Stability is one of the most crucial elements of social security systems. Although the United States is famous – some might say notorious – for drastic changes to its socio-economic structure (including its welfare programmes), its Social Security is the most secure and unchanged public pension programme among major Western countries. In the restructuring age of welfare states, public pensions have been reformed several times in Japan and various European countries, with an overhaul of benefits and taxes. However, Social Security in the US has not undergone such reforms in the three decades since the Social Security Act was amended in 1983, but has experienced relatively better financial conditions. This paper investigates the extent to which Social Security has remained stable during a time when welfare states are going through a crisis. The comparative analysis for stability consists of three steps: (1) a simple evaluation of the frequency of reforms among six countries; (2) a comparison of the scales of parametric adjustment and components of structural reform; and (3) confirming current financial sustainability to check for ‘false stability’ using individual government reports. This paper also studies stability factors, including an institutional design that is durable in this changing environment.