It is pretty obvious that people are more likely to believe things if the people around them believe the same thing.
In India, the vast majority of people are Hindus. This means, among other things, that they believe that when we die we are reincarnated. Hindu has been the dominant religion in this area for something like 4000 years. In the Americas, Europe, and Africa there are relatively few Hindus. It cannot be an accident that so many Hindus are concentrated in a particular region. When people there (and everywhere else) form their religious beliefs, they are heavily influenced by the people around them. If people picked their religion mainly by criteria independent of the people around them, an American should be just as likely to choose Hinduism as an Indian, and an Indian should be just as likely to choose Christianity as an American.
If people in different parts of the world believe conflicting things, a large percentage of them must be wrong in their beliefs. When you and I believe what we do primarily because that is what the people around us believe, then the odds are pretty good that we'll be wrong in a lot of cases.
Even if people all over the world believe the same thing they still can be wrong. Five hundred years ago it was commonly believed that the earth was the center of the universe, staying fixed while the sun, moon, and stars moved. Women were assumed to be unsuitable for leadership positions and people who were schizophrenic were assumed to be possessed by demons. When virtually everyone believes the same thing, there is very little chance that we will even consider believing something else. And even if that view is wrong, it will still be propagated from generation to generation.
Why do most people accept cultural beliefs?
If you ask people if they believe in something just because everybody else does, very few of them would answer "yes", although it is very clear that this is how things actually work in practice. If we were just told that 95% of our fellow citizens believed that touching toads gave you warts, we might be perfectly willing to question that belief. But we are not normally told beliefs as if there is some doubt about them. Instead someone might just tell us in a matter-of-fact way that toads cause warts, or they might provide some sort of evidence, like "Don't touch that toad. Mary got lots of warts from picking up toads!" Of course it was the speaker's presumption, based on the cultural belief, that handling the toad was the cause of Mary's problem.
Most of what people tell us, like "The supermarket closes at nine o'clock" or "I have an older brother" is not intended as deception and is based on easily verified information. As a result, we are usually told the truth. Even when we are told things that are not true, it is rare that we detect the falsehood. Liars don't like to tell us things that could easily be disproved, and people making honest mistakes often wouldn't have made the mistake if it was easy to find out otherwise. Experience tells us, therefore, that most of the time what people tell us is true. Unless statements contradict our current beliefs, we often accept them without question.
Have cultural beliefs withstood the "test of time"?
We may sometimes feel that cultural principles have withstood the test of time. If these principles were not correct or did not work well, they would not have been passed along for many generations.
It is not uncommon for people to assume that wisdom was greater in earlier times. If we were brought up a certain way and we find things have changed, we may have difficulty adapting to the changes and therefore prefer the earlier conditions. On the other hand someone brought up under the new conditions might be equally uncomfortable if we went back to the old way. The new way of doing things may be better or it may be worse, but our judgment is likely to be biased towards the what we are used to.
On a larger scale, we may assume that our cultural traditions must be good or else they wouldn't have survived. We might reason that cultures compete with each other and only those with beneficial traditions survive. It is particularly tempting for Americans to assume their own traditions must be beneficial since the United States has been very successful in recent history. This view isn't totally unreasonable. Certainly the success or failure of a culture is in part due to how well its traditional beliefs and practices work. The Shakers, a 19th century religious group that believed all sex was immoral even for reproduction, predictably died out. Communism as practiced in the Soviet Union turned out to be too economically inefficient for it to compete with other industrialized nations and so that system eventually collapsed. The success of the United States is probably due in part to its leadership in promoting freedom and democracy.
On the other hand, every culture has many traits involving economic practices, education, ethnic prejudices, moral codes, taboos, religion, family structure, diet and so on. A few of these could be very bad, but as long as they are not disastrous, other traits could enable the society to thrive. Another problem is that the steady advance of science and technology now changes the world so quickly that a cultural tradition that may have been very useful a century ago might now be a drawback. Finally, the success of a culture may depend on things that are almost entirely coincidental, such as the weather in a key military conflict, a famine, or the actions of a particular leader.
Societies can therefore thrive despite some bad traditions, and since few people question their cultural traditions, the bad ones get passed from generation to generation along with the good.
Wisdom of the ancients?
