Recent work in linguistics has challenged the very core assumptions of what the design principles of language are, including recursion (Everett, 2005; Karlsson, 2007), language universals (Evans & Levinson, 2009) and arbitrariness (Dingemanse, 2012; Hinton et al., 1994). In the latter line of research, Winter et al. (2022) suggested that alveolar trills (i.e., the way /r/ is often produced, e.g., in Russian or Spanish) are associated with roughness, and that this association is based on an iconic relation that links auditory amplitude modulation with the haptic experience of exploring rough surfaces. They surveyed the vocabulary of different languages and found that the likelihood of a word containing an /r/ is much higher if that word has a “rough” meaning (e.g., abrasive). Importantly, this pattern was not observed for languages that used another version of an /r/ sound – called rhotics in phonology – such as a uvular fricative [ʀ] or an alveolar approximant [ɹ].

Anselme et al. (2025) questioned Winter et al.’s (2022) coding methodology for rhotic variants (which is a more general issue in illustrations of the IPA; see Anselme et al., 2023). Their analysis found an association of any rhotic, not just trilled /r/, with roughness in lexicons of various languages. We see two explanations for this finding. The first explanation relies on sound change. Rhotic sound change often moves away from the alveolar trill (Lancien et al., 2023; Mitterer et al., 2025; Sebregts, 2015; Wiese, 2001), and the alveolar trill is often considered the ur-rhotic (Chabot, 2019; Wiese, 2003). Therefore, it is conceivable that these associations might arise while a language uses an alveolar trill for the rhotic, and then the association between the rhotic and roughness remains in the language even when the phonetic substance of the rhotic changes. This is, in fact, how Winter et al. (2022) explain the relation between roughness and rhotics in English, which mostly uses approximants for /r/. Another possibility is that the phonological properties of the rhotic phoneme /r/, rather than the phonetic properties of the alveolar trill [r], are associated with roughness (Anselme et al., 2025). In the current study, we aim to provide relevant experimental evidence bearing on this issue.

The distinction between the phonetic and phonological properties of a phoneme is particularly important for rhotics, which form an unusual class. Dutch, for instance, is said to have 21 different allophones for /r/ (Sebregts et al., 2023), some of which have little in common, both acoustically and articulatorily (such as a uvular fricative and an alveolar approximant). Ladefoged and Maddieson (1996, p. 217) see no other basis for these sounds forming a class other than orthography, that is, being written with the letter <r> or the Greek letter rho.

More recent work has suggested that rhotics are held together either by a sort of family resemblance, which can be understood in terms of diachronic changes (Sebregts, 2015), or by phonological principles (Chabot, 2019). Regarding the former, alveolar and uvular trills are obviously acoustically related; both have amplitude modulation in the signal. Within each place of articulation, lenition processes can turn trills into fricatives or approximants (Iskarous & Kavitskaya, 2018). In this way, a link between sounds as apparently different as alveolar approximants and uvular trills can be made. A different approach to explain the relatedness of rhotics is phonological in nature and argues that rhotics are sounds that easily cluster with other consonants, independent of their phonetic substance. Evidence for this claim comes, for instance, from loanword adaptations, where Moroccan Arabic uses an alveolar tap for French loans containing an /r/, even though Moroccan Arabic contains back fricatives that are much closer to the uvular fricative that French uses for /r/. This indicates that rhotics are understood in terms of their phonological properties, independent of their phonetic substance.

One study (Ćwiek et al., 2024) provided further evidence for a link between the alveolar trill and roughness. The study used a sequential matching procedure between auditory stimuli that are best described as long, syllabic versions of an alveolar trill or a lateral /l/ and smooth and jagged lines. This is similar to studies that investigated an association between high vowels and voiceless stops with jagged objects, as well as an association between round vowels and voiced consonants with round objects (often labelled the “kiki-bouba” effect; see Bremner et al., 2013; Köhler, 1947; Ramachandran & Hubbard, 2001). Participants saw the two lines and, on a given trial, heard a syllabic version of either [r̩] or [l̩] and were then asked to choose which line better matched the auditory stimulus, with the instruction to focus on how the lines feel to the touch. Over a thousand participants from different languages participated in this two-trial study. The results show a very large effect size in matching (close to 90%) that was slightly lowered when the participants’ native language used an alveolar trill as primary allophone. However, the design of the study confounds the phonological and phonetic properties of the alveolar trill. That is, participants had a choice between an /l/-bearing stimulus and an [r]-bearing stimulus. However, the [r]-bearing syllable was also /r/-bearing.1 Therefore, the results do not allow us to distinguish between the claims that the alveolar trill, or rhotic segments in general, are associated with roughness.

