Although the ICD and DSM differentiate between different psychiatric disorders, these often share symptoms, risk factors, and treatments. This was a population-based, case–control, sibling study examining familial clustering of all psychiatric disorders and low IQ, using data from the Israel Draft-Board Registry on all Jewish adolescents assessed between 1998 and 2014.

We identified all cases with autism spectrum disorder (ASD, N = 2128), severe intellectual disability (ID, N = 9572), attention-deficit hyperactive disorder (ADHD) (N = 3272), psychotic (N = 7902), mood (N = 9704), anxiety (N = 10 606), personality (N = 24 816), or substance/alcohol abuse (N = 791) disorders, and low IQ (⩾2 SDs below the population mean, N = 31 186). Non-CNS control disorders were adolescents with Type-1 diabetes (N = 2427), hernia (N = 29 558) or hematological malignancies (N = 931). Each case was matched with 10 age-matched controls selected at random from the Draft-Board Registry, with replacement, and for each case and matched controls, we ascertained all full siblings. The main outcome measure was the relative recurrence risk (RRR) of the sibling of a case having the same (within-disorder RRR) or a different (across-disorder RRR) disorder.

Within-disorder RRRs were increased for all diagnostic categories, ranging from 11.53 [95% confidence interval (CI): 9.23–14.40] for ASD to 2.93 (95% CI: 2.80–3.07) for personality disorders. The median across-disorder RRR between any pair of psychiatric disorders was 2.16 (95% CI: 1.45–2.43); the median RRR between low IQ and any psychiatric disorder was 1.37 (95% CI: 0.93–1.98). There was no consistent increase in across-disorder RRRs between the non-CNS disorders and psychiatric disorders and/or low IQ.

These large population-based study findings suggest shared etiologies among most psychiatric disorders, and low IQ.

Previous studies reported an association between advanced paternal age at birth and increased risk for schizophrenia and bipolar disorder. While some hypothesize that this association is caused by de-novo mutations in paternal spermatozoa, others cite factors associated with psycho-social characteristics of fathers who have children at a late age. This study aims to test these hypotheses.

A historical-prospective, population-based cohort study, performed by linking the Israeli Draft Board Registry and the Israeli National Psychiatric Hospitalization Registry (N = 916 439; 4488 with schizophrenia, 883 with bipolar disorder). Odds ratios (OR) and two-sided 95% confidence intervals (CI) were calculated by logistic regression models, using paternal age as predictor and risk for later hospitalizations for schizophrenia or bipolar disorder as outcome measure. Models were first fitted unadjusted, then adjusted for paternal age at birth of the first child.

In the unadjusted model, offspring of fathers aged 45 and above at birth had increased risk of schizophrenia (OR = 1.71, 95% CI 1.49–1.99) and bipolar disorder (OR = 1.63, 95% CI 1.16–2.24). However, taking into account paternal age at birth of first child, advanced paternal age was no longer associated with increased risk of schizophrenia (OR = 0.60, 95% CI 0.48–0.79) or bipolar disorder (OR = 1.03, 95% CI 0.56–1.90).

Controlling for paternal age at birth of the first offspring, advanced paternal age does not predict increased risk for schizophrenia or bipolar disorder. These data indicate that the association between advanced paternal age and having an offspring with schizophrenia and bipolar disorder is likely due to psychos-social factors, or common genetic variation associated with delayed initial fatherhood.