Grammatical gender and noun class systems categorise nouns into classes, reflected by agreement on dependents words, such as demonstratives, adjectives and numerals (see, e.g., Aikhenvald, 2006; Corbett, 1991; Hockett, 1958; Katamba & Stonham, 2006, amongst others). In these systems, nouns that trigger the same agreement patterns are sometimes referred to as sharing agreement class, or gender. Nouns that themselves share morphophonological properties are in the same form or deriflection class (Güldemann & Fiedler, 2019).1 In some languages, these largely overlap, but in others, they can diverge. In such cases, not all nouns which share some morphophonological feature (e.g., a prefix or suffix) will necessarily have the same gender. For example, in French, most nouns that end in –ette trigger feminine agreement on their dependents, but there are some (e.g., squelette ‘skeleton’) which trigger masculine agreement. In Bantu languages where gender is, in principle, determined by singular-plural agreement class pairs, most nouns are marked with a nominal prefix. Those which share a prefix are in the same form class, but as in French, not necessarily the same gender. Table 1 below provides a summary description of the noun classification system of the Bantu language Kîîtharaka. This table shows, for example, that some nouns with the prefix mû- are in agreement classes 1/2 (gender A), but others with the same mû- prefix (hence, same form class), are in classes 3/4 (gender B).2

In addition, there are exceptional cases when the nominal prefix is not the one typically expected given the class marked by the agreement prefix. For example, in (1) and (2), a noun with a prefix that is in form class 5 and 9, respectively, triggers agreement class 1.

At the same time, as in all noun class systems (Corbett, 1991), there is evidence in Kîîtharaka that at least some nouns in a given gender category share aspects of meaning. For instance, most human nouns are in gender A (see Table 1 for an example). As with morphophonology, though, there are exceptions (e.g., see (3)).

This lack of perfect correspondence between the form of the noun, or its meaning, and the agreement it triggers highlights a longstanding question in research on gender and noun class systems: what exactly determines the agreement class of a noun? That is, what productive rules do speakers of a language form regarding the features of a noun that might predict its gender or class? While it is often argued that gender is stored as a lexical feature of the noun (e.g., see V. M. Carstens, 1991, for claims about Bantu in particular), there is, nevertheless, robust evidence that learners and speakers make use of such cues productively, e.g., to determine the gender of novel nouns (or loan words) (e.g., Demuth, 1992; Gagliardi & Lidz, 2014; Karmiloff-Smith, 1981; Pérez-Pereira, 1991, and others.). Morphophonological features, other phonological properties of nouns and noun semantics have all been argued to function as potential cues to class (see e.g., Corbett, 1991; Kihm, n.d.; Kramer, 2015; Mathieu et al., 2018; Ralli, 2002, amongst others). However, establishing precisely what features productively determine the agreement a noun takes in a given language is not trivial, particularly given potentially high levels of exceptions (see, e.g., Amidu, 1997; Castagneto, 2017; Corbett, 1991; Kanampiu et al., To appear; Rice, 2006), and the possibility of a default class into which novel nouns can be classified regardless of form or meaning (e.g., see Dingemanse, 2006; Richardson, 1967).

This leaves us with fundamental theoretical and empirical questions, like how to determine which cues are productive for class assignment amidst such exceptions, and whether speakers of a language use these cues in a consistent way or not. Traditional approaches to Bantu nominal classes, for instance, have heavily borrowed from the so-called Proto-Bantu schema, an arguably semantically-centred system that characterises the noun classes in Bantu based on semantic features like Human or Animate and more abstract ones like Paired-body parts, Liquid masses, Cohesive mass and so on (see, for instance, Table 2). While some of these features may be relevant for some classes (e.g., human or animates for class 1/2), semantic regularity in most of the classes is imperfect at best – typically, there are either nouns that cut across diverse semantic domains or some that share features with nouns in different classes, e.g., Diminutives, Augmentatives and Inanimates in Table 2.

