Most non-governmental Americans that are employed as well as the employers of these Americans are subject to the Social Security payroll withholding tax which mandates that 6.2% of their paycheck for the employee as well as 6.2% matched by the employer is earmarked into the Social Security Administration for distribution at their retirement. Not too surprisingly, this amount of money withheld equates to a very large number, in which as reported by nasi.org that "In 2016, the Social Security program received $958 billion in income."
The money that each individual contributed to the Society Security Administration is for the most part, transferred to those that are currently retired in order to pay those people their benefits that they have accumulated during their lifetime of employment. This also means that those that currently contribute to the Social Security Administration have no say as to how those monies or excess monies are invested while they are so employed, of which, those funds are actually invested into special Treasury issues, of which, as reported by nasi.org, "The average interest rate on the portfolio held by the Social Security trust fund was 3.2 percent in 2016."
All of the above, seems like a rather dubious way to fund each person's retirement, for social security doesn't really take each individual's social security withholding tax and then invests it on behalf of the taxpayer, so that it will grow and accumulate each year, making it something akin to what most people do with their retirement plans, but rather, the Social Security system is some strange hybrid of a "pay as you go" program, which basically means that the current generation of workers that are having their paychecks reduced by Social Security withholding, are actually having that money, more or less, directly being paid out to those that are already retired, so that the current generation of workers, are paying for the retired generation of workers.
This would indicate that those that believe that their Social Security taxes are actually being held or placed into some sort of secure account, for their future benefit are sadly mistaken, though the Social Security Administration does provide each taxpayer an accounting of their projected Social Security monthly stipend, such a payment as that, is only as good as how solvent the Social Security Administration of these monies is, at the time of that particular individuals' retirement period.
On the other hand, federal employees have a true retirement savings plan known as the Thrift Savings Plan (TSP), of which, this plan allows each individual to self-direct the investment of their retirement monies into a choice of six different types of funds, of which, each of these funds offers a tradeoff of potential appreciation v. risk, and therefore includes stock index investments as well as governmental securities investment. For the years 2008-2017, the Ten year compounded return of these funds ranged from a high of a +9.37% annual return, which is stupendous, to a low of a +2.23% annual return, of which, a given individual's overall return is based upon that individual selections of the funds so available for them.
All those that are mandated to contribute to the Social Security Program have absolutely no say as to the investment of those monies paid, because firstly those monies aren't actually set aside into their own personal account, and even if they were, the Social Security Administration invests those excess monies from the pay as you go program solely into special Treasuries. This means, in effect, those contributing to the Social Security Program, have no control over how their money is invested or allocated, and despite what some Social Security Administration paper stipulates that they are entitled to upon their retirement, whether they will ever actually receive what they are anticipating in their retirement years.