In this paper, I investigate to what degree phonetic uniformity in diachronic vowels shifts can be accounted for in terms of a shared phonetic implementation rule of phonological features [6, 10], versus a shared social evaluation of the phonetic realizations [19]. I take a particular focus on the parallel fronting and subsequent retraction of the GOOSE, GOAT and MOUTH vowels, as well as the raising of the preconsonantal FACE and pre-voiceless PRICE vowels in Philadelphia, drawing data from the Philadelphia Neighborhood Corpus [15]. Using generalized additive models [21] I fit models for these vowels accounting for gender, date of birth, educational attainment, and vowel duration using tensor product smooths. Looking at the correlation of the byspeaker random intercepts, back vowel fronting appears to be highly correlated, thus likely phonologically grounded, while FACE and PRICE raising is not, thus likely socially grounded.