As has been stated, code-switching consists of the juxtaposition of multiple languages within a single speech event. In many communities, code-switching is regarded as typical in-group linguistic behaviour (e.g., Poplack 1980; Poplack 1988; González-Vilbazo & López 2011; Adamou & Shen 2019; Gosselin & Sabourin 2021; etc.). Switching can occur both inter-sententially (between sentences) or intra-sententially (within sentences); the use of the intra-sentential code-switching has been linked to high proficiency or balanced language-skills among speakers (e.g., Poplack 1980; Koban 2013). The examples in (3) (from Valdés Kroff, Dussias, Gerfen, Perrotti & Bajo 2017) depict both switching types, respectively.

As researchers recognized the systematicity of code-switches, a multitude of (primarily syntactic) language mixing constraints were introduced. For instance, the Equivalence Constraint (3), brought forth by Poplack (1980), claimed that code-switching boundaries are prohibited where such a switch may give rise to an illicit word-order in one of the two languages. Following the Equivalence Constraint, a series of language mixing constraints were proposed: the Closed class constraint (Joshi 1982), the Government constraint (Di Sciullo, Muysken & Singh 1986), the Matrix language frame (Myers-Scotton 1997), the Functional head constraint (Belazi et al. 1994), among others. While these theories often possess reasonable predictive power, most have been critiqued by proponents of the constraint-free approach (e.g., MacSwan 2000; 2014; MacSwan & Colina 2014). This is because the Equivalence Constraint and many of its successors directly refer to the act of code-switching and therefore impose upon it an otherwise vacuous ‘third grammar’. By contrast, constraint-free formalism maintains that code-switching does not (and should not) possess constraints that are independent from monolingual speech. As it turns out, Chomsky’s minimalism is well equipped to carry out a constraint-free analysis of code-switching (MacSwan 2000; 2014).

In minimalism, patterns observed from linguistic data are preferentially elucidated by the most simple and elegant assumptions. Consider Chomsky’s generally accepted hypothesis that the (monolingual) human language faculty is made up of A) a lexicon and B) a computational system (C_HL) (see Chomsky 1995). In brief, C_HL is responsible for Selecting lexical arrays from the lexicon, combining them into a syntactic structure with the operation Merge, and then Spelling-Out the derivation to logical form (LF: the interpretative level of language) and phonological form (PF: the sensorimotor aspect of language).

Adopting the constraint-free approach for code-switching data, MacSwan (2000) extended Chomsky’s minimalistic view of the human language (see Figure 1, left panel) to the bilingual language faculty. The bilingual version of the model (see Figure 1, right panel) includes two differences. First, there are two lexicons available to the bilingual speaker. Second, bilinguals possess discrete phonological systems, which are compartmentalized before PF. Just like the monolingual language faculty, however, bilinguals are thought to construct sentences within a single syntactic component.

This system very simply accounts for code-switching while avoiding switching-specific grammars: in the bilingual language faculty, C_HL can select lexical items from multiple lexicons and combine them into a single syntactic construction. On the other hand, a bilingual individual can easily speak in a single language by selecting items from a single lexicon instead of two. Thus, by adopting a minimalist framework, MacSwan convincingly illustrated that an adequate code-switching grammar should not explicitly or conceptually refer to the act of using multiple languages (an insight previously broached by Mahootian 1993; Belazi et al. 1994). Instead, his model is anchored in the following assumptions (MacSwan 2000: 43):

In short, assuming that two languages, L_x and L_y, are involved in a mixed sentence, and that these languages each possess their own grammar, G_x and G_y, any code-switched sentence is determined by G_x ∪ G_y. We do not need to know the arbitrary identity of any L, and we do not need to generate a switching-specific grammar.

