You may not have heard of it but there's an International Social Security Association (ISSA). It recently released a report on the performance of larger social security reserve funds, most of which are mostly invested in stocks and non-governmental debt obligations. The nations involved aren't identified by name. I'm not sure if the U.S. Social Security Trust Funds are included in the report. In any case, what the report shows to me is huge variability. Note that the time period selected is after the 2008 crash. This was a time period in which stock and bond markets went up significantly. Below is a table from the report.