the ability to comprehend and produce grammatical morphemes. These grammatical morphemes include those that indicate tense, number marking and subject-verb agreement (Rice, Wexler & Cleave 1995; Clahsen, Bartke & Göllner 1997; de Jong 1999; Wexler, Schaeffer & Bol 2004; Rispens & Been 2007; Spoelman & Bol 2012). For example, Clahsen et al. (1997) compare 9 English-speaking children with SLI to 6 German-speaking children with SLI aged between 5 and 8 years old on subject-verb agreement and tense. To explain the children’s problems with these verbal morphological features they formulate the Agreement Deficit Hypothesis: “Formal features which do not have a semantic interpretation, specifically phi-features of verbs, cause acquisition problems for children with SLI” (Clahsen et al. 1997: 151). Spoelman & Bol (2012) state that verbal morphology, including subject-verb agreement, is most affected in children with SLI, at least in morphologically poor languages such as English and Dutch. In their study, they collected audio-recordings from 10 children with SLI (aged 4–7) and compared them to 10 age-matched TD children and found impairments in subject-verb agreement and in verb argument structure. For instance, children with SLI often omit the past tense morpheme (-ed in English, -te/-de in Dutch) at ages at which TD children no longer do this.

Although the impairment in morphosyntax in children with SLI has been well-documented in the verbal domain (e.g. tense and subject-verb agreement), less is known about the nominal domain. Nominal morphology includes, for instance, grammatical gender in Dutch (neuter definite article het (‘the’) vs. common definite article de (‘the’)), adjectival inflection in Dutch noun phrases such as het mooie huis (‘the beautiful house’) vs. een mooi huis (‘a beautiful house’), and the plural morpheme (-s in English, -s or –(e)n in Dutch), one of the foci of the current study. Nevertheless, there are a few studies on nominal morphology in SLI. For example, Rice & Oetting (1993) investigated spontaneous language samples of 81 5-year-old English-speaking children with SLI and found that they produced the plural inflection in obligatory contexts less often as compared to a control group of 92 MLU-matched TD children, suggesting difficulties in the area of nominal morphosyntax. Furthermore, in a sentence completion study by Novogrodsky & Kreiser (2015), 15 Hebrew-speaking children with SLI (aged 8–14) were compared to a group of language-matched and a group of age-matched controls on their ability to complete sentences with the morphologically correct noun. Although the results show no differences between the SLI group and the language-matched TD group, the SLI group performed more poorly than the age-matched TD group. In addition, error analyses reveal that, even though no group differences between TD language-matched and SLI were found, children with SLI produced more morphological errors as compared to other error types (e.g. semantic or phonological errors). Thus, according to the authors, children with SLI in this study “presented incomplete morphological knowledge” of nouns (Novogrodsky & Kreiser 2015: 821).

As for the lexicon, many studies report smaller vocabularies than TD age-mates for children with SLI (e.g. Rice 1991; Conti-Ramsden & Jones 1997; Leonard 1998; Hick et al. 2002). Furthermore, some studies show that lexical access can be slower in children with SLI (Seiger-Gardner & Schwartz 2008). Nevertheless, it is not clear whether children with SLI have difficulties with the lexical-semantics and the features of the words that they have acquired. As storing of features on words is quite different from grammatical computation, this may not be as impaired as morphosyntax in SLI.

2.4 Previous acquisition studies on the mass-count distinction in children with specific language impairment

Rice, Cleave & Oetting (2000) investigated the interpretation of syntactic cues in a group of 20 children with SLI (aged approximately 5 years) to two groups of TD children, one matched on mean length of utterance (approximately 3 years) and one matched on chronological age on a naming task. Participants watched a video containing a story in which a total of sixteen novel count and mass nouns were introduced, together with sixteen unfamiliar objects that served as potential referents for the novel nouns. There were two versions of the video, rendering two conditions: one with syntactic cues (e.g. Look, I found a keelwug and some blick) and one with neutral syntax (e.g. Look, I found the keelwug and the blick). After viewing the video, participants were required to map the novel words to the unfamiliar objects. Results revealed that only the 5-year-old TD children were able to pick up on the syntactic cues, reflected by significantly higher performance in the cued condition than in the neutral condition, whereas the 5-year-old children with SLI (and the 3-year-old TD children) were not. This raises the question as to whether older children with SLI are better at using syntactic cues to distinguish between mass and count.

