Are there words that are universally understood, across all countries and cultures? A team of linguists has proposed one: “huh.” 
 
 Huh? 
 
 In a paper published on Friday in the journal PLOS One, researchers at the Max Planck Institute for Psycholinguistics in the Netherlands announced that they had found strikingly similar versions in languages scattered across five continents, suggesting that “Huh?” is a universal word. 
 
 The study, conducted by Mark Dingemanse, Francisco Torreira and Nick Enfield, closely examined variations of the word — defined as “a simple syllable with a low-front central vowel, glottal onset consonant, if any, and questioning intonation” — in 10 languages, including Dutch, Icelandic, Mandarin Chinese, the West African Siwu and the Australian aboriginal Murrinh-Patha. 
 
 The researchers also looked at other words and expressions used to elicit clarification during conversation, a function that linguists refer to as “other-initiated repair.” But only “Huh?,” they write, occurs across languages whose phonetic patterns otherwise vary greatly. ||||| A word like 'Huh?' —used when one has not caught what someone just said—appears to be universal: it is found to have very similar form and function in languages across the globe. This is one of the findings of a major cross-linguistic study by researchers Mark Dingemanse, Francisco Torreira and Nick Enfield, at the Max Planck Institute for Psycholinguistics in Nijmegen, the Netherlands. The study was published in the journal PLOS ONE. 
 
 It might seem frivolous to carry out scientific research on a word like 'Huh?' But in fact this little word is an indispensable tool in human communication. Without words like this we would be unable to signal when we have problems with hearing or understanding what was said, and our conversations would be constantly derailed by communicative mishaps. The research is part of a larger investigation of language and social interaction funded by the European Research Council. 
 
 Dingemanse and colleagues studied languages from around the world and found that all of them have a word with a near-identical sound and function as English 'Huh?' This is remarkable because usually, words in unrelated languages sound completely different. Compare, for example, these very different-sounding words for 'dog': inu in Japanese, chien in French, dog in English. One might object that this suggests that 'Huh?' is not a word at all. But in a careful phonetic comparison, Dingemanse and colleagues find that it is. Although 'Huh?' is much more similar across languages than words normally should be, it does differ across languages in systematic ways. 'Huh?' is not like those human sounds that happen to be universal because they are innate, such as sneezing or crying. It is a word that has to be learned in subtly different forms in each language. 
 
 Why is 'Huh?' so similar across languages? To understand this, Dingemanse and colleagues studied the specific context in which this word occurs. In human communication, when we are somehow unable to respond appropriately, we need an escape hatch: a way to quickly signal the problem. This signal has to be easy to produce in situations when you're literally at a loss to say something; and it has to be a questioning word to make clear that the first speaker must now speak again. Since these functional requirements are fundamentally the same across languages, they may cause spoken languages to converge on the same solution: a simple, minimal, quick-to-produce questioning syllable like English 'Huh?', Mandarin Chinese 'A?', Spanish 'E?', Lao 'A?', or Dutch 'He?'. 
 
 The basic principle is well-known from evolutionary biology: when different species live in similar conditions they can independently evolve similar traits, a phenomenon known as convergent evolution. For example, sharks and dolphins have different evolutionary origins but similar body plans, because they live in the same aquatic environment. In the same way, Dingemanse and colleagues propose that words may converge on similar forms when they occur in strongly similar conversational 'environments'. A clear effect of this conversational ecology on the specific shape of linguistic expressions has not been observed before. 
 
 Although 'Huh?' may almost seem primitive in its simplicity, a word with this function is not found in our closest evolutionary cousins. Only humans have communication systems in which complex thoughts can be expressed and communicative mishaps can be solved on the spot. Even a humble word like 'Huh?' can teach us a lot about our nature as ultrasocial animals. ||||| Humans speak many languages, but we may be united in our confusion. A new study examined languages from around the world and discovered what they say could be a universal word: "Huh?" 
 
 Researchers traveled to cities and remote villages on five continents, visiting native speakers of 10 very different languages. Their nearly 200 recordings of casual conversations revealed that there are versions of "Huh?" in every language they studied — and they sound remarkably similar. 
 
 While it may seem like a throwaway word, "Huh?" is the glue that holds a broken conversation together, the globe-trotting team reported Friday in the journal PLOS ONE. The fact that it appears over and over reveals a remarkable case of "convergent evolution" in language, they added. 
 
