The study has a small sample size, but bear with me.
In the control group (people who simply thought they were participating in an experiment) there was no difference in pre and post training intelligence. In the experimental group (who were told they were training to increase their intelligence) an increase in intelligence was measured (5-10 IQ points) after only one hour of training.
In general, exercises designed to improve intelligence take several hours of training over the course of several weeks to yield a measurable effect. So the researchers designed the experiment to remove training based improvement.
Why do this?
Several attempts to measure ‘cognitive improvement games’ advertise to participants in a way that may prime them for placebo improvements (or appeal to people for reasons related to their beliefs about intelligence thus creating a selection bias by recruiting people who really want the training to work). But, as I mentioned, people apparently improved. Now, I don’t care about brain training games. Just use Khan Academy, Duo Lingo, and learn to write software. Your brain will improve.
What interests me is what this study might imply about beliefs and mindset. If people can be persuaded to improve at an intelligence test by being primed to believe they have engaged in an activity that made them smarter, how could teachers, counselors, parents, ministers, and others leverage such a finding? We know that the coaching effect is very powerful for athletes. Could it hold true for education? It certainly makes more sense than the self-esteem movement. Replacing: “You’re smart and you can do it,” with “this brain training will make you smarter if you do it” could be a useful mindset technique.
Anyway, there are genetic limits to IQ. But within that set range, getting enough sleep, eating a healthy diet, exercising, and, perhaps, belief can yield minor improvements. I find Arthur Whimbey’s research on this compelling. Prior to computer games, like we have today, he found that teaching people to think sequentially (out loud or writing out their thought processes) led to increased IQ scores and performance in school. This accords with Ritche’s claim that education also increases IQ.
But again, this study is small potatoes in terms of evidence. Either way, fake it till you make it is the best strategy.
 Cyrus K. Foroughi et al., “Placebo Effects in Cognitive Training,” Proceedings of the National Academy of Sciences (June 20, 2016), 1.