CHICAGO, March 12, 2013 /PRNewswire/ -- A majority of our nation's middle-income Americans are relying heavily on Social Security to fund their retirement; however, there are gaps in understanding how it works, according to a new study released by the Bankers Life and Casualty Company Center for a Secure Retirement ® (CSR). Nearly three-fourths (72 percent) say Social Security benefits make up at least half or more of their retirement income, which exceeds the national average of 65 percent, reported by the Social Security Administration. In fact, 29 percent count on Social Security for 75 percent or more of their retirement income. Although Social Security has traditionally been the cornerstone of retirement income for Americans of all income levels, for those with household incomes between $25,000 and $50,000, one in ten (10 percent) rely on Social Security for all of their retirement income. Despite relying on Social Security to fund much of their retirement income, knowledge of specific benefits varies widely among middle-income Americans. The Longevity Risk and Reward for Middle-Income Americans study , which focused on 500 Americans ages 55 to 75 with an annual household income of between $25,000 and $75,000, found one in three (34 percent) do not understand that delaying when they start to collect Social Security benefits can increase their future benefit amount. Furthermore, nearly half (47 percent) incorrectly believe that an annual cost-of-living increase to their Social Security benefits is guaranteed and 36 percent falsely believe that full Social Security benefits start with their 65 th birthday. In addition to gaps in understanding of benefits, some middle-income Americans are also not paying attention to their individual Social Security statements. One in three (35 percent) middle-income Americans age 55 and older who are not yet receiving Social Security do not know what their monthly benefit amount will be when they retire.