However James Holland Jones says that this hypothesis may not be the whole story. People use language to communicate to others whether a treatment is actually working so one would expect the quackery treatments not to be taken up as often as proven remedies.
Eating a vulture won't clear a bad case of syphilis nor will a drink made of rotting snakes treat leprosy, but these and other bogus medical treatments spread precisely because they don't work. That's the counterintuitive finding of a mathematical model of medical quackery.
Ineffective treatments don't cure an illness, so sufferers demonstrate them to more people than those who recovery quickly after taking real medicines.
"The assumption is that when people pick up treatments to try, they're basically observing other people," says Mark Tanaka, a mathematical biologist at the University of New South Wales in Sydney, who led the study. "People don't necessarily know that what somebody is trying is going to work."
The World Health Organization is demanding better proof that folk medicines work before they can be approved. And the Malaysian government has rejected more than a third of the 25,000 applications to register traditional medicines it has received because the treatments are ineffective or dangerous.
Despite these efforts, quack medicine persists around the world. Some Nigerians treat malaria with witchcraft, a South African health minister recently claimed that garlic and beetroot treat HIV, and western health stores brim with unproven treatments for almost any disease imaginable. For instance St John's wort does nothing for attention deficit hyperactive disorder in children, a recent placebo-controlled trial concluded.
To understand why these quack medical treatments persist in the face of better proven remedies, Tanaka applied mathematical models used to measure evolutionary fitness to medical treatments.
His model accounted for factors including the rate of conversion to a treatment, the effectiveness of a treatment, the rate at which people abandon a treatment, the odds of recovering naturally, and the chances of dying. The model starts with a single person demonstrating a treatment – rubbish or not – and measures how many people are influenced to go on to give the treatment a try.
Under a wide range of conditions, quack treatments garnered more converts than proven hypothetical medicines that offer quicker recovery, Tanaka found. "The very fact that they don't work mean that people that use them stay sick longer" and demonstrate a treatment to more people, he says.
Bad treatments don't always win out. Recurring diseases are more likely to promote effective treatments than rare diseases because repeated demonstration weeds out bad treatments, Tanaka found.
But is this model valid in cultures where evidence-based medicine predominates, and government groups such as the US Food and Drug Administration vet most medical treatments?
Tanaka thinks so, pointing to the popularity of alternative medicines and the debate over the effectiveness of FDA-approved drugs. "In many situations people will just observe and copy anyway, regardless what the official information is," he says.
And in some cases, one peer-reviewed study may conclude that a drug works, while another shows it doesn't. "Even where there is a bit of clinical research, we don't really know yet whether at lot of medicines are effective," he says.
"I think it's an interesting idea. It's quite clever", James Holland Jones, a biological anthropologist at Stanford University in California, says of the model. However, language allows people to vet unproven remedies without trying them, he adds – that is, you can just ask if a treatment was effective. "You don't necessarily have to copy everything."
Journal reference: PLoS-ONE (DOI: 10.1371/journal.pone.0005192.s001)