Table 2.

The study characteristics of the five eligible studies included in the systematic review

Source	No. of patients (N)		Age range	Inclusion criteria	Exclusion criteria	Diagnostic tools
Peralta et al. (2011) 33	112		15–65 years	Patients admitted for treatment of first episode of psychosis and met a DSM‐IV diagnosis of schizophrenia, schizophreniform or schizoaffective disorder. Patients had no previous exposition to antipsychotics and an available biological mother.	A history of drug dependence, IQ < 70, evidence of organic brain disorder or any other meaningful medical illness.	An 18‐item checklist that included DSM‐IV criteria for ADHD was completed by mothers, regarding the ages of 6–10 years. This was then reviewed by researchers who also enquired about age of onset, severity and prior ADHD diagnoses
Dalteg et al. (2014) 31	Gotland Sample 1 inpatient ward	119	16–68 years	In‐patients who met DSM‐IV criteria for SCZ	The samples excluded drug‐induced psychoses	Subjects were interviewed by experienced psychologist and a clinical researcher about childhood symptoms. The self‐report Malmo questionnaire assessed adult ADHD (includes the 18 DSM‐IV criteria)
	Aftercare Sample 2 forensic	149	20–75 years	Men with a DSM‐IV‐TR SCZ diagnosis were recruited from forensic psychiatric clinics when released from compulsory care.		All patients were assessed by experienced and trained interviewers. Retrospective cADHD diagnosis was based on the presence of a typical pattern of problems (at time of data collection, there was no ADHD SCID interview)
	Aftercare Sample 3 ‐general clinics	98	19–63 years	Control patients were of the same age, sex and released from general psychiatry clinics near Sample 2 clinics and had a DSM‐IV‐TR SCZ diagnosis
Rubino et al. (2009) 34	197		35.5 mean (SD = 11.1)	In‐patients with DSM‐IV diagnoses of adult‐onset SCZ	Schizophreniform, schizoaffective and delusional disorders were excluded, patients scoring less than 24 on the Mini Mental State Examination and with clinical evidence of brain damage/disease	Two senior psychiatrists employed the SCID interview for current diagnosis. The semistructured interview K‐SADS‐PL was used to retrospectively identify Axis I disorders in childhood and adolescence
Hallerbäck et al. (2014) 30	41		28.9 mean (SD = 4.6)	All participants provided informed consent and were recruited from the only adult psychiatric clinic in Värmland, Sweden.	Their clinical diagnosis had to be confirmed by the SCID‐I, or participants were excluded	With a SCID interview, participants were asked about prior diagnostic assessments, and specifically whether they had been examined for, or had a clinical diagnosis of ADHD
Donev et al. (2011) 32	27		18–44 years	All patients had a schizophrenic disorder according to ICD‐10 criteria. Informed consent was obtained, and tests were carried out a few days before discharge (likely to be in remission)	Exclusion criteria included any severe psychotic symptoms, medication‐induced psychomotor retardation, severe schizophrenic residual symptoms or a comorbid affective disorder	The 25‐item version of the WURS was used to measure ADHD, with a cut‐off at 30