We investigated this issue in Maltese, a Semitic language spoken in Malta. A recent corpus study on Maltese (Mitterer et al., 2025) indicated that alveolar trills and alveolar approximants occur as free allophones and are roughly equiprobable in the onset position. While there is a general trend that speakers who are more English-dominant use more approximants, there is a large amount of intra-speaker variation, so that most speakers will use both approximants and trills for /r/, even within the same syllabic position. We even observed, in a phrase such as rajt rota (Engl. ‘I saw a bike’), that one /r/ is produced as a trill and the other as an approximant.

This provides an ideal testing ground for investigating the sound-symbolic properties of the alveolar trill. First, the two variants are encountered frequently by Maltese listeners, that is, in linguistic terms, they are both unmarked. Secondly, the two allophones differ acoustically, so that only the trill is obviously related to roughness, given the fast amplitude modulation. The approximant, on the other hand, is a rather sonorant consonant, with full voicing that usually reaches an amplitude that is not strongly different from surrounding vowels. These properties are similar to /l/.2 That is, phonetically, there is no reason to associate approximants with roughness. Note how that would be different for a uvular fricative, which, as an unvoiced sound, can be conceived as naturally rougher.

Since it is challenging to get hundreds of Maltese speakers for online studies (e.g., at the time of writing, hardly any can be found on the Prolific platform), we adapted the task to make it a multi-trial study. Figure 1 shows the basic task setup, in which participants choose one of two different nonwords as a name for a given product, with the additional instruction that the name should reflect how it feels to touch the object. This follows on a long tradition of product naming in sound-symbolism research (Klink, 2000; Lowrey & Shrum, 2007; for a recent overview, see Arora et al., 2022).

The products were typically perceived as either rough or smooth to touch, and one of the nonwords contained /l/, while the other contained /r/. It varied over trials whether the /r/ was produced as [ɹ] or [r]. The experimental design, hence, uses the two factors Product Surface (smooth vs. rough) and Allophone of /r/ (approximant vs. trill). The dependent variable is whether participants choose the /r/-bearing or /l/-bearing (non-)word for the product. If rhotics in general are associated with rough objects, a main effect of the product-surface factor is expected, with more /r/-word choices for rough-surface products. If particularly alveolar trills are associated with roughness, there should be an interaction between the two factors, with a stronger difference in /r/-choices between smooth and rough products if the /r/ is a trill. As Figure 1 already indicates, we did not use minimal pairs, because the use of minimal pairs generates a demand characteristic that the experiment is about /l/ vs. /r/ (see 2.2 for details)

Demand characteristics means that the experiment suggests to the participant what they should react to. An interesting example is supplied in slope perception, in which initial reports suggested that wearing a heavy backpack makes a slope look steeper (Bhalla & Proffitt, 1999). In these experiments, participants were supplied with the backpack with no obvious reason for it. Durgin et al. (2009) argued that participants could, hence, expect that the backpack was supposed to influence their reactions. Indeed, when the backpack was justified as measuring equipment, its influence on slope perception disappeared.

In a similar way, Ćwiek et al. (2024) presented syllabic versions of the consonants only and asked participants to select either a jagged or a smooth line. Participants are, hence, invited to elaborate on the relation between sound and touch, and have to focus on the one speech sound presented during the trial. This creates a similar demand characteristic, as in experiments on slope perception with and without a backpack (for further discussions of overly confirming research strategies in sound-symbolism research, see Dingemanse et al., 2016). To provide an analogy, if we ask whether Maltese listeners use epenthetic glottal stops as signs for prosodic boundaries, we could simply present the same phrase with and without glottal stop. This creates the demand characteristic that glottal stop should influence the response. To prevent that, Mitterer et al. (2021) presented stimuli with final lengthening as an additional cue, so that listeners could do the task without using the information from the glottal stop. For the experiment in Cwiek et al. (2024), a similar strategy would have been to use stimuli such as /V1rV1/ vs. /V2lV2/, with the identity of the vowels counterbalanced over participants. In this case, participants would not have been forced to consider [r] crucial to the task. Along these lines, the current experiment uses stimuli that differ in an aspect other than just between /l/ and /r/. That is, participants are not forced to consider that /r/ and /l/ are crucial to the task.

