Why is Danish so difficult to understand for fellow Scandinavians?

It has consistently been shown that among the three mainland Scandinavian languages, Danish is most difficult to understand for fellow Scandinavians. Recent research suggests that Danish is spoken significantly faster than Norwegian and Swedish. This finding might partly explain the asymmetric intelligibility among Scandinavian languages. However, since fast speech goes hand in hand with a high amount of speech reduction, the question arises whether the high speech rate as such impairs intelligibility, or the high amount of reduction. In this paper we tear apart these two factors by auditorily presenting 168 Norwegian- and Swedish-speaking participants with 50 monotonised nonsense sentences in four conditions (quick and unclear, slow and clear, quick and clear, slow and unclear) in a translation task. Our results suggest that speech rate has a larger impact on the intelligibility of monotonised speech than naturally occurring reduction.