Young people's views on the acceptability and feasibility of loneliness interventions for their age group.

BACKGROUND: Loneliness is a frequently reported problem for young people aged 16 to 24 years old. A variety of interventions have been developed (but in most cases not extensively evaluated) to try and tackle loneliness in this age group. These include interpersonal, intrapersonal, and social approaches that vary in their content and mechanisms of action. The current study aimed to qualitatively examine young peoples' views on the acceptability and feasibility of different loneliness interventions. METHODS: Young people from 16 to 24 years old living in the United Kingdom who self-identified as having experienced loneliness were recruited to participate in this study. We conducted semi-structured qualitative interviews to assess their views on the acceptability and feasibility of loneliness interventions for their age group. Interviews were analysed using inductive reflexive thematic analysis. RESULTS: Our analysis of 23 individual interview transcripts identified six themes. These related to the appropriate stage for intervention and how different types of strategies may be best suited to particular contexts; the key facilitators and barriers to engaging young people in an intervention; considerations for optimising the delivery of an intervention; divergent views on technology use in strategies to manage loneliness; the scope of an intervention and whether it takes a targeted or general approach; and the idea of combining different options within an intervention to allow tailoring to individual preferences and nature of loneliness. CONCLUSIONS: These findings demonstrate the need for continued development of individualised interventions designed to help manage loneliness in this age group. Future loneliness strategies should be co-produced with young people to ensure that they suit the varying needs of this population.

The prevalence and severity of loneliness and deficits in perceived social support among who have received a 'personality disorder' diagnosis or have relevant traits: a systematic review.

BACKGROUND: Loneliness and struggles with unmet social needs are a common experience among people with 'personality disorder' diagnoses/traits. Given the impact of loneliness and poor perceived social support on mental health, and the importance of a sense of belonging for recovery, a systematic review examining the prevalence/severity of loneliness and deficits in perceived social support among people with 'personality disorder' diagnoses/traits is an essential step towards developing an intervention targeting the social needs of people with diagnoses/traits 'personality disorder'. Despite an extensive literature on loneliness and deficits of perceived social support among people with 'personality disorder' diagnosis/traits, to date there has been no systematic review of this evidence. METHOD: We conducted a systematic review synthesising quantitative data on the prevalence/severity of loneliness and deficits of perceived social support among people with diagnoses/traits of 'personality disorder' in comparison with other clinical groups and the general population. We searched Medline, Embase, PsycINFO, Web of Social Science, Google scholar and Ethos British Library from inception to December 2021. We conducted quality appraisals using the Joanna Briggs Critical appraisal tools and rated the certainty of evidence using the Grading of Recommendation, Assessment, Development and Evaluation approach. A narrative synthesis was used describing the direction and strength of associations prioritising high quality studies. FINDINGS: A final set of 70 studies are included in this review, most of which are cross-sectional studies (n = 55), based in the United States (51%) and focused on community samples. Our synthesis of evidence found that, across all types of 'personality disorders' (except 'narcissistic personality' traits), people with traits associated with 'personality disorder' or meeting criteria for a diagnosis of 'personality disorder', have higher levels of loneliness, lower perceived relationship satisfaction, and poorer social support than the general population or other clinical samples. CONCLUSION: The quality of evidence is judged as low quality. However, given the distressing nature of loneliness and the known negative effects of loneliness on mental health and recovery, it is important for future research to explore mechanisms by which loneliness may exacerbate 'personality disorder' symptoms and the impact this has on recovery.

'Just snap out of it' - the experience of loneliness in women with perinatal depression: a Meta-synthesis of qualitative studies.