Based on the question of whether alveolar trills in particular or rhotics in general are related to roughness, we investigated this relationship, using a product-naming task with rough and smooth products and alveolar trills and approximants in Maltese, where both allophones are common and, therefore, unmarked. This creates a strong foundation for examining potential bottom-up effects of trills without the confounding influence of a frequency bias for one of the allophones. The results indicated a role for rhotics in general and not particularly for trills.

First of all, the current results confirm a sound-symbolic relation between /r/ and roughness in a paradigm in which the relation between dependent and independent variables was less obvious to participants, thereby lowering the demand characteristics of the experiment. At this juncture, it might be useful to unpack the term sound symbolism. On the one hand, it is a useful umbrella term, but on the other hand, it is somewhat imprecise. It conflates iconic relationships that are based on cross-modal associations, as assumed for the kiki-bouba phenomenon, and systematic relationships that are based on a co-variation of phonetic properties of words and their semantic or even syntactic properties that are language-specific (Monaghan et al., 2007).

This distinction may also help to explain the current finding. As reviewed above, Winter et al. (2022) argued that the relation between roughness and /r/ is based on English having used the alveolar trill in the past, with the relation now remaining in the language. This account explains only the relation in the English lexicon, but leaves open the question whether the relation between /r/ and roughness is relevant for current speakers of English. The results of a study by Bergen (2004) indicate that this might be the case. He showed that English listeners learn the phonaestheme gl- in the absence of an obvious iconic relationship. Relatedly, Farmer et al. (2006) present evidence that listeners are sensitive to (language-specific) phonological cues to word class. This suggests that speakers of English may also learn (implicitly) the relation between /r/ and roughness, independent of the allophone used for /r/. Since all of our participants were fluent in English, they hence may have learned the relationship. That is, exposure to a lexicon that contains the “/r/ is rough” relation may be sufficient for listeners to learn that link. If that is the case, it is, however, still noteworthy that the putative iconic relationship does not add anything to the systematic one for our Maltese listeners. That is, when there is a systematic relationship between /r/ and roughness, the potential iconic relation between [r] and roughness does not seem to contribute a lot anymore in language processing.

It may be argued that the lack of clear correlation between participant and item characteristics and the choice of /r/ for rough items may be informative of the difference between an iconic or a systematic relationship. For instance, it may be argued that exposure to Maltese should increase the likelihood of choosing /r/ for rough items. However, we do not agree with this prediction, because English clearly also has a relation between /r/ and roughness (see Winter et al., 2022). Since Maltese is also likely to have a systematic relationship between /r/ and roughness (being fed by Italian and Arabic words), relative exposure to both languages is unlikely to influence the association of /r/ with roughness in the mental lexicon of our Maltese participants. On the other hand, it may be argued that the lack of correlation between item roughness within category and choices for /r/ indicates a categorical and not an iconic relationship. Independent of the outcome, which showed no clear association between the two variables, we would argue that even a positive correlation would simply show that the product images activate the concepts “rough” and “smooth” to a different degree, which then modulates the fit of a “rough” name. The correlation does, in our view, not speak to the reason why /r/ is considered rough by participants.

Considering the field of L2 speech perception, it is not surprising if we consider that perception of speech sounds is strongly influenced by learning, which is especially visible in the countless demonstrations of failures in L2 speech perception (Cutler, 2012). The finding is also reminiscent of an article on cross-modal associations by Mitterer and Jesse (2010). They tested the association between a sustained piano sound and a “systematic” co-variate (i.e., seeing the key being pressed) and the “iconic” co-variate (seeing the hammer hitting the key). They found a stronger cross-modal association for the learned pattern with the key than for the hammer. Importantly, the key press is, in fact, an “anti-iconic” correlate of a sustained piano sound, since, usually, keeping in contact with a surface leads to a muffled sound (as it does on a vibraphone). Our current results dovetail well with this finding that cross-modal associations are dominated by learning and, once learning is established, iconic relations seem to play a minor role.