 "Huh?" is a much-maligned utterance in English. It's seen as a filler word, little more than what's called a "conversational grunt," like mm-hmm. But it plays a crucial role in conversations, said Herbert Clark, a psychologist at Stanford University who studies language. 
 
 When one person misses a bit of information and the line of communication breaks, there needs to be a quick, easy and effective way to fix it, he said. 
 
 "You can't have a conversation without the ability to make repairs," said Clark, who wasn't involved in the study. "It is a universal need, no matter what kind of conversation you have." 
 
 Without something like "Huh?" a conversation could be quickly and irreversibly derailed at the slightest misunderstanding. That would be bad news for a highly social species that relies on good communication to survive. 
 
 For this study, scientists from the Max Planck Institute for Psycholinguistics in the Netherlands set out to show that "Huh?" had earned the status of a full-fledged word, though an admittedly odd one. They also wanted to see whether other languages had a similar word with a similar function. 
 
 The problem is that "Huh?" often seems like such an unimportant feature of language that it's not well documented, said Nick Enfield, a linguistic anthropologist who worked on the study. The word doesn't crop up much in linguistic literature because researchers who record speakers of remote languages often ignore such forgettable filler. 
 
 The scientists knew that to find out whether "Huh?" had counterparts in other languages, they'd have to go looking themselves. So they headed to remote villages in Ecuador, Laos, Ghana and Australia and spent weeks getting acquainted with the locals. They felt they had to gain people's trust before they could record natural, casual conversations — and perhaps catch a few instances of "Huh?" in its natural environment. 
 
 "The kind of conversations we collected were just the kind of conversations you and I would have at the breakfast table or in the evening when we're doing our handicrafts," Enfield said. 
 
 The "Huh?"-hunters also visited family homes in Italy, Russia and Taiwan as well as laboratories in Spain and the Netherlands. The languages studied were Cha'palaa, Dutch, Icelandic, Italian, Lao, Mandarin Chinese, Murriny Patha, Russian, Siwu and Spanish. (English wasn't included in the study.) 
 
 Across these languages, they found a remarkable similarity among the "Huhs?" All the words had a single syllable, and they were typically limited to a low-front vowel, something akin to an "ah" or an "eh." 
 
 Sometimes this simple word started with a consonant, as does the English "Huh?" or the Dutch "Heh?" (Spellings are approximate.) Across all 10 languages, there were at least 64 simple consonants to choose from, but the word always started with an H or a glottal stop — the sound in the middle of the English "uh-oh." 
 
 Every version of "Huh?" was clearly a word because it passed two key tests, the scientists said: Each "Huh?" had to be learned by speakers, and each version always followed the rules of its language. For example, English speakers ask questions with rising tones, so when they say "Huh?" their voices rise. Icelandic speakers' voices fall when they ask a question, and sure enough, the tone goes down as they ask, "Ha?" (To an English speaker, this tone would sound like a statement of fact: "Huh.") 
 
 "It's amazing," said Tanya Stivers, a sociologist at UCLA who was not involved in the study. "You do see that it's slightly different ... and that it seems to adapt to the specific language. I think that's fascinating." 
 
 After all, Stivers pointed out, words with the same meaning sound very different in different languages: "Apple" in English is "manzana" in Spanish, "ringo" in Japanese and "saib" in Urdu. Why wouldn't "Huh?" also sound completely different across unrelated languages, they wrote — say, "bi" or "rororo"? 
 
 The Dutch researchers think it's because the word developed in a specific environment for a specific need — quickly trying to fix a broken conversation by getting the speaker to fill in the listener's blank. 
 
 A low-front vowel in the "ah" or "eh" families involves minimal effort, compared with to a high vowel like "ee" or a lip-rounder like "oo." The same can be said for a glottal stop or a "h" — hardly any mouth movement is needed to make those sounds. This allows speakers to very quickly signal that they missed a bit of information, and request it again. (In response, the other speaker will typically repeat what they just said, sometimes modifying it for good measure.) 
 
 The linguists borrowed a term from biology to describe this phenomenon: "convergent evolution." Just as sharks and dolphins developed the same body plan to thrive in the water even though they're from very different lineages, all languages have developed a "Huh?" because it's so useful for solving a particular problem, researchers said. 
 