Most importantly, the semantically-based characterizations of Bantu are often not founded on robust analysis of synchronic empirical data. Rather, in many cases, these are descriptive approaches either based on (non-systematically picked) examples, or on native-speaker intuitions alone (see, e.g., Msaka, 2019; Worsley, 1954, for similar views and other examples). Recently, quantitative corpus-based approaches have contributed to a better understanding of cues to gender in individual Bantu languages. For example, corpus studies on Zulu and Sesotho have found that nouns with certain semantic features are indeed more frequent in some classes than others (see, e.g., Demuth, 2000; Ngcobo, 2010, and others). This suggests these features may be productively used by speakers of these languages as cues to class. Studies on Kîîtharaka have used quantitative data, including formal measures of morphological productivity (specifically, the Tolerance Principle, Yang, 2016) to show that semantic features appear to be productive for only a few classes (e.g., class 1/2) (e.g., Kanampiu et al., To appear) (see Section 2 for more details on the Tolerance Principle). By contrast, (morpho)phonology appears to be the most relevant cue for the remaining classes in both Kîîtharaka, and other Bantu languages Demuth (see e.g., 2000, for Sesotho); Msaka (2019, for Chichewa); Kanampiu et al. (To appear, for Kîîtharaka). While corpus data helps make empirically informed predictions about which rules may (substantially) motivate the classification of nouns, an important question that remains to be answered is whether this reflects speakers’ own language knowledge. In other words, it leaves open whether observation of human behaviour would align with corpus data.

A complementary approach is to use behavioral experiments with native speakers to assess production of gender agreement based on targeted semantic or morphophonological features. If speakers indeed make use of such information – i.e., the form of the noun or meaning – as cues to noun class, systematic patterns of behaviour in how they respond to given stimuli should be observed. Adults and even children have been found to use both noun internal and syntactic distributional information (i.e., agreeing dependents) in making decisions about grammatical categories. For example, participants can predict the likely category or class of the word, given evidence about its syntactic context or agreeing dependents (in both natural and artificial language experiments: see e.g., Gagliardi & Lidz, 2014; Redington et al., 1998; Reeder et al., 2013). As mentioned above, in the absence of syntactic distributional information, speakers (both children and adults) can and do predict the noun class or gender of novel nouns from semantic or morphophonological information (again, in both natural and artificial language experiments, see, e.g., Frigo & McDonald, 1998; Gagliardi & Lidz, 2014; Karmiloff-Smith, 1981). Additionally, evidence suggests that different sources of information may not be used in the same way. Several experimental studies have found that, unlike adults, children acquiring nominal classes have a preference for phonological cues, even in the presence of more statistically reliable semantic cues (see, e.g., Bates et al., 1995; Culbertson et al., 2019; Gagliardi & Lidz, 2014; Karmiloff-Smith, 1981; Pérez-Pereira, 1991).

In Bantu, similar studies have found that children at certain ages have access to phonological restrictions characterizing the use of nominal prefixes (e.g., Demuth, 1992; Demuth & Ellis, 2010), suggesting that phonology is an early-available cue (see Demuth, 2000). For instance, in investigating acquisition of noun class prefixes in Sesotho, Demuth & Ellis (2010) found that children’s early use (and omission) of noun class prefixes was determined by phonological features of nouns.

More recently, Lawyer et al. (2024) used behavioral experiments to investigate whether speakers of Kinyarwanda are sensitive to semantics or morphophonology when classifying nouns. The study used a triadic method (following Burton & Kirk, 1976) in which participants are shown triads of three nouns and asked to pick the odd one out. In each triad, two nouns shared either a semantic feature, a morphophonological feature, or both, with the remaining noun being different from the others. For instance, take the triad iki-bwana ‘puppy’, in-gurube ‘pig’ and umu-kindo ‘palm tree’. Here, iki-bwana ‘puppy’ and in-gurube ‘pig’ share similar semantic features (i.e., they are animals and, specifically, mammals), while umu-kindo ‘palm tree’ is a plant. Likewise, in the triad umu-kubuzo ‘broom’, umu-shushwe ‘rat’, and in-dimu ‘lemon’, all bear different semantic properties, but the first two share a prefix. The assumption is that speakers should be able to pick out either or both of these differences, if indeed they pay attention to such features when making noun class decisions. The authors find a significant preference for semantics overall. In other words, most participants were successful at picking out semantically distinct words in triads, and used this kind of information even in trials where morphophonology could also be used.

