There is a substantial body of literature on environmental risk associated with schizophrenia. Most research has largely been conducted in Europe and North America, with little representation of the rest of the world; hence generalisability of findings is questionable. For this reason, we performed a mapping review of studies on environmental risk for schizophrenia spectrum disorders, recording the country where they were conducted, and we linked our findings with publicly available data to identify correlates with the uneven global distribution. Our aim was to evaluate how universal is the ‘common knowledge’ of environmental risk for psychosis collating the availability of evidence across different countries and to generate suggestions for future research identifying gaps in evidence.

We performed a systematic search and mapping of studies in the PubMed and PsycINFO electronic databases reporting on exposure to environmental risk for schizophrenia including obstetric complications, paternal age, migration, urbanicity, childhood trauma, and cannabis use and subsequent onset of schizophrenia spectrum disorders. This search focused on articles published from the date of the first available publication until 31 May 2023. We recorded the country where they were conducted. We downloaded publicly available data on population size, measures of wealth, medical provisions, research investment, and of quality research outputs per country and performed regression analyses of each predictor with the number of studies and recruited cases in each country.

We identified 308 publications that included a sample size of 445,000 patients with schizophrenia spectrum disorders. The majority were conducted in northern Europe and North America, with large parts of the world totally unrepresented. In the associations between the number of environmental risk studies for schizophrenia with potential predictors, we found that neither population nor wealth or research investment were strong predictors of research outputs in the field. Interestingly, the stronger correlations were found for number of researchers per population and for indicators of top-end scientific achievements, such as number of Nobel laureates per country.

Our results demonstrate a gap of knowledge due to the underrepresentation of studies on environmental risk of schizophrenia spectrum disorders in large parts of the world. This has implications not only in the generalisability of any findings from research conducted in the Northern hemisphere but also in our ability to progress in efforts to make causal inferences about biological pathways to schizophrenia. These findings reinforce the need to focus research on populations that are underrepresented in research and underserved in health care.

A link between older age of fatherhood and an increased risk of schizophrenia was detected in 1958. Since then, 10 studies attempted to replicate this result with different methods, on samples with different origins, using different age classes. Defining a cut-off at which the risk is significantly increased in the offspring could have an important impact on public health.

A meta-analysis (Meta Win®) was performed, assessing the mean effect size for each age class, taking into account the difference in age class references, and the study design.

An increased risk is detected when paternal age is below 20 (compared to 20–24), over 35 (compared to below 35), 39 (compared to less than 30), and 54 years old (compared to less than 25). Interestingly, 35 years appears nevertheless to be the lowest cut-off where the OR is always above 1, whatever the age class reference, and the smallest value where offspring of fathers below or above this age have a significantly different risk of schizophrenia.

No threshold can be precisely defined, but convergent elements indicate ages below or above 35 years. Using homogeneous age ranges in future studies could help to clarify a precise threshold.

Advancing paternal age has been linked to psychiatric disorders. These associations might be caused by the increased number of de novo mutations transmitted to offspring of older men. It has also been suggested that the associations are confounded by a genetic liability for psychiatric disorders in parents. The aim of this study was to indirectly test the confounding hypotheses by examining if there is a genetic component to advancing paternal age and if men with a genetic liability for psychiatric disorders have children at older ages.

We examined the genetic component to advancing paternal age by utilizing the twin model in a cohort of male twins (N = 14 679). We also studied ages at childbirth in men with or without schizophrenia, bipolar disorder and/or autism spectrum disorder. Ages were examined in: (1) healthy men, (2) affected men, (3) healthy men with an affected sibling, (4) men with healthy spouses, (5) men with affected spouses, and (6) men with healthy spouses with an affected sibling.

The twin analyses showed that late fatherhood is under genetic influence (heritability = 0.33). However, affected men or men with affected spouses did not have children at older ages. The same was found for healthy individuals with affected siblings. Instead, these men were generally having children at younger ages.

Although there is a genetic component influencing late fatherhood, our data suggest that the associations are not explained by psychiatric disorders or a genetic liability for psychiatric disorders in the parent.

Advanced paternal age is associated with increased risk of schizophrenia. This study aimed to explore whether older paternal age is associated with earlier onset among co-affected schizophrenia sib-pairs with the same familial predisposition.

A total of 1297 patients with schizophrenia from 630 families, which were ascertained to have at least two siblings affected, throughout Taiwan were interviewed using the Diagnostic Interview for Genetic Studies. Both inter-family comparisons, a hierarchical regression model allowing for familial dependence and adjusting for confounders, and within-family comparisons, examining the consistency between onset order and birth order, were performed.

An inverted U shape was observed between paternal age and onset of schizophrenia. Affected offspring with paternal age of 20–24 years had the oldest onset. As paternal age increased over 25 years, older paternal age exhibited a linear decrease in the onset of schizophrenia. On average, the onset was lowered by 1.5 years for paternal age of 25–29 years and by 5.5 years for paternal age ⩾50 years (p = 0.04; trend test). The proportion of younger siblings with earlier onset (58%) was larger than that of older siblings with earlier onset (42%) (p = 0.0002).

These findings indicate that paternal age older than 25 years and younger than 20 years were both associated with earlier onset among familial schizophrenia cases. The associations of advanced paternal age with both increased susceptibility to schizophrenia and earlier onset of schizophrenia are consistent with the rate of increases in spontaneous mutations in sperm as men age.

A consistent association between paternal age and their offspring's risk of schizophrenia has been observed, with no independent association with maternal age. The relationship of paternal and maternal ages with risk of bipolar affective disorders (BPAD) in the offspring is less clear. The present study aimed at testing the hypothesis that paternal age is associated with their offspring's risk of BPAD, whereas maternal age is not.

This population-based cohort study was conducted with individuals born in Sweden during 1973–1980 and still resident there at age 16 years. Outcome was first hospital admission with a diagnosis of BPAD. Hazard ratios (HRs) were calculated using Cox's proportional hazard regression.

After adjustment for all potential confounding variables except maternal age, the HR for risk of BPAD for each 10-year increase in paternal age was 1.28 [95% confidence interval (CI) 1.11–1.48], but this fell to 1.20 (95% CI 0.97–1.48) after adjusting for maternal age. A similar result was found for maternal age and risk of BPAD [HR 1.30 (95% CI 1.08–1.56) before adjustment for paternal age, HR 1.12 (95% CI 0.86–1.45) after adjustment]. The HR associated with having either parent aged 30 years or over was 1.26 (95% CI 1.01–1.57) and it was 1.45 (95% CI 1.16–1.81) if both parents were >30 years.

Unlike schizophrenia, the risk of BPAD seems to be associated with both paternal and maternal ages.