Older children were investigated in Froud & Van Der Lely (2008), who explored the mass-count distinction in 17 English-speaking children with SLI (aged 8;0 to 15;6), two groups of younger TD children (mean age 6;2 and 7;4), and a group of chronologically age-matched TD controls. Employing a production task, children were presented with novel nouns with a simple CVC structure (e.g. dap) together with potential syntactic and semantic cues (whether the novel nouns referred to an object or a substance). Syntactic cues could signal mass (some dap) or count readings (a dap). It was argued that in the TD children, the integration of syntactic and semantic cues matured over time, while the children with SLI performed unlike any of the control groups, as they are not able to discriminate between mass and count novel nouns.

From these studies we know that English-speaking children with SLI have difficulties picking up syntactic cues in the input that are associated with the distinction between mass and count nouns: the determiner a (used to introduce count nouns) and the quantifier some (used to introduce mass nouns). However, neither of these studies investigated whether children with SLI are able to use bound nominal morphology, such as the plural marker in distinguishing mass from count. Additionally, they did not address potential differences in interpretation between classical and flexible count and mass nouns, or object-mass nouns. It is these different types of count and mass nouns and the plural morpheme that we focus on in the present study.

2.5 Hypotheses and predictions

The current study addresses two main questions, one regarding the interpretation of mass and count nouns by children with SLI and one regarding the theoretical underpinnings of mass and count nouns. Our developmental hypothesis is that children with SLI are mainly impaired in morphosyntax, including nominal morphosyntax, but less so in terms of lexical-semantics. Our theoretical hypothesis is that the interpretation of flexible nouns relies solely on morphosyntax and that the interpretation of classical mass and count nouns follows from a combination of their morphosyntactic environment and convention/world knowledge. In contrast, we hypothesize that the interpretation of object-mass nouns is lexically determined by their lexical [+individual] feature. Combining these developmental and theoretical hypotheses, we predict that Dutch-speaking children with SLI:

1. have most severe problems with the interpretation of flexible mass and count nouns as compared to their TD age controls;

2. have fewer problems with classical mass and count nouns, but are not TD-like in their interpretation on classical nouns;

3. have no problems in their interpretation of object mass nouns and will thus perform similar to their TD age controls.

A secondary aim of the present study is to explore what general background measures potentially underlie the individual variation in performance on the QJT and to investigate whether this is different for TD children versus children with SLI. As these investigations are explorative in nature, we do not have any specific hypotheses regarding our secondary aim.

4.1 Group results

The data in Figure 4 provide the proportions of target responses of both groups in the different conditions that were tested in the QJT. Recall that the target response in the flexible count, classical count and object mass conditions is a judgment based on number of individual items, whereas judgments should be based on overall volume in the flexible mass and classical mass conditions. Figure 2 does not include the children’s performance on filler items, but this was at ceiling in both groups (TD: M = 99.6%, SD = 2.4%, SLI: M = 98.2%, SD = 5.6%).

Figure 4

Percentages correct (+/– 1 SE) of TD and SLI groups on the experimental conditions in the QJT.

Overall, the performance of the children with SLI on the experimental conditions ranges from a mean of 67.5% (SD = 27%) on flexible mass nouns to 86.6% (SD = 28.5%) on classical count nouns. The TD children systematically appear to score higher, ranging from a mean of 89.9% (SD = 16.3%) on flexible mass nouns to 99.1% (SD = 4.7%) on classical count nouns. Both the SLI and TD groups perform above chance (50%) on all experimental conditions (p < .01). To answer our two main questions, we compared the children’s performance on the different experimental conditions.