Once the syntactic structure (the overt component) is derived, it must surface as a phonological/phonetic form. The monolingual model simply sends the sensorimotor elements of the derivation to the unique phonological system in the configuration. The bilingual language faculty, on the other hand, possesses more than one lexical set, and thus, more than one phonological system. In a monolingual utterance, bilinguals can easily respect the phonology of any of their languages: items from each lexicon are merely rendered to their corresponding phonological system. Contrastively, during code-switching, all lexical items must be parsed by their appropriate phonological systems after Spell-Out.1 Hence, to give rise to a felicitous code-switched speech segment, we assume that the phonological systems (PS) within the bilingual architecture are sequestered (PSx, PSy) before PF. Note that this architecture starkly contrasts other (primarily non-lexicalist) models (see also González-Vilbazo & López 2012; Alexiadou, Lohndal, Åfarli & Grimstad 2015; Alexiadou & Lohndal 2018; Stefanich, Cabrelli, Hilderman & Archibald 2019; etc.) which posit that bilinguals possess fully integrated grammars, including at the level of phonology (López 2020).

To recapitulate, MacSwan’s model of the bilingual language faculty implies that code-switching is possible at the syntactic level, but that code-switching at PF is unavailable. This means that each segment involved in the switch respects the phonology of the lexical set it originates from, and furthermore, that there is no singular alternate phonological grammar that can account for the mixed utterance in its entirety. Presently, it may be asked exactly why the syntactic and phonological interfaces, by nature, differ within the architecture of the bilingual system. That is, what motivated MacSwan to posit that syntactic grammars can be combined, while the phonological counterparts could not? Essentially, this conjecture is based on the computational features of each interface—regardless of whether the system is monolingual or bilingual.

Within the syntactic interface, operations can take place at any given time. For instance, inserting a post-nominal adjective into a derivation before introducing a noun will still give rise to a correct surface form, as the lexical item is labeled with [adj. +post-nom.]. Based on this imperviousness to underlying order relations, there is no concern in stipulating that items from code-switched sentences are syntactically derived from an assembled set (i.e., a single lexical array constructed by C_HL).

On the other hand, phonological rules or constraints have been noted as being order-dependent at least since Bromberger and Halle (1989), who penned the Principle in (5). Since phonological grammars are sensitive to ordering, it would not be parsimonious to assume that code-switching is characterized by the united phonologies of the languages involved in the switch. Consider, for instance, a set of phonological rules {R₁, R₂, R₃}. In PS_x of L_x, R₁>R₂,R₃; in PS_y of L_y, R₁,R₃>R₂. What would take place if Spell-Out introduced a code-switched sentence composed of L_x and L_y into a combined PS? At the very least, the result of such a computation is challenging to obtain (though see Chapter 6 of López 2020 for a potential analysis).

Thus, the separation of the phonological systems within the architecture of the bilingual mind directly falls from the “imperfection” of phonology, notably its poor computational properties (Chomsky 2002: 118). It is a consequence of the order-specific nature of the phonological interface; if phonological systems were united in the model, their respective orders could be compromised. As strikingly noted by MacSwan & Colina (2014: 188), this entails that “language switching is phonological switching”, and nothing more. Such an observation falls into line with conceptualizations of language which claim that variation is confined to the externalization of language (Chomsky, Gallego & Ott 2019), while the internal language remains largely invariant.

The PF Interface Condition (PFIC; previously the PF Disjunction Theorem; MacSwan 2000: 45), spelled out in (6), accounts for the separation of the bilingual’s phonologies as per the computational properties of this interface (MacSwan & Colina 2014: 191). MacSwan and Colina assert (6.iv) that phonological switching is prohibited within words but not between words. This behaviour is based on the following assumption: on the way to PF, sensorimotor material is minimally parsed at the word level (see MacSwan & Colina 2014 for a discussion). Recent data does in fact indicate that word-internal phonological switching is virtually unattested, even if an acceptable morphological switch has occurred (Bessett 2017; see also MacSwan & Colina 2014; Stefanich & Cabrelli Amaro 2018).