BACKGROUND: Pregnancy and the arrival of a new baby is a time of great transition and upheaval. Women often experience social isolation and loneliness at this time and may develop depression, particularly in the postnatal period. Qualitative studies have reported that loneliness is also a feature of perinatal depression. However, until now there has been no attempt to synthesise research exploring the links between loneliness and perinatal depression. This study's aim was to explore existing qualitative evidence to answer two research questions: What are the experiences of loneliness for women with perinatal depression? What helps and what makes loneliness worse for women with perinatal depression? METHODS: A qualitative meta-synthesis retrieved primary qualitative studies relevant to the research questions. Four electronic databases were systematically searched (Ovid MEDLINE®; PsycINFO; Embase; Web of Science). Papers were screened according to pre-defined inclusion criteria and assigned a quality score. Thematic analysis was used to identify major overarching themes in the literature. RESULTS: Twenty-seven relevant qualitative studies were included. Themes relating to the interaction between perinatal depression and loneliness included self-isolation and hiding symptoms due to stigma of perinatal depression and fear of judgement as a 'bad mother'; a sudden sense of emotional disconnection after birth; and a mismatch between expected and actual support provided by partner, family and community. There was also a double burden of loneliness for women from disadvantaged communities, due to increased stigma and decreased social support. Validation and understanding from healthcare professionals, peer support from other mothers with experience of perinatal depression, and practical and emotional family support were all important factors that could ameliorate loneliness. CONCLUSIONS: Loneliness appears to play a central role in the experience of perinatal depression based on the frequency with which it emerged in women's accounts. The findings provide a foundation for the development of further theories about the role of loneliness in perinatal depression and evidence in which future psychological and social intervention design processes can be rooted. Addressing stigma and offering culturally appropriate professional and peer support are potential targets for interventions that could help women with perinatal depression, particularly in disadvantaged communities, feel less lonely. TRIAL REGISTRATION: Prospero registration: https://www.crd.york.ac.uk/prospero/display_record.php? RecordID = 251,936.

Associations between constructs related to social relationships and mental health conditions and symptoms: an umbrella review.

BACKGROUND: Loneliness and social isolation are increasingly recognised as prevalent among people with mental health problems, and as potential targets for interventions to improve quality of life and outcomes, as well as for preventive strategies. Understanding the relationship between quality and quantity of social relationships and a range of mental health conditions is a helpful step towards development of such interventions. PURPOSE: Our aim was to give an overview of associations between constructs related to social relationships (including loneliness and social isolation) and diagnosed mental conditions and mental health symptoms, as reported in systematic reviews of observational studies. METHODS: For this umbrella review (systematic review of systematic reviews) we searched five databases (PsycINFO, MEDLINE, EMBASE, CINAHL, Web of Science) and relevant online resources (PROSPERO, Campbell Collaboration, Joanna Briggs Institute Evidence Synthesis Journal). We included systematic reviews of studies of associations between constructs related to social relationships and mental health diagnoses or psychiatric symptom severity, in clinical or general population samples. We also included reviews of general population studies investigating the relationship between loneliness and risk of onset of mental health problems. RESULTS: We identified 53 relevant systematic reviews, including them in a narrative synthesis. We found evidence regarding associations between (i) loneliness, social isolation, social support, social network size and composition, and individual-level social capital and (ii) diagnoses of mental health conditions and severity of various mental health symptoms. Depression (including post-natal) and psychosis were most often reported on, with few systematic reviews on eating disorders or post-traumatic stress disorder (PTSD), and only four related to anxiety. Social support was the most commonly included social construct. Our findings were limited by low quality of reviews and their inclusion of mainly cross-sectional evidence. CONCLUSION: Good quality evidence is needed on a wider range of social constructs, on conditions other than depression, and on longitudinal relationships between social constructs and mental health symptoms and conditions.

The experience of loneliness among people with a "personality disorder" diagnosis or traits: a qualitative meta-synthesis.

