It's one of the biggest assets, 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 by all rights should be a major component of any retirement income plan.

Consider: The average monthly Social Security retirement benefit, after a 5.8 percent cost-of-living increase, 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 about $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 best to start collecting benefits -- as early as age 62 for unreduced benefit to as late as age 70 for enhanced benefits, or anywhere in between -- can hinge on many factors.

Aside from any immediate need for money, these factors include 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 trade-offs," said Carolyn Clancy, an executive from Fidelity Investments. A recent online survey commissioned by Fidelity shows many Americans lack the basic knowledge to understand these trade-offs and make informed decisions.

A vast majority (85 percent) of 300 61-year-olds surveyed did identify age 62 as the earliest they can start collecting reduced 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 of eligibility 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 that a non-working 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 that a surviving spouse could be eligible to receive the Social Security benefit of the deceased spouse if it was larger than the survivor's own benefit.

Also, 45 percent of the 61-year-olds say they plan to start taking benefits as soon as they are eligible at age 62. The most common reason given was that they need the money.

Such an action would lower their benefits permanently. Among those planning to collect as soon as possible, 73 percent didn't have a retirement income plan. So perhaps there is another way to bridge the income gap until full retirement age that they didn't consider.

Also, just 22 percent said they knew exactly how much their benefits would be -- and 26 percent had no idea. And yet Social Security has been mailing Americans an annual benefits estimate since 1997, and this year the agency introduced an improved benefits estimator at www.socialsecurity.gov/estimator). Fidelity also has launched a site, www.socialsecurity.com/socialsecurity, that while obviously commercial does include valuable educational information.