L2 difficulties in the perception of tones: Phonological universals or domain-general aptitude?

What makes Mandarin tones challenging for second language (L2) learners? Several recent studies suggest that two phonological universals, the Obligatory Contour Principle and the Tonal Markedness Scale, may constrain the L2 acquisition of Mandarin lexical tones, regardless of learners' first language. We assessed the role of these universals in L2 tonal acquisition with a perceptual testing protocol, which implemented a number of methodological and analytical improvements relatively to previous studies: larger sample sizes, better matched stimuli, and the use of Bayesian statistics to assess support for the null hypothesis. The results provided evidence for the null effects of both phonological universals. Instead, a clear determinant of tonal identification accuracy was participants' pitch acuity, suggesting that domain-general auditory processing underlies the learning of L2 phonological categories. All materials, data, and code are publicly available in the OSF repository at https://osf.io/ezadw.