 "'Huh?' has almost certainly been independently invented many, many, times," said Mark Pagel, who studies language evolution at the University of Reading in England and was not involved in the PLOS ONE study. "And that is why it appears universal." 
 
 amina.khan@latimes.com ||||| Corrections The PLOS ONE Staff Correction: Is "Huh?" a Universal Word? Conversational Infrastructure and the Convergent Evolution of Linguistic Items. PLoS ONE 9(4): e94620. doi: 10.1371/journal.pone.0094620 | View correction 
 
 Abstract A word like Huh?–used as a repair initiator when, for example, one has not clearly heard what someone just said– is found in roughly the same form and function in spoken languages across the globe. We investigate it in naturally occurring conversations in ten languages and present evidence and arguments for two distinct claims: that Huh? is universal, and that it is a word. In support of the first, we show that the similarities in form and function of this interjection across languages are much greater than expected by chance. In support of the second claim we show that it is a lexical, conventionalised form that has to be learnt, unlike grunts or emotional cries. We discuss possible reasons for the cross-linguistic similarity and propose an account in terms of convergent evolution. Huh? is a universal word not because it is innate but because it is shaped by selective pressures in an interactional environment that all languages share: that of other-initiated repair. Our proposal enhances evolutionary models of language change by suggesting that conversational infrastructure can drive the convergent cultural evolution of linguistic items. 
 
 Citation: Dingemanse M, Torreira F, Enfield NJ (2013) Is “Huh?” a Universal Word? Conversational Infrastructure and the Convergent Evolution of Linguistic Items. PLoS ONE 8(11): e78273. doi:10.1371/journal.pone.0078273 Editor: Johan J. Bolhuis, Utrecht University, The Netherlands Received: July 13, 2013; Accepted: September 18, 2013; Published: November 8, 2013 Copyright: © 2013 Dingemanse et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study was supported by European Research Council grants 240853 (N.J.E. and M.D.) and 269484 (F.T.). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. 
 
 Materials and Methods We collected 196 instances of the interjection for other-initiated repair (henceforth OIR interjection) in videotaped recordings of conversation in a worldwide sample of 10 languages (mean instances per language = 19.6, sd = 7.5). We used field recordings of maximally informal conversation because most written sources do not offer enough phonetic detail and people's intuitions about their behaviour can be different from their actual behaviour [16]. We examined at least ten tokens per language to find out whether or not the same articulatory target is aimed for within and across languages. All data were collected in accordance with protocols approved by the ethical review board of the Seventh EU Framework (240853 HSSLU). Informed consent was obtained from all participants according to standard practices [17], [18]. The data were anonymised and unlinked and there is no possibility of identification. We used a two-stage approach to comparative analysis of the tokens. In an auditory analysis, we collected phonetic auditory judgements of interjection tokens by three annotators and combined them into graded measures along five phonetic dimensions for every single token (see File S1). In an instrumental analysis, we took acoustic measurements on a subset of tokens and used these to verify the auditory judgements for selected dimensions. The combination of auditory and instrumental approaches enabled us to carry out an analysis that is ecologically valid and well controlled. In the auditory analysis, all interjection tokens (n = 196) were presented one by one in random order to three annotators independently. No separate information about language or recording was provided. Annotators listened to the audio clips with spectrograms and pitch tracks available on screen, and coded every token for five phonetic dimensions selected on the basis of preliminary observations of the range of variation: closure, nasality, vowel quality, intonation, and onset (see SI). Articulatory gestures in spoken language are essentially gradient [19]. Therefore, the coding results were combined into cumulative measures per token per dimension, allowing us to measure and display the variation in, for instance, vowel quality or consonant onset by language. In the instrumental analysis, we took acoustic measurements of intonation and the first two vowel formants for languages in which token quantity and acoustic quality permitted this, namely Spanish and Cha'palaa. For Spanish, all tokens came from laboratory recordings of casual conversation [20]; for Cha'palaa, the large number of tokens in the field recordings permitted instrumental analysis. Some acoustically inferior interjection tokens (due to overlapping speech or ambient noise) and some tokens spoken by children were discarded. In total, 13 Cha'palaa tokens and 12 Spanish tokens were analysed instrumentally. Pitch values throughout each interjection were computed, and formant values of vowels were measured at the point of maximum intensity using the Burg method implemented in the software Praat [21]. 
 
