In a study designed to assess the experimental methods of earlier brain-training studies, researchers found that sampling bias and the placebo effect explained the positive results seen in the past. “Indeed, to our knowledge, the rigor of double-blind randomized clinical trials is nonexistent in this research area,” the authors report. They even suggest that the overblown claims from brain training companies may have created a positive feedback loop, convincing people that brain training works and biasing follow-up research on the topic.
“The specter of a placebo may arise in any intervention when the desired outcome is known to the participant—an intervention like cognitive training,” the authors note. 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 conclude that past research is basically bunk. (Source)