Beyond the few studies mentioned above, there is relatively little work exploring how children and adult speakers of Bantu languages use semantic information to classify nouns, and no systematic experimental work testing the use of morphophonological cues in adult speakers. Here, we conduct two experiments investigating how adult speakers of Kîîtharaka make use of semantic and morphophonological features to predict the agreement class of novel nouns. Before reporting these experiments, we first briefly describe some key features of the noun class system of Kîîtharaka.

Kîîtharaka is an Eastern Central Kenya Bantu language spoken mainly by Atharaka people in Tharaka-Nithi County.3 The language is mutually intelligible with other closely related languages in the Meru cluster of Bantu languages, though there are varying degrees of lexical and mophophonological differences (see, e.g., Kanana, 2011). According to descriptive accounts, the language has 17 noun classes (defined using the traditional class pairs, based on nominal prefixes), often argued to be based on abstract semantic features (as in BTL, 1993; waMberia, 1993). However, in a more recent analysis, Kanampiu et al. (To appear) showed that, as in many other Bantu languages, nominal prefixes do not perfectly predict agreement classes. In the terminology of Corbett (1991), this system exhibits crossed relations between form class and agreement class. Following Güldemann & Fiedler (2019), Kanampiu et al. (To appear) establish 8 form class pairs, or deriflection classes (i.e., groups of nouns sharing morphophonological features in the singular and plural) and 8 agreement class pairs, or genders (i.e., groups of nouns triggering the same singular/plural agreement).4 In a recent study, Kanampiu et al. (To appear) applied the Tolerance Principle – a theory of morphological productivity (Yang, 2016, MS) – to a corpus of Kîîtharaka nouns to evaluate the productivity of selected semantic and morphophonological cues predicted to motivate nominal classification in Kîîtharaka. The Tolerance Principle is a theory of how and when learners form generalizations. It assumes that learners can store both productive rules and also a list of exceptional forms not determined by productive rules. To evaluate a potential rule, a learner considers whether the items in the lexicon that follow that rule meet a productivity threshold, taking into account the potential number of forms that would need to be listed as exceptional. The threshold, θN, is calculated using the formula N/lnN, where (N) is the number of items that follow the rule. A rule is productive if the number of exceptions (e) does not exceed θN. While the Tolerance Principle is primarily intended as a model of learning, it has also been applied to adult corpora to make predictions about productive rules (see, e.g., Björnsdóttir, 2023; Kodner, 2020; Yang, 2016, and others).

Kanampiu et al. (To appear) found that of twenty-three tested semantic features, only seven of them – Human, Trees, Augmentative, Pejorative, Manner, Diminutive and Infinitive – were productive. On the other hand, all morphophonological features tested were found to be productive; with the exception of the nasal prefix n-, all other singular and plural prefixes were productive, according to the Tolerance Principle. This study, therefore, suggests that (i) despite the existence of exceptions, there are specific semantic and morphophological cues which should, in principle, be productive for speakers; however, (ii) morphophonology is likely to be a more robust cue to the agreement class (or gender) a noun belongs to than semantics.

It is worth noting that evaluative semantic features – Diminutive, Augmentative, and Pejorative – can function derivationally in Kîîtharaka. In other words, while some nouns are always in the same evaluative gender class (due to an inherent property, like their size or social stigma), it is also possible to use the evaluative prefixes and corresponding agreement on nouns that are typically in other classes. Such derivation usage is in order to express or highlight one of these evaluative features (for additional discussion of these classes in Bantu and other languages, see Di Garbo, 2013). We will return to this below. However, the key question we address in this article is whether the predictions of productivity reported in Kanampiu et al. (To appear), in fact, align with what speakers are actually sensitive to when making noun class decisions. In other words, we ask whether, when shown a novel object with a particular semantic feature, or a novel noun with a particular prefix, speakers provide agreeing forms in a systematic way, and in line with the predictions of Kanampiu et al. (To appear). If they produce systematic responses for a given feature, then we assume this feature is productive (see 3.4 for details on how we establish what counts as systematic). We, therefore, conducted a series of psycholinguistic experiments to test whether speakers are sensitive to the semantic and morphophonological cues investigated in this previous research.