 Discussion Is huh? a word? In work on English conversations, the interjection huh? has been characterised as a “non-lexical token” [9] or a “non-lexical conversational sound” [8]. Yet our phonetic analysis shows that despite the overall similarity across languages, the OIR interjection is systematically calibrated to the language system in which it is integrated. This motivates the question whether huh? is a word. Two key characteristics of words are ‘integration’ – they are items in larger linguistic systems, and ‘conventionalisation’ – one cannot know them without having learnt them. Non-linguistic vocalisations like crying or grunting are the opposite on both counts: they are not integrated in linguistic systems, and one does not need to learn them to know them. Integration. In all languages investigated, the sound of the OIR interjection shows some degree of calibration to local linguistic systems. Vowel targets are language-specific and appear to be drawn to existing phonemic targets, e.g. /e/ for Spanish and /a/ for Cha'palaa. Intonation melodies appear to be linked to the interrogative prosodic system, which may differ from language to language. The occurrence and quality of consonant onsets is related to the consonant inventory of the language. The interjection is also part of a larger paradigm of expressions for the other-initiation of repair, including, in English, other items like what? and pardon? [5], [15]. Huh? is thus an item integrated in several linguistic subsystems, from segmental and prosodic phonology to conversational structure. Conventionalisation. Huh? exhibits linguistic conventions that speakers need to learn in order to use the form properly. A learner of Spanish has to know that repair is initiated with the mid front unrounded vowel “e↗”, a learner of Cha'palaa has to know that the form is more like “a ” with falling intonation, and a learner of Dutch has to know that a glottal fricative at onset is common: “h ”. Its acquisition follows a normal trajectory, at least in American English-speaking children [37]. Second language learners' reports confirm that the precise form of this interjection has to be learnt, and that intuitions are not necessarily a reliable guide in this process [38]. Perhaps there is a continuum from non-linguistic vocalisations like sneezing and crying to prototypical conventional lexical items like bless you and pain [39]. Our evidence suggests that huh? is more on the word side of that continuum. Based on the fact that huh? is integrated in multiple linguistic subsystems and conventionalised in language-specific ways we conclude that huh? a lexical word. Is huh? universal? Although there is systematic calibration to specific language systems, the bandwidth of the variation of OIR interjections across languages is exceedingly narrow. In all languages investigated, it is a monosyllable with at most a glottal onset consonant, an unrounded low front central vowel, and questioning intonation. Narrow bandwidth of variation. We have already shown that the uniformity of the interjections is in striking contrast to the question words that languages can recruit for the same function (Table 1). Another way to appreciate the small range of cross-linguistic variation exhibited by this form is to consider it in context of the possibility space for words in spoken languages. Across languages, words can consist of one or more syllables, but the OIR interjection was never longer than one syllable in the languages we have studied, even in those like Murrinh-Patha, for which phonological words are generally longer than one syllable. Across languages, syllables can have rich internal structure, but the only structure attested in the OIR interjection is (C) V, i.e. a vowel V with an optional onset consonant C, even in languages like Dutch, where CVC syllables are common. Similarity in vowels and consonants. Strong constraints on variation are also seen in the vowels and consonants employed. Vowel space can be depicted as a two-dimensional plane formed by height and backness. On average, languages have around 6 vowel phonemes [40], which tend to be maximally spread across this space to increase perceptual distinctiveness [41]. Given this fact, it is striking that the vowels of OIR interjection tokens are only found in the low front central corner of vowel space (Figure 2), and that on a third dimension of lip rounding OIR interjections are only found on the ‘unrounded’ side. Consonants are articulated at different locations throughout the vocal tract (lips, teeth, alveolar ridge, palate, velum, uvula, pharynx, epiglottis) and with different manners of articulation, from plosives, nasals and trills to taps, fricatives, and glides – a multidimensional possibility space in which the International Phonetic Alphabet records at least 64 simple phonemic consonants (and three times as many complex variants) attested in the world's languages [4]. Out of this enormous range of possibilities, only two basic sounds, the glottal consonants [ʔ] and [h], are found in the OIR interjection across languages. Such limited variation and striking similarity across languages is wholly unexpected on the basis of the principle of the arbitrariness of the sign. Does this mean that huh? is a universal word? We propose a qualified yes. Qualified, because huh? is clearly not phonetically the same word across languages – if Cha'palaa tokens were cross-spliced into Spanish dialog, Spanish speakers would likely be confused. What appears to be universal is the function of this interjection along with a set of constraints determining its form. Other-initiated repair sequences have been found in all languages investigated so far, and no language appears to lack an interjection for this function. Thus huh? is universal in the sense that a short, questioning interjection like it with the function of initiating repair is likely to be attested in all natural spoken languages. Possible explanations As we have seen, huh? is so common as to be practically universal, and yet calibrated to specific language systems such that it qualifies as a word. The language-specific nature of words is of course expected; it is the strong similarity that is in need of an explanation. Why do we find basically the same form –something like huh?