Critics of the Bush administration plan to reform Social Security with personal accounts have a seemingly endless supply of reasons why it can’t possibly work. You know the litany: It’s too risky. It’s too expensive. It’s too complicated. The critics never mention that there’s already a government-administered retirement system that has shown for over 15 years that personal accounts are prudent, inexpensive, and simple. It’s the Thrift Savings Plan of the United States federal government, currently serving 3.3 million government employees. The years since Thrift was first offered in 1987 couldn’t make for a better laboratory to crash-test a...