We also should not count on ancient people being particularly wise. Until relatively modern times, virtually any sort of education, including reading and writing, was limited to a privileged few. Before the invention of the printing press, books were rare and expensive. Sources of information like radio, television, newspapers, magazines, and the internet were not available. Governments were undemocratic and free speech and other rights were not generally enjoyed. Rules and customs were likely to promote the interests of the powerful and not the common people. Slavery, suppression of women, and other forms of bigotry were widely accepted. Science was primitive and most scientific instruments had not been invented. For the most part, philosophical understanding was at a huge disadvantage compared to what we have today.
Even though a few historical thinkers stand out as being far wiser than most of those before them, we should should not assume that their ideas were more sound than those of the thinkers of today who have far more resources at their disposal. While Socrates and Shakespeare and Jefferson and Lincoln may have been outstanding in their time, they were all fallible and had far less in terms of resources to build on than the top minds and talents of today.
How do we free ourselves from cultural assumptions?
Even if we try, freeing ourselves from our cultural prejudices is particularly difficult because each culture is likely to have developed rationalizations for its positions. We will hear the arguments as to why the prevailing belief is right much more frequently and in more detail than we will hear arguments for opposing positions. People may be very reluctant to express contrary opinions for fear of ridicule or persecution. It can be personally dangerous to express dissenting opinions in many cultures, and the religious concept of "blasphemy" is aimed at preventing people from expressing or perhaps even thinking ideas that are contrary to the established doctrine. Even without official persecution, our upbringing may make us feel that we are mentally unhealthy for even thinking thoughts that question strong cultural principles.
If we care about truth, we don't want to believe things just because the people around us say so. We need to ask ourselves why the people around us believe these things. We must question their rationalizations, which are often based on other cultural assumptions. We have to really ask ourselves if we would have reached the same conclusion on our own, based on evidence that is reliable. If beliefs are not shared by people in other cultures, we might question them to find the reasons they believe differently and compare the evidence for each view.
It is particularly difficult to avoid a cultural assumption if we have no idea we are making that assumption. Unfortunately there is no way we can systematically reconsider all the ideas we take for granted. Probably the best we can do is listen for disputes and before we join in with the majority that ridicules some unpopular idea, double check whether that idea has some chance of being right. I personally never find it very satisfying to join in with the vast majority on any issue. If the majority is right they don't need me. If the minority turns out to be right, they need all the help they can get.
We should always wary of the term "common sense". If somebody says "it's just common sense" they are telling us that something is a cultural assumption. The fact that they choose to use that phrase rather than presenting supporting evidence suggests they can't think of a good practical reason for the belief. While common sense may often give us the right answer it can never hurt to make sure there is valid reasoning behind it.
We might sometimes learn about our assumptions by listening to children. Not yet knowing all the cultural beliefs, they will sometimes deal with a new situation using unbiased reasoning. If we hear a dispute between a child and an adult, we should not immediately assume the adult is right. Sometimes adults will be supporting an unsound position based on their cultural beliefs. The child's argument may give us an insight into our own cultural biases.
Naturally we benefit greatly from the learning of people who have come before us, and many things widely believed by our society are true. However if we only want to believe things that are reliably true, we cannot count on the assumption that our cultural traditions and beliefs are always good ones. We must analyze each question for ourselves and come to the best conclusion possible based on the knowledge and reasoning we can apply. Often we will not be able to come to a firm conclusion, but that is far better than adopting a firm stance on something that might be false.
Recognizing false assumptions doesn't require rebellion
Finally, a word about what recognizing cultural assumptions does not mean. Just because we may feel that some tradition or belief is not valid doesn't mean that it is a good idea to violate that tradition or ridicule that belief. Occasionally we should be willing to risk personal hardship in order to take a moral stand, as Rosa Parks did when she refused to sit in the back of the bus in order to take a stand against racial discrimination. Much of the time, however, it is sensible to conform to rules even if we don't agree with them.
A man may decide that the practice of wearing a suit and tie for a job interview is unnecessary, but that doesn't make it wise for him to go to an interview without one. He probably stands a much better chance of getting the job if he conforms. If at some time we are the ones conducting interviews, however, we may benefit by recognizing that the poorly dressed but talented applicant should be hired in preference to the well dressed dull one. We might also point out to others that it is shallow to judge people by the way they dress.
It is almost always beneficial to recognize the truth and to know when claims are unsupported. How you should use that knowledge depends on you and the situation.