In the early years if the 20th century, something happened in American homes that changed the way adolescent girls felt about 
their appearance. And the effects became a runaway train that hasn’t slowed down since.
The change? Electric lights and running water came into bathrooms, and along with it sinks. And above the sink: a mirror! Prior to that time, the only opportunities for girls to get a close look at their faces was through mirrors in stores or their reflections in a clean window. So it was difficult to obsess about something you couldn’t see easily and routinely. But mirrors changed all that.
They created a new type of self-appraisal that was totally external. The previous Victorian Age had it’s warts, but during that era good character and spiritual qualities were emphasized over external qualities like good looks. Becoming a better person involved getting out of yourself and being there for others vs. focusing on yourself and improving your appearance. Adolescent girls started spending more and more time grooming themselves, and this self-scrutiny mostly involved the hair and face. And this in turn created more angst about acne.
The medical field, and especially the new specialty of dermatology, had made the connection between sexual diseases like syphilis and skin lesions, and acne was lumped in and viewed as a sign of moral failure. Medicine also developed it’s germ theory around this same time, and pimples were associated with being dirty and lower class. Thus care of the skin of teenagers became an integral part of the middle class female ideal. More products came on the market rapidly, and the cosmetics industry went off.
The culture also brought more magazines designed for girls and women, motion pictures, and also pictures in mass-marketed ads. Beauty began to be seen as more of a visual quality, and an expectation to look perfect and have perfect skin arose as a middle class aspiration for adolescent girls. The advertising industry noted the insecurities of girls at this age, and played to it by framing new products as ways to become more popular, beautiful, and thus self-confident. And of course girls bought in.
It’s sad that we have conditioned girls to focus on their looks above their character. And it’s a shame that beauty is primarily associated with external aspects of a girl instead of what’s really important: intelligence, integrity, service, compassion, spirituality, and leadership to name just a few. It’s amazing to think that something as innocuous as a bathroom mirror could be the impetus for such a radical shift in how girls view themselves and form their identity.