It's probably your most valuable retirement asset, but how much do you know about it?

It's one of the biggest if not the biggest asset for millions of Americans in or near retirement. Despite a global financial crisis, it has kept all its value. But we have little practical knowledge of this asset - assuming we even think about how to get the most out of it. I am taking about Social Security retirement benefits, which should be a major component of any retirement plan.

Consider: The average monthly Social Security retirement benefit will be about $1,153 in 2009. To receive that much inflation-adjusted income for life, a 65-year-old man would have to pay an insurance company a lump-sum premium of $204,000 for an immediate annuity, and a 65-year-old woman about $225,000, based on the lowest quotes I found from highly rated companies.

Is your IRA or 401(k) worth that much? In addition, Social Security offers attractive spousal and survivor benefits.

Clearly, we should pay attention to the ins and outs of Social Security. The decision of when to start collecting benefits - as early as age 62 for reduced benefits to as late as age 70 for enhanced benefits - can hinge on many factors, including how long you expect to live, your tax bracket, whether you're still working, and whether you are single or married.

"Social Security-related decisions can be complex, and there can be tradeoffs," said Carolyn Clancy, a Fidelity Investments executive. But a recent online survey commissioned by Fidelity shows many Americans lack the basic knowledge to make informed decisions.

Eighty-five percent of 61-year-olds surveyed (300 of them) did identify age 62 as the earliest they can collect benefits. But 56 percent didn't know when they would receive unreduced benefits if they waited to collect. (The answer is age 66 for anyone born between 1943 and 1954. After that, the age rises by two months every year until it becomes age 67 for those born in 1960 or later.)

More than half didn't know we have to file for benefits three months before we want to start receiving them. Almost a third believed incorrectly that Social Security benefits are not taxed. (Up to 85 percent of benefits may be taxed, depending on what other income we have.)

Nearly three-quarters didn't know a nonworking or lesser-earning spouse could be eligible for benefits based on the work record of the higher-earning spouse. More than half didn't know a surviving spouse could be eligible to receive the Social Security benefit of the deceased spouse if it was larger.

Also, 45 percent said they plan to start taking benefits as soon as they are eligible. The most common reason given was that they will need the money. But that would lower their benefits permanently.

Just 22 percent said they knew exactly how much their benefits would be, and 26 percent had no idea. Yet Social Security has been mailing each of us an annual benefits estimate since 1997.

Humberto Cruz is a columnist for the South Florida Sun-Sentinel.