How does this relate to the findings of Ćwiek et al. (2024)? How do speakers of all the languages tested in Ćwiek and colleagues associate a trill with roughness? We see at least two explanations for this finding. Research on cross-modal associations (Guzman-Martinez et al., 2012) argues for a relation between visual-spatial and auditory frequencies that is mediated by tactile roughness. The apparent problem with linking visual-spatial and auditory frequency is that their denominators differ: time in the auditory domain and space in the visual domain. Guzman-Martinez et al. (2012) argued that this link is achieved by assuming that humans explore surfaces with their hands at roughly similar speeds, which gives the spatial component a temporal dimension. However, the question remains how to conceptualize these associations. Is it the case that cross-modal associations influence early perceptual processing (van Wassenhove et al., 2005), or are the two streams processed independently and then merged into a percept (Massaro, 1998)? This is related to the question whether knowledge influences perception in a top-down fashion, which also questions the modularity of perceptual processes. And, indeed, many of the pitfalls identified by Firestone and Scholl (2016) for top-down modulation of perception also apply to cross-modal interactions. Early interactions often turn out to be based on relatively general-purpose attentional mechanisms (Vroomen & Stekelenburg, 2010).

The results in Ćwiek et al. (2024) do not force the conclusion of early cross-modal interactions, as participants were explicitly invited to search for an analogy. This issue is aggravated by the fact that the stimulus for the /r/-bearing syllable was not a typical example of an alveolar trill as it occurs in normal, conversational speech. Ladefoged and Maddieson (1996, p. 218) report that alveolar trills usually contain two to three contacts with geminates, maximally having six to eight contacts. Inspection of Figure 1 in Ćwiek et al. (2024) indicates that their stimulus had about 17 contacts and is, therefore, not an ecologically valid example of an [r]. In addition to this unusual rendition of the trill, participants were required to focus on the properties of one speech sound given the nature of the syllabic consonant stimuli. This may have created demand characteristics; in other words, Ćwiek et al. (2024) employed what Firestone and Scholl (2016) termed an overly confirmatory research strategy.

Another explanation for the results of Ćwiek et al. (2024) may be found in a learning account. The association between rhotics and roughness might be caused by the learning of speech sounds. Learning seems to be a more potent force in speech perception than the bottom-up characteristics of speech sounds. Countless demonstrations of failures of L2 speech perception (see Cutler, 2012, for a review) show that speech sound differences that are obvious to L1 listeners, and, hence, perceivable bottom-up, are intractable for L2 learners. The Maltese listeners in the current study may have learned to equate the alveolar trill and the alveolar approximant, and this learning may be potent enough to override any allophonic effect of the alveolar trill. Such a learning-based effect would also explain why Anselme et al. (2025) observed the association of roughness with rhotics overall. Given that the most observed sound changes for rhotics are usually away from the alveolar trill, it is conceivable that most languages –or at least one of their ancestral stages– once made use of an alveolar trill. This historic relation may underlie the association between rhotics and roughness, originating from the phonetic properties of the trill. This, then, led to a learning bias that rhotics as a phonological class should be associated with roughness, even when the phonetic substance of that rhotic changed. Due to an initial association of the alveolar trill with roughness, the lexicon provides a source for learning. Consequently, /r/ then works as a learned cue for roughness, independent of the phonetic implementation.

How can we investigate to what extent any potential iconic relationship adds to (or exists above) the systematic relationship? One way to do so is to see whether sub-phonemic detail enhances the effect of a cross-modal match. Importantly, though, this should be achieved in a design in which the sub-phonemic detail remains subtle –rather subtle manipulations have been shown to influence lexical access (Salverda et al., 2003)– and the task should allow participants to succeed without relying on the sub-phonemic manipulation (i.e., other cues support the decision).

Another possible way to distinguish symbolic and iconic relations would be to investigate whether learning of new words for objects can be based on consistent association between object properties and word properties, even if they are “anti-iconic”. For instance, if participants learn that, in an artificial language, voiceless stops are associated with round objects, would this create a bias that makes it easier for them to learn new words following this pattern, even though it conflicts with the well-known kiki-bouba effect? Cross-modal associations can be learned, and learned associations even seem stronger than natural ones (Mitterer & Jesse, 2010). Overall, this indicates that sound symbolism may rely much more on what Hinton et al. (1994) called “conventional sound symbolism”, somewhat counter to the idea of bottom-up determined synaesthetic relationships (Ramachandran & Hubbard, 2001), even if there is a small natural cross-modal association.