– everywhere and not, say, bi in one language and rororo in the next? We consider two possible explanations. The first is that huh? is similar across languages because it is an innate grunt. The second is that it is similar as a result of convergent evolution. Empirical evidence supports the second. Innateness. On one account, huh? may be similar across languages because it is a natural symptom with a biological basis, like laughs and screams – a “grunt” [8], [10]. Such qualifications, common in the wider literature on interjections, place huh? in a position close to instinctive cries [7], [42]. This would be one explanation for its similarity: it is innate, therefore all humans share it, therefore it assumes roughly the same form in all languages. This view is as hard to support as it is to discount, but we note four doubts. No known phylogenetic precursor. Whereas laughter and pain cries (and by extension the conventionalized interjections associated with them) have demonstrable phylogenetic precursors in other mammals [43]–[46], there is, to the best of our knowledge, no evidence for an animal precursor of huh?. Nor is it obvious what the function and biological survival value of this precursor would be in primates which lack the kind of shared intentionality that underlies human cooperative communication [47], [48]. Not an involuntary response. Grunts and other non-linguistic vocalisations such as sneezes and pain cries are typically direct, involuntary responses to stimuli [49]. In contrast, the OIR interjection is selected for a specific purpose at a specific juncture in conversation from a larger system of alternative formats for initiating repair [15], [50]. A greater degree of agency over utterance and selection is characteristic of linguistic rather than instinctive expressions. Acquired like a normal word. Whereas non-linguistic vocalisations like sneezes, cries and smiles are present at birth or develop soon after [51], [52], the acquisition of huh? follows a trajectory that is very similar to that of other linguistic items. In American English-speaking children, it is employed and responded to somewhat erratically at 2.5 years but perfectly at 5 years [37]. Related to this, the variability of laughs and screams appears to be much greater than what we find for huh?, and is not as strongly regimented by language [44], [53]–[55]. Parsimony. In terms of evolutionary history, language is a recent arrival that shows clear signs of being a bio-cultural hybrid: a complex adaptive system in continuous cultural evolution within a landscape of cognitive, cultural, and communicative factors [56]–[59]. Although some of our linguistic abilities are no doubt underpinned by genetic infrastructure, positing innateness for specific linguistic items would hardly be realistic given the timescale involved. Strong cultural universals do not necessarily imply strong innate biases [60] and strong innate biases are unlikely to evolve in cultural systems [61]. If there is a mechanism that can explain cross-linguistic similarity on a more proximate timescale, without resorting to genetic encoding, this is preferred on scientific principles of parsimony. Convergence. A more plausible mechanism for the cross-linguistic similarity of huh? is convergent cultural evolution. This proposal sees huh? not as an arbitrary grunt but as a product of cultural evolution in the adaptive context of its interactional environment. The basic principle is well-known from biology: similar environmental constraints have led to the independent evolution of similar body plans in sharks and dolphins, and in the placental mammals of North-America and the marsupials of Australia. Likewise, we propose that the similarity of huh? in unrelated and distantly related languages is due to the fact that it is found in a strongly similar environment in all these languages. What is this environment like? Conversations are built out of sequences of communicative moves between speakers [62], [63]. These moves –or ‘turns at talk’– are allocated in systematic ways and bear special sequential relations to each other [64], [65]. For instance, a question sets up an expectation that the addressee will provide a fitted response –in this case an answer– in the next move. Speakers inspect moves for their fittedness and aim to minimize gaps and overlaps between them. Speaker change most often takes only between 100–300 milliseconds, and deviations from the timing target can be treated as problematic [66], [67]. In order for this tight timing to work, planning a next turn often has to start well before the end of the preceding turn [68], [69]. Trouble in hearing or understanding is a regular feature of conversation [5], [47]. In the case of such trouble, planning and producing a fitted and timely response will be harder (indeed at times impossible), but the pressure to produce one will be just as strong. Given these pressures of turn-taking and formulation in conversation, a signal that indicates trouble should be minimal and easy to deploy. At the same time, given the communicative importance of indicating trouble (which if not solved might derail the conversation), such a signal should also clearly indicate a knowledge deficit and push for a response. These requirements are met rather precisely in the combination of minimal effort and questioning prosody that characterises the OIR interjection across languages. Minimal effort. Many of the formal aspects of the OIR interjection minimize articulatory effort. The codaless monosyllable is the least marked syllable type across languages [68], [70]. The glottal onset, where present, is simply some constriction at the narrowest place in the vocal tract, and the unrounded low front central vowel is close to the neutral state of the articulators – both requiring minimal encoding, planning, and articulation [34]. Additionally, for Spanish phonetic corpus studies show that the vowel target of the interjection is the most frequently attested vowel [71], making retrieval, planning, and production easier [72]. These features render the OIR interjection well-fitted to the interactional environment of other-initiated repair. For the person initiating repair, the OIR interjection is quickly deployable from intention to articulation [68], and therefore easy to produce even under conditions of cognitive duress. For the addressee, the minimal form is a word that is unlike most content words and therefore –by Darwin's principle of antithesis [45]– a good