The thing about common sense is that it isn't particularly common... and I suspect it never has been.  

If we are to believe science, and I mostly do, our personalities consist of one part genetically inherited predispositions and another, the accumulation of everything we have seen, heard and felt – the sum total of our experiences.  The first part includes those often suppressed, (or even denied) primitive predispositions, which are displayed in our powerful sexual inclinations and ethnocentric and xenophobic attitudes.  As unpopular as they have become, at least in polite society, they did help our predecessors survive a challenging environment long enough to produce and rear offspring, thereby ensuring a foothold in all following generations.  Success at both survival and procreation was the entire agenda during that era and those who developed the best mechanisms passed them along.  Since we are all descended from those original populations we can thank (or blame), them for most of what we have in common.  The sense part is a bit more mysterious. 

As our population multiplied and moved to every corner of the Earth, in the process meeting and overcoming incredible and diverse challenges, culture became the primary power in our individual development – layers of “nurture” were being piled on top of our human “nature”.  Common sense is usually viewed as the ability to see the obvious but what is obvious in one culture may not be in another. 

For instance: The culture I grew up in suggests that marriage is a partnership based on sharing for the mutual benefit of both partners. While some cultures treat marriage as a form of ownership - a husband owns his wife and uses her as he sees fit.  The values I grew up with oppose such beliefs but my culture also acknowledges that people have a right to their differences.  Indeed, if everyone were the same there would be no need for us to exist. (And life would be so boring.)  We would have nothing to offer each other.  Diversity is the lifeblood of any species and because we face a greater variety of risks than other creatures we need a greater variety of differences to promote our survival.  I understand the theory but understanding is not the same as agreeing.  

We have choices.  I believe everyone has the ability to choose, at least on a primary level, what values govern their lives.  That is not to suggest the choice is either easy or fair because choosing against your culture is often accomplished at high cost.  However, accepting the necessity for real equality - regardless of gender, race, social status or birth nation - even when opposed by our culture and our primitive predispositions - is necessary if we are to move forward as a human race.  We may live in a world full of constantly amazing technological achievements but when mixed with those often denied feelings, held in the deep dark recesses of our mind, the possibility exists the combination will precipitate a highly dangerous threat in a crowded, compact and unfair world.  

Most people understand that the ultimate goal of human society rests on the attainment of individual freedom.  Fortunately, (or unfortunately, depending on your personal beliefs), this is only achievable in an environment where the support for constitutional equality is strong.  Freedom means living in as open a society as possible and that makes us vulnerable.  If we talk about freedom and practice favouritism and bigotry we incite passions and even hatred in those against whom we discriminate, and when our ever growing technology is combined with our desire to live in an unrestrained society, we are put in jeopardy on a massive scale.  Every day we discover more and more ways to fit truly lethal and dangerous substances into smaller and smaller packages.  And we all know well that passionate and committed individuals may eventually find ways to achieve their goals, regardless of the security in place.  The best chance for humanity's future rests with creating a lifestyle where trust exists in the people and institutions that lead us - and trust comes from seeing real equality and fairness in action.  In other words, real freedom requires us not just to have the tools in place to make each individual constitutionally and functionally equal with all others... it requires that we use them.  If that is not the case the best we can hope for is a well designed bunker.     

Over the course of my life I have witnessed many forms of evil perpetuated by individuals and groups - up to and including nations - and as a result I have become convinced that humanity is nothing special in the grand scheme of things... especially since much of our greatness is a result of being able to establish the criteria and ignore the downsides.  Individuals, communities, races, religions, nations, all humanity; we constantly manufacture stories and excuses to justify our selfishness while gushing about the few positive attributes we have. Over time this has given each of us a history which has become our truth.  But, as every thinking person knows, it's not true, even if we are willing to die in its defence. For some strange reason we have learned that bragging about how great we are as a people is acceptable, whereas saying other people are less than us is unacceptable, yet we make no effort to recognize the inherent irrationality within this contradiction.  Lies are the real enemy of freedom.  Finding and exposing them has to be our new common sense.