The thing that immediately jumps to mind is that the first syllable is more likely to be stressed, and stressed syllables are usually pronounced longer. From t

The thing that immediately jumps to mind is that the first syllable is more likely to be stressed, and stressed syllables are usually pronounced longer. From the paper: > Our corpus data are not annotated for suprasegmental features such as stress or tone. However, on the basis of available phonological descriptions, only 4 of the 51 languages can with some certainty be considered to have fixed initial word stress, while most other languages are either tone languages or stress languages with non-initial stress It sounds like they account for languages that always have initial stress, but not for languages that simply have mostly initial stress or at least some bias towards it, e.g. German. There's also no mention of secondary stress, which plays a big role in English for example. It also sounds like they're assuming that if a language is tonal, then there must be no stress-like suprasegmental features which can affect syllable duration, which is untrue. I'm inclined to believe the results anyway and it's a really interesting finding, but they would need to redo the study fully controlling for stress to get meaningful results in my opinion.