The results cast doubt on the booming billion dollar industry that has resulted in online training, apps and television shows, that tap into dark fears about ageing, and the promise of maintaining brain function.

The researchers urge the brain training industry to "temper its claims" until more is known, and say most brain training websites make explicit claims that are not currently supported by research.

In the new study,  published on Monday in the Proceedings of the National Academy of Sciences, a group of researchers argue that the "desire to become smarter may blind us to the role of placebo effects."

Advertisements for brain training  — for example, NeuroNation.com argues the "more you train, the smarter you can be" — may attract individuals who are more likely to expect positive results than those who are more sceptical, said researchers.

To test this theory, the researchers got a group to play an hour long memory game that was designed to identify and remember a pattern.

The 50 participants were split into two groups, which were recruited differently.

One group received a flyer advertising "brain training and cognitive enhancement". It said numerous studies had shown memory exercises can increase fluid intelligence. It also included an impressive-looking scholarly citation to studies on brain training.

The second didn't mention brain training. It simply asked them to "participate in a study" and promised participants extra course credits.

When participants contacted the researchers about the study which promised to improve their brains, the responses "stacked the deck toward seeing the effects." For instance, they were informed they could benefit from this kind of training.

The others, who saw the advertisement with the promise of course credits, weren't told anything about the potential benefits.

All participants then participated in a cognitive training task similar to those offered by companies that sell this sort of training.

After one hour of cognitive training, individuals who responded to the suggestive flyer promising improvements in fluid intelligence showed improvements equivalent to a 1- to 10-point increase on a standard IQ test.

In contrast, those people who responded to the non-suggestive flyer showed no improvement, said the research.

"The spectre of a placebo may arise in any intervention when the desired outcome is known to the participant — an intervention like cognitive training," the authors said.

When coupled with evidence that "people tend to hold strong implicit beliefs regarding whether or not intelligence is malleable" and that those beliefs may skew research findings, the authors concluded that past research is basically bunk, said a report on the ArsTechnica website.

Researchers said the findings "provide an alternative explanation for effects observed in the cognitive-training literature and the brain-training industry, revealing the need to account for confounds in future research."

The study was designed by Cyrus Foroughi, a PhD candidate working with Dr. Boehm-Davis in the Applied Performance Research Lab of George Mason University in Virginia, USA.

The authors say the placebo effect isn't worthless. But more research was needed where participants didn't self-select their group or know the point of the study.

"By using such methods, we can begin to understand whether true training effects exist and are generalisable to samples (and perhaps populations) beyond those who expect to improve," the authors argue.

The benefits of brain training have been promoted by a range of companies in Australia. They were also highlighted in the two part series by Todd Sampson, the non–executive chairman of communication company Leo Burnett, called Redesign My Brain, aired on ABC. Mr Sampson is also a Fairfax board member. He was approached for a comment, but Fairfax was told he was out of the country and unavailable.

Earlier this year, Lumosity, the largest brain training company in the world was fined US$2 million by the US Federal Trade Commission for deceptive advertising after exaggerating the claims of the efficacy of brain training.

"Lumosity preyed on consumers' fears about age-related cognitive decline, suggesting their games could stave off memory loss, dementia, and even Alzheimer's disease," Jessica Rich, director of the FTC's Bureau of Consumer Protection, said. "Lumosity simply did not have the science to back up its ads."