Seems pretty intuitive:
The way you gain people’s trust is to earn it over time by repeatedly proving that you deserve it. That, or grow a beard.
A recent study in the Journal of Marketing Communications found that men with beards were deemed more credible than those who were clean-shaven. The study showed participants pictures of men endorsing certain products. In some photos, the men were clean-shaven. In others, the same men had beards. Participants thought the men with beards had greater expertise and were significantly more trustworthy when they were endorsing products like cell phones and toothpaste.
This appears to be a evolutionary response to the self-evidently exceptional nature of men who can grow beards and choose to do so. In the hunter-gatherer period, the choice of toothpaste could mean life or death; those who failed to follow the advice of bearded men were “selected out.”