Before continuing, it is important to distinguish code-switching from nonce-borrowing (and attested loanwords). According to original theory, code-switching, whether inter- or intra-sentential, involves unique processes (e.g., Grosjean 1995). Unlike switched segments, borrowed words are said to be morphologically, and often phonologically integrated into the target language (Poplack & Meechan 1998; though see Poplack, et al. 2020 for recent data). Since borrowings (and not code-switches) may be parsed by the phonological system of the recipient language (e.g., ‘rid-er’ is pronounced as French-like /ʁɑ͡ɪde/, even though it originates from the English riding /ɹɑ͡ɪdɪŋ/), these types of items do not give us much insight into the architecture of the bilingual’s phonological systems. From a computational perspective, sentences featuring borrowed words or loan words are not at all different from single-language speech; these items will not be considered in the empirical analyses of this paper.

In the subsequent section, past phonetic studies examining code-switching will be reviewed; these indirectly support MacSwan’s model of the bilingual language faculty.

A moderately large literature has investigated whether bilinguals abruptly switch between the phonetics of their languages when they produce mixed utterances, or alternatively, whether the phonetic systems somehow converge towards a new schema that is dissimilar from that of both languages in unilingual contexts. The results from these studies remain conflicting.

Early findings examining the voice-onset time (VOT) of French-English code-switchers indicate that bilinguals shift from one phonetic system to the next, with absolutely no phonetic remnant of the opposite language after a switching boundary (Grosjean & Miller 1994). These results have seldom been replicated (though see Brown 2015; Olson 2013 (bilingual contexts) for comparable conclusions). For the most part, studies have observed some sort of effect between the languages involved in the code-switch.

On one hand, some research indicates that code-switched speech may actually be hyper-articulated. This result has primarily been found for suprasegmental features, such as pitch (Olson 2012; see also Olson & Ortega-Llebaria 2010) and stress, as indicated by vowel length (Olson, 2012; Muldner et al. 2019). In other words, the suprasegmental features of the languages involved in the switch may diverge from each other, possibly in order to ensure maximal distance between their acoustic features. Hyper-articulation of code-switched items may also occur since switches are said to be “less predictable” (Olson 2012: 451) than single-language speech (see Bell, Brenier, Gregory, Girand & Jurafsky 2009 for supporting results). In any case, hyper-articulation is not problematic to MacSwan’s model of the bilingual language faculty; it still presupposes that bilinguals have encapsulated phonological systems and that code-switches are not phonologically integrated.

On the other hand, several studies indicate that phonetic convergence does indeed take place between languages involved in a code-switch; this is by far the most common finding (Bullock, Toribio, Davis & Botero 2004; Bullock, Toribio, González & Dalola 2006; Antoniou, Best, Tyler & Kroos 2011; Balukas & Koops 2015; Piccinini & Arvanti 2015; López 2012; Ojeda, de Prada Pérez & Wayland 2018). For instance, when participants produced switches between Spanish and English, the VOT values of stops in these languages may shift towards each other. Interestingly, this shift is often asymmetric: it is primarily observed within the language that contains a broader VOT spectrum. Thus, English VOT may shift towards shorter Spanish-like values, since these segments can afford to shift while respecting their own phonological thresholds (Bullock et al. 2006; Balukas & Koops 2015; Piccinini & Arviniti 2015).

Despite these complex and often antithetical findings, one result remains unchanged: the presence of phonetic convergence does not result in a merge of the systems. In fact, the phonetic shift never extends as far as the respective phonological categorization boundaries of each language. For example, Balukas and Koops (2015) observed a durational shift for English voiceless stops in code-switched compared to monolingual speech, notably towards shorter Spanish-like VOT values. Nevertheless, the VOTs of the code-switched English stops still remained within the typical English range (see also Bullock et al. 2004; 2006; Antoniou et al. 2011; Piccinini & Arvaniti 2015, etc.). This means that the often-observed phonetic convergence is, as its name implies, simply phonetic in nature. It does not negate the duality of the phonological systems according MacSwan’s model of the bilingual language faculty.