We report two experiments with native Kîîtharaka speakers. These experiments follow previous research which makes use of artificial lexical items to test knowledge of nominal classes in adults and children (e.g., Bates et al., 1995; Berko, 1958; Frigo & McDonald, 1998; Gagliardi & Lidz, 2014; Karmiloff-Smith, 1981; Pérez-Pereira, 1991).5 Our experiments sought to investigate whether adult speakers of Kîîtharaka are sensitive to particular kinds of cues – i.e., the morphophonological features of the noun, and the meaning of the noun – when producing syntactic agreement on adnominal modifiers. As we set out in Section 1, we take agreement as indicative of gender assignment. If speakers consistently use a particular semantic or morphophonological feature when providing nominal agreement, then novel nouns with that feature should be assigned the same agreement, hence, belonging to the same agreement class. Although assignment to a single agreement class does not necessarily indicate the gender of a noun (i.e., gender is a consistent pairing of agreement classes), we, nevertheless, make that simplifying assumption here.6 On the other hand, if speakers do not consistently use a particular feature when classifying nouns, no such regularity should be observed, or nouns will be assigned to a default gender. In the latter case, this is likely to be gender E (i.e., class 9/10 agreement). This is the agreement class associated with most loan nouns denoting non-humans, more particularly, those whose noun-initial (morpho)phonological make-up does not correspond to a possible prefix in Kîîtharaka. This kind of behavior is also found in many other Bantu languages, and we expect the same here (see, e.g., Demuth, 2000; Demuth & Weschler, 2012; Diercks, 2012; Rose & Demuth, 2006).

Experiment 1 tested whether Kîîtharaka speakers are sensitive to semantic cues depicted in images corresponding to novel nouns. A total of nine semantic features were investigated, following previous research in Kîîtharaka (Kanampiu et al., To appear) and related Bantu languages more generally. The set of features is shown in Table 3. These features target 6 genders in Kîîtharaka, corresponding to seven different singular agreement classes: 1, 3, 5, 7, 11, 12.7 As noted in Section 2, these particular semantic features were investigated in a corpus of Kîîtharaka (see Kanampiu et al., To appear), and only a subset of them were predicted to be productive based on the Tolerance Principle: the evaluative features Diminutive, Augmentative, and Pejorative; and Human. Here we test whether these predictions are, in fact, borne out for Kîîtharaka speakers tasked with producing agreeing modifiers of novel nouns. Our experimental protocol received ethical approval from the Linguistics and English Language departmental Ethics Committee.

In contrast to semantic features, Kanampiu et al. (To appear) predict that all nominal prefixes are productive morphophonological cues to class in Kîîtharaka. In Experiment 2, we test these features. Specifically, we test whether Kîîtharaka speakers use nominal prefixes on a novel noun stem (i.e., a beginning segment or syllable) to predict the form of an agreeing dependent, in the absence of semantic information. We tested both singular and plural prefixes, repeated in Table 11 below. As in Experiment 1, we use the agreement prefix provided by participants as an indicator of the class participants assign a novel noun to. If participants use noun beginnings in this way, we predict that they will consistently assign the predicted agreement pattern to nominal dependents. Our experimental protocol received ethical approval from the Linguistics and English Language departmental Ethics Committee.

Based on the results of Experiments 1 and 2, our findings suggest a sharp difference in accuracy based on the semantic features compared to the morphophonological features tested. This is summarized in Figure 9, which shows average accuracy across the productive semantic features and block 1 of the morphophonological experiment.

Considering that semantics and morphophonology are the main factors which interact to determine class and or gender more generally (see e.g., Aikhenvald, 2012; Corbett, 1991; Katamba & Stonham, 2006, and others), this comparison suggests that in Kîîtharaka, morphophonology may generally be the preferred cue type. However, this claim can not be confirmed here since these two cue types were tested separately. We leave this to future research.

