A study by Kar at el (2018), concludes that “Fuller lips relative to the width of the face and greater vermilion height are perceived as attractive in women”. The vertical width of the upper lip is regarded as the most prominent feature in smile aesthetics.
A study by McNamara from the School of Dentistry, University of Michigan found the thickness of an upper lip to be the most attractive aspect of a pleasant smile.
Full lips have long been desired by women because they signify youth and femininity. Evolutionary psychology also suggests that women with a full pout are often associated with a strong mating potential. One of the most visible signs of ageing is a pair of thin and flat lips. This is because as collagen production slows down, your once-luscious lips become deflated. This Sexually dimorphic feature is an indication of reproductive health (Fink, & Neave, 2005) .
Kar, M., Muluk, N. B., Bafaqeeh, S. A., & Cingi, C. (2018). Is it possible to define the ideal lips?. Acta otorhinolaryngologica Italica : organo ufficiale della Societa italiana di otorinolaringologia e chirurgia cervico-facciale, 38 (1), 67–72. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5952987/
McNamara, L., McNamara Jr, J. A., Ackerman, M. B., Baccetti, T., (2008). Hard- and soft-tissue contributions to the esthetics of the posed smile in growing patients seeking orthodontic treatment. American Journal of Orthodontics and Dentofacial Orthopedics. 133 (4), 491-499. https://www.sciencedirect.com/science/article/abs/pii/S0889540607013170
Fink, B., & Neave, N. (2005). The biology of facial beauty. International Journal of Cosmetic Science, 27 (6), 317-325. https://onlinelibrary.wiley.com/doi/10.1111/j.1467-2494.2005.00286.x