Ted Benna was working for an insurance firm outside Philadelphia about 40 years ago when he figured out how to use an obscure provision of a 1978 tax law—section “401(k)”—and turn it into an employee retirement account for contributions from both employees and employers. He designed it as a fringe benefit for banks that wanted to save taxes when they transferred bonuses to employees. In the years since, however, it’s grown into much more—it’s now the dominant way that Americans save for retirement, helping them collectively amass trillions.