To compare these feature types statistically, we collapsed data from the productive features of Experiment 1 (Human, Augmentative, Pejorative, and Diminutive), and from all morphophonological features in block 1.25 We then fit a logistic mixed-effects regression model with accuracy as a binary dependent variable and experiment as the predictor variable, along with by-participant and by-item random intercepts. The predictor variable was treatment (dummy) coded, with Experiment 1 as the reference level. The model reveals a statistically significant difference between Experiment 1 and 2 (β = 2.8190, SE = 0.3663, z = 7.696, p < 0.0001). This confirms that participants are significantly more sensitive to morphophonology than semantics overall, even when considering only those semantic features which were used productively.

In Bantu languages, both nominal prefixes and the semantics of the noun (both inherent, e.g., human, and evaluative, e.g., diminutive) are considered to determine the agreement class and therefore gender of the noun. However, recent quantitative studies in Bantu languages have shown that both of these cues may have varying degrees of relevance in particular Bantu languages (e.g., Kanampiu et al., To appear; Lawyer et al., 2024; Msaka, 2019). This study aimed to provide behavioral evidence to establish whether speakers of Kîîtharaka, an Eastern-central Bantu language, make use of both the nominal prefixes and specific semantic features of the noun in classifying novel nouns. The study involved two experiments: Experiment 1 tested whether speakers were sensitive to a set of inherent and evaluative semantic features. Experiment 2 tested for sensitivity to nominal prefixes. In both of these experiments, participants were presented with novel stimuli and required to produce class agreement on a Kîîtharaka numeral. In Experiment 1, they saw unfamiliar images highlighting one particular semantic feature accompanied by a prefix-less novel Kîîtharaka stem, along with its prefix-less numeral ‘one’, and were required to produce the missing prefixes. In Experiment 2, they saw a prefixed novel Kîîtharaka noun, along with its prefix-less numeral, in a carrier sentence and were required to produce the missing prefix.

The results show that some of the tested cues were consistently used to predict class agreement, while others were not. First, all nominal prefixes in Kîîtharaka were used productively by participants to predict class agreement on a nominal dependent in our experiment. This implies that in Kîîtharaka, the first segment of the noun, i.e., the nominal prefix, is a robust cue to the gender of a noun. Second, semantic features were, in general, less likely to be used productively to predict class agreement. Indeed, according to our findings, most features tested, such as Extended, Artefact, Narrow, etc., failed to elicit consistent agreement classes. The exceptions were Human, Fruit and the evaluative features Augmentative, Pejorative, and Diminutive, which were all found to be used productively by speakers (though to different degrees). As we argued in 3.5, the evaluative feature Augmentative is likely ambiguously expressed in Kîîtharaka, i.e., it is productively associated with two class prefixes – î- and k(g)î-. This feature was found to be robustly productive across analyses, once this pattern of usage was accounted for. Overall, then, while some semantic features are used as cues to class and gender, these features are not as robust as nominal prefixes in Kîîtharaka. Below, we discuss the implications of these results, along with a few unexpected findings.

The results of the two experiments reported here largely corroborate the findings of a corpus-based study by Kanampiu et al. (To appear), which used the Tolerance Principle to evaluate the productivity of the two types of cues in determining class and gender agreement in Kîîtharaka. While the Tolerance Principle (like other measures of productivity) can generate predictions, experimental results like these provide behavioural evidence that speakers of Kîîtharaka can and do use particular features productively. Our results suggest that many inherent semantic features are not represented by speakers as cues for classifying new nouns in the language. If such features were, at some point, relevant in the diachrony of the language, or the parent Proto-Bantu, most of these features have since lost that relevance. If this is true for Kîîtharaka, then it may be the case for many other present-day Bantu languages. Interestingly, comparing the results of Kîîtharaka to those of Kinyarwanda, as in Lawyer et al. (2024), where participants were found to be more sensitive to semantics, suggests the possibility that Bantu languages may exploit these types of cues differently.26 Studying nominal classification systems of individual languages in an empirically-grounded way is likely to reveal interesting results.

