The relationship between childhood adversity and affective instability across psychiatric disorders: A meta-analysis.

INTRODUCTION: Affective instability represents an important, transdiagnostic biobehavioural dimension of mental ill health and clinical outcome. The causes of affective instability remain unclear. This systematic review and meta-analysis evaluated the extent to which exposure to childhood adversity is associated with affective instability across psychiatric disorders, and which forms of adversity are most strongly associated with affective instability. METHODS: The review followed a published protocol (PROSPERO: CRD42020168676). Searches in Medline, Embase and PsychInfo identified studies using quantitative measures of childhood adversity and affective instability, published between January 1980 and July 2023. Data were analysed using a random effects meta-analysis separately for each outcome, namely affective lability, emotion dysregulation, and rapid cycling. The Mixed-Methods Appraisal Tool was used to appraise the quality of the literature. RESULTS: The search identified 36 studies involving 8431 participants. All reports focused on cross-sectional associations. We did not identify any prospective longitudinal research. The analysis showed small, but statistically significant effects of childhood adversity on affective lability (r = 0.09, 95% CI 0.02, 0.17), emotion dysregulation (r = 0.25, 95% CI 0.19, 0.32), and rapid cycling (OR = 1.39; 95% CI 1.14, 1.70). When considering adversity subtypes, emotional abuse showed the strongest effect on affective lability (r = 0.16, 95% CI 0.07, 0.24) and emotion dysregulation (r = 0.32, 95% CI 0.19, 0.44). Quality assessment scores were generally low. Most studies failed to control for confounding factors or offer assurances around the representativeness of the samples. CONCLUSIONS: The findings suggest that childhood adversity, particularly emotional abuse, is associated emotional instability in adulthood, but further prospective longitudinal research is needed to confirm causality. The findings have implications for the prevention and treatment of affective instability across psychiatric disorders.

The neuropsychological outcomes of non-fatal strangulation in domestic and sexual violence: A systematic review.

This systematic review draws together evidence from the literature for the pathological, neurological, cognitive, psychological, and behavioural outcomes of non-fatal strangulation in domestic and sexual violence. A systematic search of PubMed, PsycINFO, CINHAL, Proquest, ASSIA, Web of Science, WestLaw, Open Grey, and Ethos was conducted, with no date limits set, to identify eligible studies. Thirty empirical, peer-reviewed studies were found which met the inclusion criteria. Pathological changes included arterial dissection and stroke. Neurological consequences included loss of consciousness, indicating at least mild acquired brain injury, seizures, motor and speech disorders, and paralysis. Psychological outcomes included PTSD, depression, suicidality, and dissociation. Cognitive and behavioural sequelae were described less frequently, but included memory loss, increased aggression, compliance, and lack of help-seeking. However, no studies used formal neuropsychological assessment: the majority were medical case studies or based on self-report. Furthermore, few authors were able to control for possible confounds, including other physical violence and existing psychosocial difficulties. There is therefore a need for further neuropsychological research, focusing on cognitive and behavioural outcomes, using standardized tools, and control groups where possible. This is urgent, given societal normalization of strangulation, and legal systems which often do not reflect the act's severity and its consequences.

ADHD prevalence in the psychiatric population.

OBJECTIVE: This article presents a systematic literature review of the prevalence of Attention Deficit Hyperactivity Disorder (ADHD) in the adult psychiatric population. METHODS: Three databases were searched using keywords relating to ADHD and psychiatric disorders. Fifteen studies published before May 2020 met inclusion criteria. RESULTS: Prevalence rates ranged from 6.9 to 38.75%. There was considerable heterogeneity with regards to the assessment methods of ADHD and comorbidities, the sampling as well as inclusion and exclusion of psychiatric comorbidities. Research concluded that ADHD is underdiagnosed in this population. Gender ratios also varied between 1.8:1 and 1:2.5 (male:female). CONCLUSIONS: All ADHD prevalence rates for the psychiatric population were considerably higher than the 2.8% estimated for the general adult population. ADHD should be kept in mind for psychiatric patients to ensure accurate diagnosis and optimal treatment options.Key PointsADHD is considerably underdiagnosed in the psychiatric populationGender ratios between 1.8:1 and 1:2.5 (male:female) reported in researchMore research is needed to ascertain if the use of ADHD screening instruments would improve the recognition and treatment of adult ADHD in the psychiatric population.