In brief, the acoustic literature illustrates that phonetic convergence (among other possibilities) is available for code-switches, but that this shift is virtually never phonological. These results rather support the notion that code-switching does not impact the phonologies of the languages involved in the mixed utterances. Up to this point, however, mostly indirect evidence concerning this matter has been examined. In the subsequent section, the duality of the phonological systems, as postulated by MacSwan’s model of the bilingual language faculty, is assessed. Unlike previous research, this is done by examining a facet that is quite distinct from the phonetic system: that of syllabic structure.

The goal of the current section is to investigate the underlying phonological system of code-switched utterances, in order to verify MacSwan’s model of the bilingual language faculty. One way of tapping into these rather abstract notions is by comparing code-switched languages which differ within a specific aspect of their respective phonological systems. For instance, MacSwan and Colina (2014) utilized the Spanish-specific phonological rule resulting in the realization of intervocalic stops as approximants. Unilingual English displays no pattern similar to the Spanish approximant rule. These authors observed that in Spanish-English code-switches favouring the application of the Spanish rule (e.g., /migowst/ ‘mi ghost’ → */miɰowst/), stops were only realized as approximants in about 10% of the recordings (compared to 93% in unilingual Spanish). However, phonological processes were successfully triggered for segments belonging to the same language in which the process was active (e.g., Spanish voicing rule: /misgowsts/ ‘mis ghosts’ → /mizgowst/). MacSwan and Colina thus cleverly demonstrated that code-switching boundaries involve an abrupt shift in the bilingual’s phonological systems.

In the current paper, I utilize the phonotactic structure (i.e., the permissibility of certain phoneme combinations and not others; Anttila 2008) of Spanish-English code-switched speech in order to garner insight into the phonological system(s) that is—or are—employed during mixed-language utterances. The majority of the research substantiates the idea that language-specific phonotactic patterns are psychologically real (e.g., Friederici & Wessels 1993; Jusczyk, Friederici, Wessels, Svenkerud & Jusczyk 1993; Jusczyk, Luce & Charles-Luce 1994), and that they arise from speakers’ underlying grammar (Coleman & Pierrehumbert 1997; Frisch, Large & Pisoni 2000; Shademan 2006; Albright 2008; though see Bailey & Hahn 2001; Hammond 2004). This means that examining such patterns effectively allow us to hone into the phonological component of the language faculty, as modelled by Chomsky and MacSwan.

The possible empirical outcomes are presented in (7). Outcome A) supports phonological encapsulation as it is posited in current model of the bilingual language faculty (MacSwan 2000); outcome B) does not support MacSwan’s model. Rather, outcome B) may indicate a degree of integration for bilinguals’ phonological systems.

The general phonotactic differences between Spanish and English are exploited in order to test the outcomes in (7). Spanish and English have distinct constraints regarding the legality of certain syllabic structures. English has a relatively lax syllable structure; it can accommodate up to three consonants in the onset position, and even more in the coda (e.g. strengths /strɛŋ(k)θs/ ➔ CCCVC(C)CC, glimpsed /ɡlɪmpst/ ➔ CCVCCCC). Contrastively, consonant clusters within a given syllable are greatly disfavoured in Spanish (e.g., Barlow 2003). As can be seen in Table 1, this is reflected in the relative frequency of syllable types in unilingual English (Delattre & Olsen 1969) and Spanish (Villanueva-Reyes 2013).

As it turns out, the grammatical distinction between English and Spanish is easily modeled with the Optimality Theory framework (OT). In OT (see Smolensky & Prince 1993), the ranking of universal constraints forms a language-specific grammar; the numerous permutations for hierarchical constraint rankings give rise to cross-linguistic differences in phonological systems. When comparing languages with different constraints rankings (and thus different superficial phonological attributes), one can use OT to determine which system is currently active in code-switched discourse.

