The study, published on Monday in the Proceedings of the National Academy of Sciences, was carried out by cognitive scientists at George Mason University in the US. To test whether programs like NeuroNation's may be relying on the placebo effect, the researchers recruited students to take part in a pair of identical tests using a pair of flyers with subtle, but crucial, differences.

The first flyer (the one on the left) describes the study as looking at "brain training and cognitive enhancement," and promises that "numerous studies have shown working memory training can increase fluid intelligence." The second flyer (the one on the right), simply advertises a generic "study" that will earn the students course credits. No mention of brain training is made, and there are no scientifically backed promises of increased intelligence.

Twenty-five students were recruited using each flyer and then given a standardized test for fluid intelligence — the capacity for reasoning and problem solving. After this, they participated in an hour of cognitive training, and were given the same intelligence test again. The researchers found that those students that were recruited using the suggestive flyer showed an increase in their intelligent tests equal to between five and 10 IQ points, while those that answered the second, boring, flyer, showed no benefits at all. "It's strong evidence that it wasn't really a true training effect," one of the study's authors, Cyrus Foroughi, told Cosmos.

"strong evidence that it wasn't really a true training effect."

But why is this finding important? Well, the researchers also looked at 19 past studies into the effectiveness of brain-training tests, and found that 17 of them used "overt" language similar to that of their first flyer. These studies were all included in a 2015 meta-analysis that concluded that cognitive training exercises increased participants' IQ by up to four IQ — but that increase could have been simply down to the placebo effect, rather than any long-lasting increase in individuals' intelligence.

And the influence of the placebo effect is not the only challenge to the science of brain-training programs. Researchers have pointed out serious flaws with studies that claim to show their efficacy and repeatedly criticized the tests for their lack of "transfer effects." That is to say, while it seems that people who play brain-training games get better at playing certain tests, there's far less evidence that there is any benefit to their general cognitive abilities. In other words, brain-training games might be just that: games.