Bednets are one of the best ways in the world to help people. But some persistent myths about them have clouded that basic fact — and those myths can threaten to roll back the progress we’ve made on fighting malaria.

Bednets are relatively cheap to distribute, at only about $4.50 each. They protect against malaria, which affects more than 200 million people every year and kills more than 400,000. They don’t just protect the people who sleep under them; when they’re widely distributed, they disrupt the transmission of the disease, reducing overall risk.

Even with local governments, foreign governments, and foreign aid dedicated to malaria control, there isn’t enough funding to get a bednet to everyone who needs one. It is estimated that distributions have prevented more than 600million cases of malaria and saved 6.8million lives.

GiveWell is a charity evaluator that compares interventions for the global poor. They’ve looked into bednets, safe drinking water, deworming treatments, Vitamin A and iodine supplementation, labor mobility, and cash transfers. All of these interventions are promising and cost-effective ways to do good.

Bednets are consistently among the best options of the lot. $100,000 spent on bednets is expected to save the lives of 29 people, mostly children under 5. GiveWell tends to estimate that other programs are between 20 percent less effective and a tenth as effective at helping people live longer, healthier lives. And that’s comparing them to the most promising global health interventions out there.

Again and again, though, I have conversations with people who know only one thing about bednets: They are sometimes used by poor recipients for fishing. This is true, though local governments are trying to crack down on the practice. (Unfortunately, they’re doing so with stiff prison sentences for illegal fishing rather than by distributing food so people have better options.) Therefore, skeptics tend to argue that bednets are not such great lifesavers after all.

But here’s the thing: The math on bednet effectiveness takes such uses into account. Studies that groups like GiveWell rely upon are conducted by distributing malaria nets and then measuring the resulting fall in mortality rates, so those mortality figures don’t assume perfect use.

Additionally, malaria distribution organizations like the Against Malaria Foundation survey households to make sure nets are still being used. They don’t just ask people whether the nets are in use — people might lie — but go in and check. They’ve found that 80 percent to 90 percent of nets are used as intended, hanging over beds, half a year after first deployment. This isn’t surprising, as people are highly motivated not to die of malaria and won’t put nets to secondary uses lightly.

Bednets would work even better if no one was ever desperate enough to use them for fishing, but no estimates of their effectiveness assume such perfect use. Our figures for the effectiveness of bednets all reflect their effectiveness under real-world conditions.

There’s not much evidence that unapproved uses are doing harm

What about harm to fisheries from people fishing with nets? Researchers have only recently started looking into this. No one has measured detrimental effects yet, though they could emerge later.

The lack of evidence hasn’t stopped people from trying to link bednet fishing to the catastrophic collapses in fisheries in Africa, which is driven by large, often-international commercial fishing boats that operate at thousands of times the scale of individual bednet fishers.

The insecticide in anti-malarial bednets also does not have negative effects on humans, because the dosages involved are so low. It’s unclear whether there are any harmful effects from fishing with nets. (And, it’s worth noting, there is one oft-forgotten positive effect from the use of bednets for fishing: People are fed.)

This clip from Billions sums up the case against bednets, presenting three myths in 16 seconds: that it makes the fish inedible, that it’s been linked to depletion of fish stocks, and that local governments have asked to halt bednet distributions. (They haven’t.)

So why have these falsehoods about nets been so persistent?

There’s an interesting small-scale study by the Busara Center that might explain this. They asked people to split $100 between a wildlife conservation charity and one that provides clean water to low-income people. With a different group of subjects, they added a line to the description of the clean water charity, noting that the charity didn’t fight malaria. This should have been irrelevant — it was already apparent from the initial description of the charity, and none of the case for the charity’s impact rested on the assumption it’d solve malaria too. But support for the charity was significantly lower when subjects were reminded that the people affected could still get malaria.

In a second survey, they offered three options — wildlife, microfinance, and a graduation program for the very poorest people in the world. They tested an addition to the description of the microfinance program, saying it doesn’t reach the very poorest. That did provoke a drop in donations to microfinance — and people who decided to donate elsewhere gave to the wildlife charity just as often as to the program for the very poorest people in the world. In other words, learning that a charity doesn’t solve a problem causes people to take their money elsewhere — but not necessarily to a charity that does solve that problem.

This was a small online study as part of a new initiative to publish more early-stage research. It’s exploratory and shouldn’t be treated as conclusive. But it’s worrying. Irrelevant factors, or ones that researchers already considered, could be turning people off effective charities.

If donors conclude bednets are hopeless, then no charitable cause will ever survive an open discussion about how it works and how it could work better. Even worse, it doesn’t seem like donors are motivated, when they learn something negative about a charity, to seek out a better charity. Instead, as on Billions, these myths seem to sometimes get deployed as a justification to not bother giving in the first place.

Sign up for the Future Perfect newsletter. Twice a week, you’ll get a roundup of ideas and solutions for tackling our biggest challenges: improving public health, decreasing human and animal suffering, easing catastrophic risks, and — to put it simply — getting better at doing good.