The ethnic density effect in psychosis: a systematic review and multilevel meta-analysis.

BACKGROUND: An 'ethnic' or 'group' density effect in psychosis has been observed, whereby the risk of psychosis in minority group individuals is inversely related to neighbourhood-level proportions of others belonging to the same group. However, there is conflicting evidence over whether this effect differs between minority groups and limited investigation into other moderators. AIMS: To conduct a comprehensive systematic review and meta-analysis of the group density effect in psychosis and examine moderators. METHOD: Four databases were systematically searched. A narrative review was conducted and a three-level meta-analysis was performed. The potential moderating effect of crudely and specifically defined minority groups was assessed. Country, time, area size and whether studies used clinical or non-clinical outcomes were also tested as moderators. RESULTS: Thirty-two studies were included in the narrative review and ten in the meta-analysis. A 10 percentage-point decrease in own-group density was associated with a 20% increase in psychosis risk (OR = 1.20, 95% CI 1.09-1.32, P < 0.001). This was moderated by crudely defined minority groups (F6,68 = 6.86, P < 0.001), with the strongest associations observed in Black populations, followed by a White Other sample. Greater heterogeneity was observed when specific minority groups were assessed (F25,49 = 7.26, P < 0.001). CONCLUSIONS: This is the first review to provide meta-analytic evidence that the risk of psychosis posed by lower own-group density varies across minority groups, with the strongest associations observed in Black individuals. Heterogeneity in effect sizes may reflect distinctive social experiences of specific minority groups. Potential mechanisms are discussed, along with the implications of findings and suggestions for future research.

The effectiveness of physical exercise as an intervention to reduce depressive symptoms following traumatic brain injury: A meta-analysis and systematic review.

Alongside the obvious health benefits, physical exercise has been shown to have a modest anti-depressant effect for people in the general population. To the authors' knowledge, there are no current literature reviews or meta-analyses available exploring this effect for people with a traumatic brain injury (TBI). A systematic review of intervention studies utilising physical exercise and mood outcome measures for a TBI population was performed in November 2016. Baseline and outcome data were extracted for the nine studies which met the inclusion criteria. Effect sizes were calculated for the three controlled trials and six uncontrolled trials and entered into the meta-analysis. Consistent with research in non-brain injury populations, the current meta-analysis identified a small to medium effect size of physical exercise on reducing depressive symptoms in people with a TBI. This would support further rigorous trials to provide additional evidence for the efficacy of physical exercise interventions for people with TBI. Limitations of the current meta-analysis and clinical implications are discussed.

The survivability of dialectical behaviour therapy programmes: a mixed methods analysis of barriers and facilitators to implementation within UK healthcare settings.

BACKGROUND: Dialectical Behaviour Therapy (DBT) is an evidence-based intervention that has been included in the National Institute of Health and Care Excellence guidelines as a recommended treatment for Borderline Personality Disorder in the UK. However, implementing and sustaining evidence-based treatments in routine practice can be difficult to achieve. This study compared the survival of early and late adopters of DBT as well as teams trained via different training modes (on-site versus off-site), and explored factors that aided or hindered implementation of DBT into routine healthcare settings. METHODS: A mixed-method approach was used. Kaplan-Meier survival analyses were conducted to quantify and compare survivability as a measure of sustainability between early and late implementers and those trained on- and off-site. An online questionnaire based on the Consolidated Framework for Implementation Research was used to explore barriers and facilitators in implementation. A quantitative content analysis of survey responses was carried out. RESULTS: Early implementers were significantly less likely to survive than late implementers, although, the effect size was small. DBT teams trained off-site were significantly more likely to survive. The effect size for this difference was large.  An unequal amount of censored data between groups in both analyses means that findings should be considered tentative. Practitioner turnover and financing were the most frequently cited barriers to implementation. Individual characteristics of practitioners and quality of the evidence base were the most commonly reported facilitators to implementation. CONCLUSIONS: A number of common barriers and facilitators to successful implementation of DBT were found among DBT programmes. Location of DBT training may mediate programme survival.

Morphometric analysis of gray matter integrity in individuals with early-treated phenylketonuria.