signal that the other has no contentful response on offer. Questioning prosody. If ‘minimal’ were the only design requirement, the most low-effort form possible would be enough. But to carry out the work of initiating repair, the OIR interjection also has to signal a knowledge deficit and indicate that a response is needed. We have seen that the intonation of the interjection appears to be calibrated to local systems of questioning prosody. In many languages this means that it has rising intonation – a contour that requires more effort than falling intonation [73], and (in English-speaking infants) has been shown to elicit greater attention [74]. In Cha'palaa and Icelandic, where the OIR interjection has falling intonation, it has a low central unrounded vowel – the vowel that is inherently most sonorous and acoustically salient due to the wide open oral cavity [75]. We propose that the questioning prosody and the acoustic salience of the interjection render it more adaptive for the function of OIR. As a question word devoid of semantic content, it expediently returns the floor to the original speaker and signals that there is trouble to be fixed. In effect, huh? is an easy to produce, maximally underspecified question word – a tight fit of form and function found in language after language. We propose that this is the result of convergent cultural evolution: the interactional environment of other-initiated repair, present in every language investigated so far, provides a set of selective pressures that pull the interjection towards a similar form and that keep regular processes of language change from affecting the item. This process of convergent evolution explains the narrow bandwidth of the variation, but also the language-specific calibration of the items. To minimize articulatory effort, the OIR interjections of different languages will end up in the same low-effort area of the phonetic possibility space; yet to be recognised as questioning expressions, they will be calibrated to local phonological and prosodic subsystems. We use ‘convergent evolution’ as a general term for the independent evolution of similarities in form and function. When ancestral forms are known, a distinction can be made between form/function convergence in species that are closely related (‘parallel evolution’) versus in species that are not closely related (‘convergent evolution’). However, this distinction is not always consistently made in biology and recently there have been proposals to use ‘convergent evolution’ as a general term [76]. We use the term in this general sense. Our proposal accounts for the present-day cross-linguistic similarity of huh?, but has to remain agnostic as to its ultimate origins – in the absence of historical language data it is impossible to tell whether the present-day forms go back to one ancestral form (a stabilising evolution scenario [77]) or whether they arose independently in different languages (an independent convergent evolution scenario [78]). In either case, the selective pressures are the same. The convergent evolution proposal explains the forms documented so far, but also generates the prediction that in undescribed languages as well as newly emerging ones, we can expect to find a similar repair initiator that minimizes articulatory effort while making use of questioning prosody. Independently emerged sign languages of the deaf, though in a different expressive modality (visual-only instead of audio-visual), provide a good test case. Consistent with our proposal, in Argentinian Sign Language, repair can be initiated with a minimal sign that involves a raising of the eyebrows, the semiotic equivalent of questioning prosody [14],[79]. Conversational infrastructure and convergence of linguistic form. Apart from its explanatory and predictive value, the convergent evolution proposal offers a more general mechanism. For most words in most languages, there is no necessary connection between form and function. This is why words can change over time, and why we expect even words with similar functions to have different forms in unrelated languages. Accordingly, cultural evolutionary models of language change have tended to depict languages as collections of words evolving in utterances [80], [81], with various social and cognitive biases influencing transmission [82], [83] and with frequency of use as a primary factor influencing rates of change and divergence [84], [85]. However, our study points to a factor that may constrain divergence or diachronic drift: the selective pressures of specific conversational environments, which may cause convergent cultural evolution. The possibility should not be surprising. After all, words evolve in utterances in conversation, so conversational infrastructure is part of the evolutionary landscape for words. We are referring here to the sequential infrastructure that serves as the common vehicle for language use – an infrastructure that may well predate more complex forms of language and that seems largely independent of sometimes radical differences between individual languages [63], [66], [86], [87]. A clear effect of this conversational ecology on the cultural evolution of linguistic items has not, to our knowledge, been observed before. Though we have focused on huh? as a case study, the mechanism we propose has wider relevance. In our corpora, we have noted other items that are strongly similar in form and function across unrelated languages: continuers like mm/m-hm [88], hesitation markers like uh/um [89], [90], and change of state tokens like oh/ah [91]. It would be neither plausible nor parsimonious to propose that all of these have precursors in distinct innate grunts. Instead, we observe that these interjections all serve important discourse regulatory functions, and we propose that the reason they are so similar across languages is that common communicative needs and conversational infrastructure conspire to create, for each of them, a set of similar selective pressures constraining their evolution. The ultimate fit to the tight constraints of their conversational environments, these words stay put and help us conduct conversation in optimal ways. The approach followed in this study can be systematically extended to the larger set of discourse regulatory expressions and beyond, to explore further effects of conversational ecologies on language structure. 
 
