NEW YORK (Money Magazine) -- The question of why people don't save more money is a hot topic in academic circles. A third of eligible 401(k) participants never sign up, and those who do typically don't put away enough.

"We may have every intention of saving," says Brigitte Madrian, professor of public policy at Harvard University. "But most of us can't follow through."

That follow-through is what academics have been shedding light on lately, and the solution they've found is to do, well, nothing: If you make 401(k) enrollment automatic - new hires must opt out, not in - those who do nothing still save.

In a recent analysis of companies that have adopted automatic enrollment, professors at Harvard and the University of Pennsylvania found that participation levels skyrocketed to as high as 96%.

"With autopilot 401(k)s, inertia works for you instead of against you," says Harvard economics professor David Laibson, who co-authored the study. "Since people aren't inclined to take action, very few of those who are signed up automatically will choose to drop out."

The results have been so strong that Congress took notice: The recently enacted pension bill will make it easier for companies to enroll you in a 401(k) the day you start work. Letting your company automatically increase how much you put in your plan can cure low savings levels too. In a landmark experiment at one midwestern company, Thaler and UCLA accounting professor Shlomo Benartzi found that automatic contribution hikes raised average savings rates from 3.5% to 11.6%.

The lesson from all these programs is that if you're forced to save, you probably will. Still, a healthy portion of families manage to save enough money on their own.

How? These motivated planners develop a savings habit and, once accustomed to it, tend to keep it up.

Frank Maldonado certainly has. When he was just 26, Maldonado began saving 30% of the income from his dental practice. "I grew up with four brothers in an apartment in Brooklyn, and my father was a bus driver," says Maldonado, 46. "I wanted better financial security for my family."

Over the years he kept putting away that 30%.

Today he and wife Gina, 40, are raising four children in Short Hills, N.J., and their portfolio is well over seven figures. Maldonado, who never converted to online banking, still writes his monthly check to his savings plan.

"It just became a habit," he says. "When I haven't written the check, I start to feel bad."

If you have access to a 401(k), sign up immediately and save at least enough to qualify for the company match.

No 401(k)? You can set up an automatic investing program at nearly any fund company or brokerage. Then sit back and let inertia do the work for you.

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