The most widely-reported neurologic finding in individuals with early-treated phenylketonuria (PKU) is abnormality in the white matter of the brain. In contrast, much less is known regarding the impact of PKU on cortical gray matter (GM) structures. Presently, we applied advanced morphometric methods to the analysis of high-resolution structural MRI images from a sample of 19 individuals with early-treated PKU and an age- and gender-matched comparison group of 22 healthy individuals without PKU. Data analysis revealed decreased GM volume in parietal cortex for the PKU group compared with the non-PKU group. A similar trend was observed for occipital GM volume. There was no evidence of group-related differences in frontal or temporal GM volume. Within the PKU group, we also found a significant relationship between blood phenylalanine levels and GM volume for select posterior cortical sub-regions. Taken together with previous research on white matter and gray matter abnormalities in PKU, the present findings point to the posterior cortices as the primary site of neurostructural changes related to early-treated PKU.

Elevated P3b latency variability in carriers of ZNF804A risk allele for psychosis.

Increased intra-subject variability (ISV) in reaction times (RTs) is a candidate endophenotype for several psychiatric and neurological conditions, including schizophrenia. ISV reflects the degree of variability in RTs and is thought to be an index of the stability of cognition. It is generally assumed to have the same underlying physiological basis across conditions, but recent evidence raises the possibility that the neural underpinnings of ISV vary with aetiology. Combining genetics with single-trial event-related potentials is an ideal method for investigating the neural basis of ISV in groups where ISV may vary for relatively homogenous reasons. Here we examine the association between P3b latency variability and a polymorphism on the ZNF804A gene associated with psychosis. Ninety-one healthy volunteers genotyped for rs1344706, a polymorphism on ZNF804A, had electroencephalographic data recorded while carrying out three n-back tasks. Data were analysed with a single-trial approach and latency variability of the P3b was compared between the AA homozygous risk group (N=30) and C allele carriers (N=61). P3b latencies were more variable for AA carriers than C carriers. Behavioural ISV, however, was not associated with genotype. The increase in neurophysiological variability, unaccompanied by increased behavioural variability, suggests that this risk gene is associated with an attenuated form of an endophenotype associated with the psychosis phenotype. The increase in both stimulus and response-locked variability also contrasts with previous work into attention-deficit hyperactivity disorder, where only response-locked P3b variability was elevated, suggesting that increased ISV may not signify the same underlying processes in all conditions with which it is associated.

Increased reaction time variability in attention-deficit hyperactivity disorder as a response-related phenomenon: evidence from single-trial event-related potentials.

BACKGROUND: Increased intra-subject variability (ISV) in reaction times (RTs) is a promising endophenotype for attention-deficit hyperactivity disorder (ADHD) and among the most robust hallmarks of the disorder. ISV has been assumed to represent an attentional deficit, either reflecting lapses in attention or increased neural noise. Here, we use an innovative single-trial event-related potential approach to assess whether the increased ISV associated with ADHD is indeed attributable to attention, or whether it is related to response-related processing. METHODS: We measured electroencephalographic responses to working memory oddball tasks in patients with ADHD (N = 20, aged 11.3 ± 1.1) and healthy controls (N = 25, aged 11.7 ± 1.1), and analysed these data with a recently developed method of single-trial event-related potential analysis. Estimates of component latency variability were computed for the stimulus-locked and response-locked forms of the P3b and the lateralised readiness potential (LRP). RESULTS: ADHD patients showed significantly increased ISV in behavioural ISV. This increased ISV was paralleled by an increase in variability in response-locked event-related potential latencies, while variability in stimulus-locked latencies was equivalent between groups. This result held across the P3b and LRP. Latency of all components predicted RTs on a single-trial basis, confirming that all were relevant for speed of processing. CONCLUSIONS: These data suggest that the increased ISV found in ADHD could be associated with response-end, rather than stimulus-end processes, in contrast to prevailing conceptions about the endophenotype. This mental chronometric approach may also be useful for exploring whether the existing lack of specificity of ISV to particular psychiatric conditions can be improved upon.

Identification of neuromotor deficits common to autism spectrum disorder and attention deficit/hyperactivity disorder, and imitation deficits specific to autism spectrum disorder.

