I can tell you one thing for certain, declining birth rates across the entire western world are not due to Social Security and payroll taxes on Americans alone. And the fact that it is happening everywhere, is pretty good evidence that it's not Social Security's fault here.

There's a lot of research out there that answers that question. There are many reasons for declining birth rates, and those include cultural changes, changes an expectations of the standard of living, changes in family planning priorities.

If you really want to know the true answer to this, you don't need to ask a tax advisor or a politician, you need to ask a sociologist.