What researcher posited that the lowering of diphthongs is "natural"?
This change is interesting from a sociolinguistic point of view because it has apparently happened relatively recently, in the 1970s, and was pioneered by older well-educated women from the upper middle classes. The lowering of the diphthongs has long been current in many Dutch dialects, and is comparable to the English Great Vowel Shift, and the diphthongisation of long high vowels in Modern High German, which centuries earlier reached the state now found in Polder Dutch. Stroop theorizes that the lowering of open-mid to open diphthongs is a phonetically "natural" and inevitable development and that Dutch, after having diphthongised the long high vowels like German and English, "should" have lowered the diphthongs like German and English as well.