Deficits in motor and imitation abilities are a core finding in autism spectrum disorders (ASD), but impaired motor functions are also found in attention deficit/hyperactivity disorder (ADHD). Given recent theorising about potential aetiological overlap between the two disorders, the present study aimed to assess difficulties in motor performance and imitation of facial movements and meaningless gestures in a sample of 24 ADHD patients, 22 patients with ASD, and 20 typically developing children, matched for age (6-13 years) and similar in IQ (>80). Furthermore, we explored the impact of comorbid ADHD symptoms on motor and imitation performance in the ASD sample and the interrelationships between the two groups of variables in the clinical groups separately. The results show motor dysfunction was common to both disorders, but imitation deficits were specific to ASD. Together with the pattern of interrelated motor and imitation abilities, which we found exclusively in the ASD group, our findings suggest complex phenotypic, and possibly aetiological, relationships between the two neurodevelopmental conditions.

Covid and the common good: In-group out-group dynamics and Covid-19 vaccination in Wales and the United States.

Vaccination is a social act, where benefits spill-over to third parties. How we approach such social decisions is influenced by whether likely beneficiaries share salient social identities with us. This study explores these dynamics using representative survey data from two contexts: national identity groups in Wales (N = 4187) and political partisans in America (N = 4864). In both cases, those in the minority in their local area were less likely to be vaccinated. In Wales, respondents who did not identify as Welsh were less likely to be vaccinated the greater the proportion of residents of their local area identified as Welsh. In America, the vaccination rate of Biden voters fell off more steeply than that of Trump voters as the proportion of Trump voters in their county increased. Results are robust to controlling for likely confounds and sensitivity analyses. In-group out-group dynamics help to shape important health decisions.

Mindfulness as a Potential Moderator Between Child Behavior Problems and Maternal Well-Being.

Mindfulness-based programs can help lower psychological distress among parents of children with developmental disabilities. However, less is known about the functions of mindfulness in relation to parental outcomes. In a cross-sectional survey, mothers of children with developmental disabilities (N = 313) reported on their child's behavior problems, trait mindfulness, mindful parenting, and a range of outcomes (anxiety and depression symptoms, parenting stress, family satisfaction, and positive gain). Neither trait mindfulness or mindful parenting acted as moderators between child behavior problems and outcome variables, although both had main effect (compensatory) associations with parent outcomes. Benefits of mindfulness-based programs may be general rather than specifically in the context of high child behavior problems, given the lack of evidence for the moderating function of mindfulness.

Covid and the coalfield: Covid-19 vaccine hesitance in Wales and Appalachia.

BACKGROUND: Vaccine hesitancy is a barrier to Covid-19 vaccine uptake and displays a social gradient, compounding health disparities. While social gradients are a vital concept in health, they flatten distinctions between types of disadvantaged community. This paper focuses on vaccine hesitance in post-industrial and de-industrialising coalfields. The social consequences of the decline of coal mining may present barriers to vaccine uptake. METHODS: We ran parallel surveys in Wales (N = 4187) and US states overlapping with central Appalachia (N = 4864), to examine whether vaccine attitudes and uptake varied between areas with different coal mining histories. These surveys were accompanied by qualitative interviews of 36 residents of these coalfields to explore vaccination decisions and triangulate with survey data. RESULTS: Factor analysis identified four axes of attitudes in the survey data: vaccine confidence, covid scepticism, vaccine individualism, and concerned confusion. These themes were echoed in the interviews. Vaccine confidence was lower; and covid scepticism, vaccine individualism, and concerned confusion higher, in residents of areas of Wales with greater mining extent and where pits closed during certain periods. Residents of former US coal counties had lower vaccine confidence and higher covid scepticism, while those in current coal counties had greater vaccine individualism and concerned confusion. In former US coal counties and Welsh areas where pits closed since 1980, vaccine uptake was lower. Differences could not be explained by respondents' income and education. In the interviews, norms of social solidarity were often invoked by vaccinated respondents, while unvaccinated respondents did not frame decisions in the context of the industrial history of their areas. DISCUSSION: The legacy of coal-mining's decline presents barriers to public health campaigns. We show evidence of this across two historically significant coalfields. Attention is needed to avert negative public health consequences of global energy transition.

Health and Mental Health Disparities Between National Identity Groups in Wales.