4.2 A closer look at the different experimental conditions

The results for classical and flexible mass and count nouns reveal a significant effect of group (F(1, 56) = 19.683, p < .001, d = 1.21), but the main effects of condition (flexible or classical nouns) or noun type (mass or count nouns) were both non-significant (F(1, 56) = .622, p = .434, d = 0.22, and F(1) = 1.962, p = .167, d = 0.38 respectively). Importantly, the results reveal a significant interaction between group and condition (F(1, 56) = 10.486, p < .01), showing that the groups did not perform equally on flexible and classical conditions. Follow-up analyses revealed that the children with SLI are outperformed by their TD peers on both classical (t(54) = 3.101, p = .003, d = 0.83) and flexible (t(54) = 5.572, p < .001, d = 1.49) nouns. However, there is no difference in performance on flexible nouns (mass and count combined) as compared to classical nouns (mass and count combined) in the TD group (t(27) = 1.643, p = .112, d = 0.31), while the children with SLI perform significantly better on classical nouns than on flexible nouns (t(27) = 3.979, p < .001, d = 0.75).⁶ The interaction between group and noun type is not significant (F(1) = .472, p = .495), so it is not the case that the children with SLI show a response pattern to mass and count nouns different from that of the TD children.

The results on object mass nouns indicate no significant difference between the groups (F(1, 56) = 1.485, p = .228, d = 0.33). This means that the children with SLI (M = 88.4%, SD = 24%) did not score significantly lower than their TD age-mates (M = 92%, SD = 15.3%).

4.3 Individual variability

Noticeably, the group of children with SLI shows more individual variability than the control group throughout conditions. This is reflected in their range of performance and the standard deviations from the mean in the different experimental conditions and in their overall performance as shown in Table 5. The ranges within the group of TD children range from chance level (50%) to 100% accuracy, whereas in the SLI group there are children who score 0% correct on flexible count, classical count and/or object mass nouns. In fact, there is one child (aged 10) who opts for a mass interpretation for all flexible and classical nouns, regardless of the absence or presence of the plural morpheme.

As mentioned in section 2.5, our secondary aim was to explore potential sources of individual variability in the groups under investigation. To this end, we computed correlations between children’s overall performance on the QJT, their age and their performance on general language ability (CELF-CLS) and non-verbal reasoning ability (Raven percentile score). Table 6 presents the correlations between these variables for the TD and the SLI groups. In the TD group, the only variable that significantly correlated with the QJT was age (r = .497, n = 28, p = .007). The QJT performance of the children with SLI did not only significantly correlate with age (r = .512, n = 28, p = .005), but also with their general language ability (r = .498, n = 28, p = .007). No significant correlations with the children’s non-verbal reasoning ability were found (p > .1). The same correlational analyses were run without the child who adopted a “mass strategy”, as described above, but this did not change the pattern of results.

In follow-up analyses, the predictive power of our different background measures was investigated using hierarchical regression analyses for both groups. The first model, including both control variables (age and non-verbal reasoning abilities) significantly explained 33% of the variance in the TD group (F(2) = 6.172, p = .007) and 27% of the variance in the SLI group (F(2) = 4.579, p = .02). In the second model, the addition of the CELF-CLS did not significantly contribute to the model as a whole for the TD group (t = 1.139, p = .266), although the model as a whole still explains a significant amount of variance in the QJT as age remains a significant predictor of performance (37%, F(3) = 4.597, p = .011). For the SLI group, however, adding the CELF-CLS significantly contributed to explaining individual variation (t = 2.324, p = .029) and the contribution of age was no longer significant (t = 1.819, p = .081). The model now explained 40% of the total variance (F(3) = 5.391, p = .006). The Raven turned out not to be a significant predictor in and of itself in any of the models and, thus, a similar pattern of results was obtained when non-verbal reasoning abilities were left out of the regression models. These analyses confirmed the correlations mentioned above: whereas performance on the QJT was mostly predicted by age in the TD group, the SLI group’s accuracy was predicted by both age and performance on the CELF-CLS.