You can increase your intelligence(blogs.scientificamerican.com)

    This is very damaging to the case that dual n-back increases IQ. Not only do the
    better studies find a drastically smaller effect, they are not sufficiently 
    powered to find such a small effect at all, even aggregated in a meta-analysis, 
    with a power of ~11%, which is dismal indeed when compared to the usual 
    benchmark of 80%, and leads to worries that even that is too high an estimate 
    and that the active control studies are aberrant somehow in being subject to a 
    winner’s curse or subject to other biases. (Because most studies used convenient 
    passive control groups and the passive effect size is 3x larger, they in 
    aggregate are very well-powered: 94%; however, we already know how they are 
    skewed upwards, so we don’t care if we can detect a biased effect or not.) In 
    particular, Boot et al 2013 argues that active control groups do not suffice to 
    identify the true causal effect because the subjects in the active control group 
    can still have different expectations than the experimental group, and the 
    group’ differing awareness & expectations can cause differing performance on 
    tests; they suggest recording expectancies (somewhat similar to Redick et al 
    2013), checking for a dose-response relationship (see the following section for 
    whether dose-response exists for dual n-back/IQ), and using different 
    experimental designs which actively manipulate subject expectations to identify 
    how much effects are inflated by remaining placebo/expectancy effects.