Despite close links with ethnic identity and other health-relevant identities, there is surprisingly little work on national identity in the context of population health. National identity is particularly important in multi-national states, where national identity is contested and where different nationalities often reflect both distinct ethnic groups and competing civic visions of national boundaries. The present study examines health disparities between national identity groups in Wales, a constituent nation of the UK. Using data from the National Survey for Wales (n = 23,303), latent class analysis was used to identify national identity groups in Wales. Generalised linear mixed-effects models were then fitted to the data to identity disparities between groups in terms of self-reported general and mental health, both unconditionally and conditionally on several socio-demographic and geographic variables. Analyses identified five groups: Anglophone Welsh, British, Cymry Cymraeg (Welsh-speaking Welsh), English and Ethnically Diverse. Striking health disparities were found, with the Cymry Cymraeg and Ethnically Diverse groups reporting better health than the other groups, especially the Anglophone Welsh and the English. These disparities could not be accounted for by differences in demographic, socio-economic or geographic factors.

Cross-level group density interactions on mental health for cultural, but not economic, components of social class.

BACKGROUND: The ethnic density literature links mental health to ethnic minority status in one's neighbourhood. This design has also been applied to other identities, including sexual and political minorities, but surprisingly little attention has been paid to class-based identities. We use a Bourdieusian framework to examine whether a 'class density' association with mental health can be found for economic capital - people's material assets - and cultural capital - symbolic signifiers of class, such as tastes and interests. METHODS: Multi-level regression with post-stratification was used to make area-level estimates of cultural and economic capital for each middle super output area in Wales, using data from the National Survey for Wales 2017/18 (N = 11,381). These estimates were linked to independent individual-level data from the 2018/19 edition of the same survey (N = 4058) and mixed effects models containing individual-level capital, area-level capital, and their interaction were fitted, predicting whether respondents reported a mental health problem. Models were fitted unadjusted, adjusted for age and gender; for the other form of capital on the individual and area levels; and for the other capital's cross-level interaction. Multiple imputation was used to account for missing data. RESULTS: For cultural capital, a cross-level interaction was found where area-level cultural capital was protective in respondents reporting higher levels of individual-level cultural capital, but a risk factor for mental health problems in those with low individual-level cultural capital (odds ratio = 0.83, CI95% = 0.75-0.93). No such relationship was found for economic capital (odds ratio = 0.96, CI95% = 0.88-1.06). These results remained robust in adjusted models. CONCLUSIONS: The presence of a class density association with mental health for cultural capital but not economic capital suggests that symbolic signifiers of class, more than material aspects, activate class identities and drive group density phenomena. There are implications for planning policy, mental health, and political leadership.

Social capital and geographical variation in the incidence of COVID-19: an ecological study.

BACKGROUND: The new behavioural norms needed to reduce the spread of COVID-19 are likely scaffolded by social capital. Research on social capital and COVID-19 has yielded mixed results, with some studies finding it to be protective while others identifying it as a risk factor. We examined the association between social capital and COVID-19 at a finer spatial scale than previous research, and examined changes in the relationship over the course of the pandemic. METHODS: Routine COVID-19 surveillance data from Wales were linked to estimates of social capital at a small area level. Generalised linear mixed effects models predicting COVID-19 case rates across areas using social capital estimates and possible confounding variables were fitted to the data. A moving window version of the analysis explored whether this relationship varied across time. RESULTS: Areas with higher levels of social capital had lower rates of COVID-19 (rate ratio for trust=0.94, 95% CI 0.92 to 0.96; rate ratio for belonging=0.94, 95% CI 0.92 to 0.96). These associations were strongest during periods of lockdown, with evidence that social capital was less protective, and potentially even a risk factor, during periods when restrictions were eased. Trust, but not belonging, remained protective after adjusting for deprivation, population density, ethnicity and proportion population aged over 65 years. CONCLUSIONS: Social capital is an important public health resource, which should be considered in future pandemic preparedness. Its importance may be greatest during times when social activity is most restricted.

Stroke Survivors Experience Elevated Levels of Loneliness: A Multi-Year Analysis of the National Survey for Wales.

