What leads vowels to reduce, and why do they reduce to schwa in so many languages (compare the highlighted vowel in photograph vs. photography)? In an open access paper in Journal of Phonetics, they use a spoken corpus of American English, and show that all vowels reduce when their duration gets shorter, but they do not reduce toward a schwa-like position, but toward a closer-jaw position that American English used for the vowel in wanted. They show that there needs to be another attractor, perhaps the quality of being a vowel, that involves the more open jaw position that characterizes schwa.