It is an experiment that runs like a story.
Once upon a time there were four monkeys chosen for an experiment. These four monkeys were placed in a cage and a bunch of banana was tied at the top. Instinctively, a moneky tried to jump and take hold of the banana. At this time water was splashed on monkey and thus prevented from reaching banana.
Any time a monkey tried to reach the banana, water was splashed and all the monkeys 'learnt' that there is danger in trying to reach for banana.
Now, one monkey was removed from the cage and a new monkey was replaced in the cage. Knowing not the danger the new monkey tried to reach the banana. This time water splash was also not there. Yet the new monkey did not get the banana. Why?
Apparently, the three old monkeys prevented the new one to reach the banana. They sensed the danger and "taught" the new one to not try to reach the banana.
Next, another monkey was removed from the cage and a new monkey was replaced in its place.
Now, this monkey was prevented from reaching the banana by all the three (two old with experience and one new without the experience). In this manner all the four monkeys (who 'learnt' about the danger) were replaced with four new monkeys.
Lo and behold, all the new four monkeys had fear of banana even though they did not directly percieve the threat to them.
This is the conditioning of the mind of the monkeys.
Now, for the conclusion:
All of us are also like monkeys - having belief systems that are not part of our own experience. It is based on someone else's experience. It is either 'some one said so' or 'elders said so'. We would not take the risk to find out!!
Those who question this belief system have taken that additional risk and hence have a rich experience of life.