OBJECTIVE: Despite clinical observation that stroke survivors frequently experience loneliness, there is no large-scale empirical evidence to support this observation. Therefore, the primary objective of this research was to provide the first large-scale and comprehensive estimate of loneliness in the stroke survivor population. METHOD: To address this issue, we completed two preregistered analyses of a nationally representative annual survey (N > 21,000). A two-phase approach was adopted combining both exploratory (Study 1) and confirmatory (Study 2) phases. The benefit of such an approach is that replication is built into the design, which considerably strengthens the inferences that can be made. RESULTS: Across two separate cohorts, the results consistently showed that human stroke survivors report higher levels of loneliness compared with healthy individuals, and this relationship could not be accounted for by demographic factors (e.g., age, sex) or objective measures of social isolation (e.g., marital status, number of household members). CONCLUSIONS: These findings demonstrate that elevated levels of loneliness poststroke are robust in that they replicate in large nationally representative samples and cannot be reduced to objective measures of social isolation. The work has clinical and societal relevance by suggesting that loneliness poststroke is unlikely to be adequately "treated" if only the quantity and not the quality of social experiences are considered.

Ecological social capital does not predict geographical variance in increases in depression following the 2008 financial crisis.

Research suggests that the financial crisis of 2008 and its aftermath were associated with an increase in mental health problems, but there has been little research into potential protective factors. Ecological social capital is a plausible candidate given evidence of its protective status following natural disasters. Pre-crisis area-level estimates of generalized trust and sense of belonging were computed from the 2004 to 2006 waves of the Living in Wales survey (N = 43,473) for 413 neighbourhoods in Wales, using multilevel regression with post-stratification, a technique for disaggregating survey data into small area estimates. Area estimates and the planned analysis were preregistered using Open Science Framework. Data (N = 180,462) from the Welsh Health Survey (2003-2015) were then used to model whether social capital was protective against depression in general and whether it moderated the increase in depression prevalence, associated with the financial crisis. Depression rates increased post-crisis (odds ratio [OR] = 1.271), and trust was a protective factor against depression (OR = 0.775). The hypothesized interaction, however, was not significant (OR = 1.033), nor was sense of belonging (OR = 0.934) or its interaction with the crisis (OR = 1.024). Although ecological generalized trust appears to be a protective factor against depression, it did not buffer against the mental health impact of the financial crisis.

Not belonging where others do: a cross-sectional analysis of multi-level social capital interactions on health and mental well-being in Wales.

BACKGROUND: Social capital may be a social good in health terms, but it is not necessarily a universal good. Several studies have shown that while there is a positive association between ecological social capital and health in people with high individual-level social capital, this relationship is weaker or even reversed in those with low individual-level social capital. Such studies, however, have used relatively coarse levels of geography for quantifying ecological social capital. The present study looks at this relationship at a more fine-grained spatial scale. METHODS: Data from the National Survey for Wales (n=27 828, weighted mean age=48.4) were linked to previously published small-area estimates (n=410) of ecological social capital for Wales. Mixed effects models were then used to assess whether the relationship between mental well-being and self-reported health on one hand, and ecological social capital (sense of belonging) on the other, was moderated by individual-level social capital. RESULTS: The models found the same moderation of the relationship that has been demonstrated previously: Although ecological social capital is positively associated with health in respondents with high individual-level social capital, the relationship is negative in those with low individual-level social capital. CONCLUSION: This study replicates this association at a spatial scale orders of magnitude more fine-grained than had been shown previously. Ecological social capital is not an unambiguously positive factor for public health, and may be a risk factor for marginalised people.

Mental health consequences of minority political positions: The case of brexit.

The group density effect, where a group member's psychiatric risk is associated with the proportion of the local population their group comprises, demonstrates the importance of minority group status to mental health. Previous research, focusing on ethnicity, has been correlational, but newly-formed identities provide opportunities for natural experiments, with greater scope for causal inference. This study examines whether such a group density effect can be found for the novel Brexit identities of 'leaver' and 'remainer' following the UK's divisive 2016 referendum on EU membership. Mixed effects models were fitted to the Understanding Society panel survey series (N = 25,555, 19,767 for analyses controlling for pre-referendum mental health data), predicting mental health as a function of individual opinion on EU membership and local referendum results. These interacted such that those holding the local majority opinion had better mental health (Odds ratio (OR):875 [0.766- 0.9995]), compared to those in the minority. This result survived adjustment for individual and area-level economic circumstances (OR:866 [0.758-0.989]), and, strikingly, pre-referendum mental health (OR: 0.841 [0.709-0.998]), as well as a number of other potential confounding variables. The results provide evidence for rapidly forming group density effects based on de novo identities, and suggest that identity may be a causal mechanism for group density effects more broadly. They also speak to the extent of polarisation in the Brexit-era UK, and its public health consequences.