One possible solution for America's growing need for organ donors could be relatively simple, according to new research: just keep asking.

About 123,000 Americans are waiting for an organ transplant — a wait that kills 18 people each day, according to the U.S. Department of Health and Human Services. The waiting list has grown sixfold over the past two decades, while the number of transplants has onlyabout doubled from 15,756 in 1991 to 28,954 last year, also according to HHS.

Most organ donations come from people who have died, who had either registered as a donor (commonly through the DMV) or through a decision made by the next-of-kin. And transplant advocates are always looking for ways to increase donor registration.

New research shows there's one quite effective way: Ask more than once. Don't just let the decision rest on a box for people to check — or ignore — when they go to the DMV. Giving people more opportunities to donate organs and providing more information would increase registrations, according to a new working paper posted by the National Bureau of Economic Research.

Researchers surveyed 368 people with a Massachusetts driver's license or ID card, including 156 people (42.4 percent) who were already registered organ donors. Of those who weren't registered donors, 61 people in the study decided to sign up after researchers presented them with the chance to update their status. Just two people who had been registered donors asked to remove themselves from the registry.

"Put simply, asking again for organ donation generates more donors," wrote Judd Kessler of the University of Pennsylvania's Wharton School and Stanford University's Alvin Roth. They said this suggests that policymakers should look for more opportunities to keep asking this question, like on income tax forms, as the researchers said some states are considering.

So, why could asking a second time yield a better result? People may have missed the opportunity to register the first time; or, repeat requests may signal the importance of organ donation, Kessler and Roth write. The "guilt factor" may also kick in after repeat requests. And there's also the chance that people learned something that changed their minds. On that final point, Kessler and Roth found that just informing non-donors about what organs they could donate made them more willing to register.

How you ask could matter

There's also an active debate about the best way to present the opportunity to register. There's the "opt-in" approach, in which people can check a box to become a donor, or the "active choice" model, in which people must answer yes or no. About 80 percent of registry forms the researchers reviewed followed the active choice model.

California provides an interesting illustration of that distinction. In 2011, the state changed from "opt-in" to "active choice." The old opt-in form is on the top:

Researchers said the early results from California's switch to active choice seems to have had a negative effecton registrations. They point to another finding, though, that they say signals concerns with this approach.

A survey 0f 800 people found more support for the next-of-kin to donate organs if the deceased hadn't opted into a donation registry (38.1 percent), compared to 26.7 percent support if the deceased didn't register under the active choice model. Essentially, it indicates that the opt-in approach leaves some ambiguity about what the deceased may have wanted, increasing the chance of organ donation.