I tend to look askew at studies that compare the U.S. to other nations, but this one caught my eye:

Experience of 30 Nations Offer Lessons for Social Security Reform

Thirty countries have reformed their pay-as-you-go Social Security systems with personal retirement accounts (PRAs). As a result of these changes, Britain, Chile and many others have virtually no unfunded liability. By contrast, the U.S. Social Security system is facing an unfunded liability of $11 trillion - about the size of the entire U.S. economy.