While the alignment between the corpus data and our results suggests the differences between semantics and morphophonology in Kîîtharaka are real, it is also worth revisiting a methodological consideration. It could also be the case that some of the semantic features tested appeared less productive or unproductive in the experiment because of how the participants interpreted the image stimuli. Image stimuli necessarily fail to isolate individual semantic features, in contrast to morphophonological features. Participants may, therefore, have been influenced by those features. For example, as noted in 3.5, some Narrow images could also be perceived as Extended. Second, it could be that the stimuli themselves varied in how much they instantiated the target feature. For instance, some particular images might have been more successful in depicting target meanings than others for a given feature (see Figure 12). For example, the target feature Fruit, may have been more successful at eliciting this feature compared to other features depicted by (sometimes complex) artefacts. While we did our best to select the most appropriate images (e.g., by piloting), there is likely to always be an asymmetry when comparing these two classes of features. Therefore, the possibility remains that the productivity of semantic features in the experiment is under-estimated.

We also noted some unexpected patterns in Experiment 2 that are worth additional discussion. First, we saw that participants tended to associate nouns prefixed with an ambiguous 1/2 or 3/4 class prefix with gender B (class 3/4), rather than gender A (class 1/2). This is despite the fact that in the corpus analyses (see Kanampiu et al., To appear) this prefix is productive as a cue to both gender A and B. As mentioned above, this likely indicates that in the absence of semantics, participants assume that new words refer to objects, rather than humans.

It is also worth returning to the types of responses participants consistently provided when they did not provide the expected responses based on class. First, the majority of such responses were class 9/10 (gender E). This is what we predicted, since this gender is typically considered to be a default, across Bantu languages. However, we also saw minimal but consistent alternative assignment to other classes, mainly class 6 (gender C plural agreement class) and gender B (class 3/4). While further research is needed to understand whether these responses indeed reflect something non-accidental, we have suggested that these may also reflect alternative default classification. Loans or novel nouns that are featurally un/under-specified are typically admitted to one class – mostly class 9/10 in Bantu (see, e.g., Gunnink et al., 2015; Mous, 2019, and others). However, gender B is also a large, heterogenous class, and Kîîtharaka speakers may thus assume novel words are likely to fall into it, particularly when no semantic feature is present, or the semantic features are not productive enough to determine class. We have suggested that class 6 responses may have been used in Experiment 2, in particular, as a default plural class, since it is used to form collective plurals in Kîîtharaka. However, this, and the general trouble our participants experienced deriving plural forms from singulars in Experiment 2 deserves further study.

While we have found here that morphophonological features appear generally more productive than semantic features in Kîîtharaka, future research should also explore more explicitly their relative importance. For example, previous research has used experimental methods to show that when semantics and (morpho)phonology features are both present but provide conflicting cues, adult speakers tend to rely on semantics (e.g., Demuth & Ellis, 2010; Gagliardi & Lidz, 2014; Karmiloff-Smith, 1981; Pérez-Pereira, 1991). Whether this is the case for Kîîtharaka as well remains to be seen.

What determines productivity in noun class systems across languages is a longstanding question. While descriptive research suggests that both semantics and morphophonology are relevant, behavioral evidence of productivity in native speakers is missing for many languages. In this study, we set out to establish whether speakers of the Bantu language Kîîtharaka are sensitive to the semantics and the form of the noun (i.e., the nominal prefix) when providing agreement on the nominal dependents. We found speakers are robustly sensitive to a small subset of semantic features: Human, and to a lesser extent, Fruit, Narrow, and the evaluative ones Augmentative, Pejorative and Diminutive. By contrast, speakers were robustly sensitive to all morphophonological cues we tested. These experimental results provide psychological evidence for these cues, and suggest that if others may have been historically relevant (i.e., in Proto-Bantu), their relevance has waned for Kîîtharaka and potentially many other Bantu languages. Our study, therefore, shows that the behaviour of novel nouns can provide insights into productive morphophonological processes in a language, and thus highlights the importance of empirically grounded research in this area.