The benefits of brain and memory training could be all in the mind, nothing more than a placebo effect, finds a major study by American researchers.

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."