MANY problems are so complex that even if we had the money to fix them, we wouldn’t know how to do it. Fixing inner-city schools, reducing obesity, creating peace in the Middle East are just a few examples.

But some problems are frustrating in another way: we know how to fix them and we can afford to fix them, but we drop the ball. That’s the situation with a crisis facing many Americans: saving enough for retirement.

Here is one measure of the problem: A Boston College economist, Alicia H. Munnell, and her colleagues have estimated that more than half of Americans are saving too little to support an adequate lifestyle if they plan to retire at 65. Why is the situation so serious? One reason is that traditional pension plans — in which employees have almost no decisions to make — are being supplanted by defined-contribution plans like 401(k)’s. In these plans, employees have to decide for themselves how much to save and how to invest their money. For many people, being asked to solve their own retirement savings problems is like being asked to build their own cars.

To fix this, we need to do two things. First, make payroll retirement savings plans available to everyone. Then, add empirically proven design features to them, making it easier for workers to make good choices. In other words, improve the plans’ choice architecture.