Is it a psychological defense mechanism that the unsuccessful in life believe the successful had an unfair adv?

It is a common sense issue. Perhaps intelligence should be looked upon as an advantage.

If someone is more intelligent than another person chances are yes they will be more successful. Decision making abilities make you who you are. People with better decision making abilities have an advantage.

To be successful in life is fortunate, notice I said "fortunate"? Good fortune has nothing to do with mental capabilities. There are numerous psychologically handicapped people who have had the good fortune of being successful. It depends on what you call success. Sometimes its surviving what all life has thrown at you! HMMM...

It's just the truth, analyse it however you will, but there are tonnes of examples proving that those who have more are at an advantage....

Are you suggesting that WalMart doesn't have a buying advantage over a local store?

How then, would you explain the statistical evidence that people overwhelmingly remain in the same economic class that they were born into? Why are there relatively few who advance from middle class to wealth, or descend from wealth to middle class?

I think they suffer from Utopian Delusion Disorder. Some will outgrow it.