There are four subjects the polite American avoids discussing in public: Politics, Religion, Sex, and Money. The ostensible reason for this taboo is to avoid offending anyone. But here I argue that this over-correctness is a causative factor in the decline of a civilization.
Let's do money, first. As far as I know, this is a particularly American obsession. My European parents had to learn not to talk about money when they came to this country. Other places, the question, "So, how much do you make?" is as normal as "Are you married?" But in the U.S.A, we maintain a fiction of a classless society. We ask the same question only obliquely: "Where did you go to school?" is a good indicator of family income and social position. It is to the advantage of the landed class employers that their serfs employees not compare incomes, as well. By not allowing people to honestly gauge their economic value, they stay insecure. And insecurity leads to all manners of submissive behaviors, shoring up the security of the ruling classes, both secular and religious.
Sex is a more generally repressed topic. There is no stronger drive, yet we must never directly say what we feel about it. Western churches even teach that one should deny and ignore the strongest drives within ourselves, leading to all sorts of perverse (read as counter-social) behaviors. To discuss it in public would allow people to see how normal their lusts really are, removing a major source of insecurity. Minor curiosities would not blossom into obsessions and perversions. Such openness would reduce the influence of those very organizations that profit from its repression, like churches and (other) marketing firms, whose urgent short-term goals are only occasionally and accidentally in line with continuing our civilization.
Religion is a big one. People wear "subtle" symbols to let others of the same brand know they can be approached on the subject. The third eye, a cross or fish, a Koranic verse, and a star are some of the more obvious "secret" symbols. But it is a major faux pas to overtly declaim about your own faith to someone who may not agree. Unless, of course, the purpose is to stir controversy or solicit, two disreputable (completely human) drives. Again, by not knowing when and to whom you may come out,one feels insecure. This gives the leaders the upper hand. Especially when they strive to sow divisiveness, as in malignant fundamentalist sects.
Finally, politics. This is the least stringent of the social prohibitions. I think this is in part because the churches and marketing firms rule the field, anyway. In our land, there are basically two sides: The established American parties, and those who can barely tell them apart. The parties do have differences. One wants to conserve our resources, reduce capitalist predation, and protect the underclass in hopes of a better tomorrow, and the other wants government to protect the minorities (specifically the rich, the unborn, and corporations) and let God (or the invisible hand) sort out the others until the imminent judgment day.
So it occurred to me that hiding from these basic topics destabilizes civilization. Social groups balkanize into small, trusted segments that define themselves by their perceived differences. Each of the 30,000 Christian sects publicly claim the sum of all members of all denominations as supporting them, yet privately know that most of the 30,000 others are wrong and hell-bound. We have been divided, and conquered. If the people knew where they stood, and knew where the leaders stood, we would have actual checks and balances as were envisioned by our founders. Without such things, our nation may well founder.