In the present study, the relevant constraint which primarily accounts for the phonological differences between Spanish and English is *Complex (i.e., “Assign a violation for every syllable with a consonant cluster”). That is, *Complex possesses a distinct ranking in Spanish and English, being of a relative higher rank in the former than the latter (8). This simple distinction accounts for the resulting phonotactics of both languages, as can be seen in Tables 2 and 3 (loosely adapted from Misker & Anderson 2003).

In English (Table 2), *Complex is ranked lower than Dep-IO (i.e., ‘No insertion’), Peak (‘Syllables must contain vowels’) and Max-IO (‘No deletion’). Consequently, an input like /abzorbd/ ‘absorbed’ is syllabified into two units: a VC syllable, and a complex CVCCC syllable. Simply put, this pattern is obtained since English grammar ‘prefers’ outputs that preserve all of their original segments (i.e., faithful candidates), rather than outputs without consonant clusters. Crucially, this preference is determined by the outranking of Max.IO over *Complex (8a).

Contrastively, in Spanish (Table 3), an input such as /absorb-to/ ‘absorbed’ is syllabified into three units, none of which include a consonant cluster: VC, CVC and CV syllables. Syllabification occurs in this manner since consonant clusters are less desirable in Spanish than deleted segments, as determined by the ranking of * Complex over Max-IO (8b).

With only a handful of constraints, we can observe quite simply from where arise the differential phonotactic patterns in English and in Spanish. While this analysis may be slightly idealized, it demonstrates the key idea upon which the current section hinges: cross-linguistic differences in the ranking of *Complex give rise to distinct relative syllabic frequencies in Spanish and English. As such, this paper capitalizes on these distinct rankings of the *Complex constraint while examining natural speech from a Spanish-English corpus. If bilinguals do indeed possess encapsulated phonological systems (in line with MacSwan’s model and the constraint-free approach), the hierarchy of constraints in code-switched speech should respect the hierarchy of constraints of each respective language involved in the code-switch.

In line with the hypotheses, statistical models indicated that the language of production of each syllable was a significant predictor in determining syllabic frequency in intra-sentential code-switches. Context was neither a significant predictor for syllabic use in Spanish nor in English. As such, even when Maria was code-switching, the Spanish portions of her mixed utterances were Spanish-like, and the English portions of her mixed utterances were English-like (see assumption A in (12)). By contrast, context was neither a significant predictor for syllabic use in Spanish nor in English—Maria was not using an excess of complex onsets and codas in the Spanish portions of her code-switched speech, and she was not limiting these same syllabic structures in the English portions. These main findings demonstrate that Maria was actively and abruptly switching between the encapsulated phonological systems of her language.

Recall that MacSwan suggests that code-switching at the phonological form of language is unavailable, since PF is composed of hierarchical constraints: if the phonological forms were united, in a code-switching context, the order of the constraints would not be upheld. In consequence, when a derivation arrives at Spell-Out, two separate phonologies (PS_x and PS_y) are compartmentalized before PF. This means that lexical items originating from either lexicon can only be conditioned by L_x or L_y. The results from the current study thoroughly support MacSwan’s model: Maria’s language-specific orderings of *Complex were preserved during code-switching (i.e., the syllabic structure in her code-switched speech matched the syllabic structure of respective monolingual speech). Consequently, the current data also fail to support models which maintain that bilinguals possess fully integrated grammars (see López 2020); Maria displayed no convergence between her phonological systems. The interface of phonology is clearly dichotomized in the bilingual language faculty.

How can independent phonological systems be analyzed within OT? Preliminary considerations tend to support the notion of co-phonologies. The formalism of co-phonology has mostly been applied to morphologically conditioned phonological systems. In a co-phonology (e.g., Inkelas 1998; Inkelas & Zoll 2007), different morphological components (within a single language) are treated with different grammars (e.g., one for suffixes, one for roots). Co-phonologies have previously been implemented for words from different lexical classes (Inkela & Zoll 2007). From this line of thinking, it is not a huge leap to suppose that co-phonologies can be applied to different lexical sets, as in code-switching.

