Does DNB increase IQ? What factors affect the studies? Probably not: gains are driven by studies with weakest methodology like apathetic control groups.
I meta-analyze the >19 studies up to 2016 which measure IQ after an n-back intervention, finding (over all studies) a net gain (medium-sized) on the post-training IQ tests.
The size of this increase on IQ test score correlates highly with the methodological concern of whether a study used active or passive control groups. This indicates that the medium effect size is due to methodological problems and that n-back training does not increase subjects’ underlying fluid intelligence but the gains are due to the motivational effect of passive control groups (who did not train on anything) not trying as hard as the n-back-trained experimental groups on the post-tests. The remaining studies using active control groups find a small positive effect (but this may be due to matrix-test-specific training, undetected publication bias, smaller motivational effects, etc.)
I also investigate several other n-back claims, criticisms, and indicators of bias, finding:
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payment reducing performance claim: possible
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dose-response relationship of n-back training time & IQ gains claim: not found
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kind of n-back matters: not found
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publication bias criticism: not found
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speeding of IQ tests criticism: not found