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I think it's less about sincerity and more about authenticity. As a society we're more critical of cynical marketing and have seen enough scandals to make us jaded, or at least less generous in the benefit of the doubt.
Kadirov, D., Varey, R. J., & Wooliscroft, B. (2013). Authenticity: A macromarketing perspective. Journal of Macromarketing, 0276146713505774. says "Postmo-dern consumers perceive modern branding efforts to be inauthentic because they ooze with the commercial intent of their sponsors’ and this has given rise to the end of homo-economicus in marketing and the creation of "Homo-reciprocans is motivated by the desire to co-operate and improve the environment."