A co-phonology seems to correspond to what was observed in Maria’s code-switched speech: Maria abruptly alternated from one phonological system to the next when she mixed Spanish and English. In a code-switching co-phonology, each language involved in the utterance would possess its own compartmentalized phonological system. That is, phonological systems would map to a single language involved in the switch. Spoken works would thus be parsed by the appropriate system, depending on the language from which they originate.

Up to this point, the minimalist model of the bilingual language faculty (MacSwan 2000) has been supported. Utilizing the phonotactic differences between Spanish and English (i.e., regarding the *Complex constraint), the speech of a Spanish-English code-switcher was analyzed in order to determine whether she abruptly alternated between her phonological systems during code-switching. The analysis revealed differences in syllabic proportion between languages (Spanish vs. English), but not between contexts (unilingual vs. code-switched). These results reinforce the idea, in line with MacSwan’s model of bilingual language faculty, that A) distinct phonological systems are accessed during code-switching and B) that these systems correspond to those of each monolingual language involved.

We can now answer the first half of the primary research question: the bilingual language faculty is composed of two compartmentalized phonological systems. Just like MacSwan originally conjectured, “language switching is phonological switching” (MacSwan & Colina 2014: 188). In the next section, I will answer the second part of the research question. That is, I will review theoretical evidence that suggests that code-switching is composed of a single syntactic grammar. This evidence will once again be supported by in-the-field instances of Maria’s code-switched speech.

While the independence of the phonological systems is supported, this does not reflect the entire conceptualization of the bilingual language faculty. Recall that MacSwan’s model also ascertains that bilinguals (just like Chomsky’s monolingual) possess a computational system (C_HL) which combines items into a single syntactic derivation. That is, once items are chosen from the lexicon(s), the array is Spelled-Out in one fell swoop (or, potentially, in a series of phases; see López et al. 2017); structures are built when items are derivationally combined via the operation of Merge (MacSwan 2000).

Up to this point, I have taken the singularity of C_HL for granted. Nonetheless, another option is possible when it comes to the bilingual language faculty: individuals who speak more than one language may possess globally compartmentalized systems for each language, as in Figure 3 (right panel). Luis López (2017 and onward) coins this framework as separationism or the common-sense framework, since it is reflective of the way humans intuitively think about language (e.g., “I speak three languages, but my mom only speaks two.”). As MacSwan’s model also involves some degree of architectural separation (i.e., at the level of phonology), I will refer to the common-sense framework as global separationism. Global separationism entails that bilinguals’ morphosyntactic grammars5 are just as independent as their phonological systems.

Crucially, global separationism and MacSwan’s model make different assumptions with regard to multi-language structures. If global separationism is correct, the bilingual language faculty would only be able to Merge items from a single lexicon, since the C_HL is language-dependent, or language-encapsulated. Contrastively, MacSwan’s minimalist model of the bilingual language faculty allows for the combination of items originating from multiple languages.

You may think that global separationism is immediately falsified, since bilinguals evidently do combine items from more than one lexicon—that is the very core of intra-sentential code-switching, after all. However, a sceptic might argue that the mere existence of code-switching does not negate global separationism, since bilinguals may simply be building partial syntactic structures within their compartmentalized computational systems, before pronouncing these structures in juxtaposition. In other words, some may argue that code-switching has the outward appearance of a combined syntactic system, but that it is actually composed of independent structures being outputted side-by-side. This would entail that code-switching is a surface phenomenon that exists in performance but not in competence.

How can we verify whether this is the case? It seems that the solution lies in the examination of syntactic dependencies (i.e., c-command, grammatical case, complement relations), which presupposes the singularity of C_HL. The theoretical outcomes are the following.

The diverse syntactic code-switching literature, as well as Maria’s speech from the Bangor-Miami corpus, provide us with ample opportunities to verify these assumptions. Much of the argumentations provided in this section are inspired by Luis López’ generativist body of work, most notably his recent book (2020). Recall that, unlike MacSwan, López believes that the bilingual language faculty is combined at every interface, including PF. While the syllabic data from Section 3 markedly refutes the integration of a bilingual’s phonological systems, this does not mean that López’ entire model is unsupported. In fact, MacSwan and López appear to agree in their assumption that bilinguals derive code-switches from a unique syntactic subset. Maria’s naturalistic data provides more evidence for this conjecture.