 Conclusions We have presented evidence and arguments that huh?, or more precisely a short questioning interjection with the function of other-initiation of repair, is a universal word likely to be attested in similar form in all natural spoken languages. The similarity of this interjection across languages is unlikely to be specified in our genetic makeup and we argue that it is the result of convergent cultural evolution: a monosyllable with questioning prosody and all articulators in near-neutral position is the optimal fit to the sequential environment of other-initiated repair. Our proposal invites closer attention to the infrastructure for social interaction that underlies language in use, and its possible influence on language structure. It also enhances existing models of language evolution and change by providing a mechanism for the convergent cultural evolution of linguistic items: conversational environments may exert selective pressure towards the evolution of common optimised forms, calibrated to local linguistic systems. Hence, we see how the investigation of a seemingly banal everyday word –previously characterised as a grunt or dismissed as a non-lexical sound– can shed light on the emergence and motivation of linguistic signs. 
 
 Supporting Information File S1. Combined Supporting Information containing a description of the auditory analysis, the coding scheme used, and further information related to nasality and mouth aperture. doi:10.1371/journal.pone.0078273.s001 (PDF) 
 
 Acknowledgments This work was carried out in the project “Interactional Foundations of Language” within the Language and Cognition Department at the Max Planck Institute for Psycholinguistics. We thank our colleagues (individually credited in Table 2) for making interjection tokens from their corpora available for analysis. We thank Dan Dediu, Carlos Gussenhoven, Sean Roberts, and Joe Blythe for helpful comments on earlier drafts, Mark Sicoli for providing some Meso-American data points, and the PLoS One reviewers for constructive feedback.
– Listen in on any language anywhere in the world and you're bound to hear a familiar sound: "Huh?" Dutch researchers who studied 10 languages on five continents say the one-syllable utterance might be that rarest of things: a universal word. And it's no mere tic or meaningless grunt, they say in their PLOS One study: It serves the vital purpose of allowing one person to let another know quickly that he doesn't understand something. It's "the glue that holds a broken conversation together," explains the Los Angeles Times. "You can't have a conversation without the ability to make repairs," says a Stanford expert not involved with the study but intrigued by it. "It is a universal need, no matter what kind of conversation you have." The researchers found the word in widely different languages ranging from Icelandic to Mandarin Chinese to West African Siwu, reports the New York Times. And though it may sound a bit different in some tongues—more like "ah" or "eh"—it remains fundamentally the same. They chalk it up to "convergent evolution," notes Science Codex, a term from biology. It's used to explain how different species, such as dolphins and sharks, evolve in similar ways because they live in similar environments. It seems that "huh?" is the linguistic version, a tool so useful to humans everywhere that they can't live without it. (In other language news, researchers suggest that being bilingual helps ward off dementia.)