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Research Review: The strength of the genetic overlap between ADHD and other psychiatric symptoms - a systematic review and meta-analysis.

BACKGROUND
Attention-deficit/hyperactivity disorder (ADHD) frequently co-occurs with other psychiatric disorders. Twin studies have established that these co-occurrences are in part due to shared genetic risks. However, the strength of these genetic overlaps and the potential heterogeneity accounted for by type of psychiatric symptoms, age, and methods of assessment remain unclear. We conducted a systematic review to fill this gap.


METHODS
We searched PubMed, PsycINFO, Embase, and Web of Science until March 07, 2019. Genetic correlations (rg ) were used as effect size measures.


RESULTS
A total of 31 independent studies fulfilled the inclusion criteria. The pooled estimates showed that the associations between ADHD and other psychiatric symptoms were partly explained by shared genetic factors, with a pooled genetic correlation of 0.50, 95% confidence interval: 0.46-0.60. The genetic correlations (rg ) between ADHD and externalizing (rg  = .49 [0.37-0.61]), internalizing (rg  = .50 [0.39-0.69]), and neurodevelopmental (rg  = .56 [0.47-0.66]) symptoms were similar in magnitude. The genetic correlations in childhood and adulthood were rg  = .53 (0.43-0.63) and rg  = .51 (0.44-0.56), respectively. For methods of assessment, the genetic correlations were also similar in strength, self-reports rg  = .52 (0.47-0.58), other informants rg  = .55 (0.41-0.69), and combined raters rg  = .50 (0.33-0.65).


CONCLUSIONS
These findings indicate that the co-occurrence of externalizing, internalizing, and neurodevelopmental disorder symptoms in individuals with ADHD symptoms in part is due to a shared genetic risk.