Attention-deficit hyperactivity disorder (ADHD), bipolar disorder
and schizophrenia are highly heritable psychiatric disorders that
sometimes co-occur.1–4 The extent to which ADHD shares
aetiological factors with bipolar disorder and/or schizophrenia
has important implications for both clinical practice and research.
Studies suggest shared genetic susceptibility loci as well as genetic
deletions and duplications (i.e. copy number variants, CNVs) that
overlap between these disorders.5,6 However, no adequately sized
family study has explored the degree to which ADHD shares
familial risk factors with bipolar disorder and schizophrenia,
and prior family studies of ADHD and bipolar disorder indicate
that the co-occurrence of the two disorders represents a familially
distinct syndrome.7,8 Hence, we aimed to explore the extent to
which ADHD shares genetic and environmental risk factors with
bipolar disorder and schizophrenia. To this end we conducted a
total population study in Sweden of people with ADHD
(n = 61 187) and a control group. We estimated the occurrence
of bipolar disorder and schizophrenia not only in the ADHD
proband and control groups, but also in the relatives of
these individuals. To study the effect of shared genetic and
environmental factors specifically, we adjusted for the potential
existence of aetiologically distinct subsyndromes (e.g. an ADHD
plus bipolar disorder subtype) by excluding from the proband
and control groups people with schizophrenia or bipolar disorder,
and by excluding relatives of probands or controls with ADHD.9