Given that much of the evidence provided in the subsequent sections was also presented by López (2020), novel data (i.e., Maria’s data from the Bangor-Miami corpus; syntactic judgements obtained by the author) are identified as such.

According to Hoeksema (2000: 115), negative polarity items (NPIs) are linguistic units that “give rise to minimal pairs of affirmative and negative sentences”. Crucially, in order to produce grammatical sentences, NPIs must be c-commanded (see Chomsky 2014) by negative elements, like not (Giannakidou 2011). Thus, the difference between the sentences in (16) is that any is licensed (or triggered) in (16a) but not in (16b).

Past data (and novel judgements)6 indicate that sentences containing opposite-language NPIs and negation triggers are acceptable, as indicated in (17–19). In each case, the NPI is bolded and the license is underlined. Under a globally separatist point of view, such sentences should be ill-formed, since syntactic relations like c-command can only take place within the language-compartmentalized C_HL. On the other hand, MacSwan’s model predicts that such constructions should be entirely acceptable, since lexical arrays are built into a single syntactic structure and can thus Command one another.

In each of these cases, a syntactic dependency is established between an NPI and a trigger that originate from distinct lexical sets. That is, a negative particle from L_x licences an NPI from L_y. Thus, the singularity of the syntactic component is supported: the examples above clearly demonstrate that code-switching is not simply the juxtaposition of two partial syntactic structures constructed in language-dependent C_HL. In actuality, the lexical items from the code-switched language pairs are united into a combined array, from which the unique C_HL can build a single syntactic construction.

Grammatical case is associated to the syntactic or grammatical function of an item within a given sentence. Many languages possess complex case systems, wherein grammatical case is indicated by an inflectional morpheme; the sentence becomes unacceptable in the absence of case marking or when the wrong case is assigned. For instance, in (20b) bedny- ‘poor’ is assigned the genitive case instead of the dative case (as in 20a), and the resulting sentence is ill-formed.

If case assignment occurs between a verb from L_x and an item from L_y, we can say that there is a syntactic dependency between these items, and that they co-exist within the same C_HL. Past evidence suggests that these syntactic dependencies do indeed take place in code-switching (21–24). For instance, in the sluicing constructions in (21), the verb ‘to threaten’ assigns the dative case in German (21a), but the accusative case in Spanish (21b). However, in the code-switch, the German remnant ‘who’ must possess the accusative case (21c). This judgement was confirmed by an acceptability task (González-Vilbazo & Ramos 2011). López (2020) also gives evidence that these types of constructions occur (23–24).

Altogether, these data illustrate that grammatical case assignment can take place between items from different languages. As case assignment entails syntactic dependency (assumption A), it is clear that the intra-sentential code-switches shown above were derived from a singular syntactic construction. The evidence above therefore supports the singularity of the syntactic component, in line with MacSwan’s formulation of the bilingual language faculty.

López (e.g. 2020) originally introduced the ‘Mixed Selection’ problem by examining code-switching between light verbs and infinitives. To generalize this concept, I assume that Mixed Selection has occurred when an item from L_x selects one from L_y as its complement, or vice versa.

In the current section, the second half of MacSwan’s model of the bilingual language faculty was supported: code-switching is in fact governed by a singular morphosyntactic grammar. The evidence from the literature and Maria’s spontaneous speech shows that syntactic dependencies do indeed take place between different-language items. This is not predicted by global separationism, which presupposes globally encapsulated systems for each language known by a bilingual or multilingual.

We can now answer the second half of the primary research question. Contra the “common-sense” hypothesis, the bilingual language faculty is composed of a unique computational system, wherein syntactic constructions are built.