As gwern points out in his meta-analysis of Dual N-Back [1], and his Dual N-Back FAQ [2], the results of Dual N-Back have been very mixed. Some studies show massive improvement. Other studies show minor improvement, while some studies show no improvement at all. It is by no means clear in the literature that Dual N-Back actually helps more than an active placebo.
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.
That said, there isn't any evidence of
harm, either, so at worst, you'll waste your time if it doesn't work out for you.
[1] http://www.gwern.net/DNB%20meta-analysis
[2] http://www.gwern.net/DNB%20FAQ