BACKGROUND: Loneliness is prevalent among people with a "personality disorder" diagnosis or who have related personality traits, but the experience of loneliness among people with "personality disorder" diagnoses/traits has not been well described. A qualitative approach has potential to help understand the experience of loneliness among people with "personality disorder" diagnoses/traits, and to develop interventions that promote recovery. We therefore aimed to synthesise the qualitative literature relevant to this topic. METHOD: We conducted a meta-synthesis of qualitative studies exploring the subjective experience of loneliness as reported by people with "personality disorder" diagnoses/traits. We searched four databases using pre-formulated search terms, selected eligible articles, appraised the quality of each, and analyzed data from eligible studies using thematic synthesis. RESULT: We identified 39 articles that described the experience of loneliness in people with "personality disorder" diagnoses/traits. From extracted data, we identified seven themes: (1) disconnection and emptiness: a "haunting alienation", (2) alienation arising from childhood experiences, (3) thwarted desire for closeness and connection, (4) paradox: for both closeness and distance, (5) experiences of existential loneliness, (6) recovery, embedded in a social world, and (7) group therapy: a setback. Our results suggest that for our sample early alienating and traumatic experiences may pave the way for experiences of loneliness, which further exacerbate "personality disorder" symptoms and distress. CONCLUSION: Despite describing a need to belong and efforts to cope with unmet social needs, people with "personality disorder" diagnoses/traits (particularly "emotionally unstable personality disorder") report experiencing an intense disconnection from other people. This seems rooted in early adversities, reinforced by later traumatic experiences. Given the apparent salience of loneliness to people with "personality disorder" diagnoses/traits, interventions focused on helping people connect with others, which may include both psychological and social components, have potential to be beneficial in reducing loneliness and promoting recovery.

Digital interventions for subjective and objective social isolation among individuals with mental health conditions: a scoping review.

BACKGROUND: Social isolation encompasses subjective and objective concepts. Both are associated with negative health consequences and are more prevalent among people with mental health problems than among the general population. To alleviate social isolation, digital interventions have potential as accessible alternatives or adjuncts to face-to-face interventions. This scoping review aimed to describe the types of digital interventions evaluated for feasibility, acceptability and effectiveness in alleviating social isolation among individuals with mental health problems, and to present an overview of the quantitative evidence yielded to inform future intervention design. METHODS: We searched five electronic databases for quantitative and mixed methods studies published between January 2000 and July 2020. Studies were included if they evaluated digital interventions for individuals with mental health conditions, had subjective and/or objective social isolation as their primary outcome, or as one of their outcomes if no primary outcome was specified. Feasibility studies were included if feasibility outcomes were the primary outcomes and social isolation was among their secondary outcomes. A narrative synthesis was conducted to present our findings. The protocol was registered on Open Science Framework (doi: https://doi.org/10.17605/OSF.IO/CNX8A ). RESULTS: Thirty-two studies were included for our review: 16 feasibility studies, seven single-group studies and nine effectiveness trials. There was great variation in the interventions, study designs and sample populations. Interventions included web-based programmes, phone-based programmes, blended interventions, socially assistive robots and virtual reality interventions. Many were feasibility studies, or otherwise not fully powered to detect an effect if one were present, thus preventing clear conclusions about clinical effectiveness. Satisfactory feasibility outcomes indicated potential for future trials to assess these interventions. CONCLUSION: Our scoping review identified a range of digital approaches utilized to alleviate social isolation among individuals with mental health disorders. Conclusions regarding clinical effectiveness cannot be reached due to variability of approaches and lack of large-scale randomized controlled trials. To make clear recommendations for digital social isolation interventions, future research needs to be based on rigorous methods and larger samples. Future studies should also focus on utilizing theory-driven approaches and improving existing approaches to advance the field.

Loneliness and the onset of new mental health problems in the general population.

PURPOSE: Loneliness is associated with poor health including premature mortality. There are cross-sectional associations with depression, anxiety, psychosis, and other mental health outcomes. However, it is not known whether loneliness is causally linked with the new onset of mental health problems in the general population. Longitudinal studies are key to understanding this relationship. We synthesized evidence from longitudinal studies investigating the relationship between loneliness and new onset of mental health problems, in the general population. METHOD: We systematically searched six electronic databases, unpublished sources, and hand-searched references, up to August 2021. We conducted a meta-analysis of eight independent cohorts and narrative synthesis of the remaining studies. RESULTS: We included 32 studies, of which the majority focused on depression. Our narrative synthesis found most studies show loneliness at baseline which is associated with the subsequent new onset of depression. The few studies on anxiety and self-harm also showed a positive association. Our meta-analysis found a pooled adjusted odds ratio of 2.33 (95% CI 1.62-3.34) for risk of new onset depression in adults who were often lonely compared with people who were not often lonely. This should be interpreted with caution given evidence of heterogeneity. CONCLUSION: Loneliness is a public mental health issue. There is growing evidence it is associated with the onset of depression and other common mental health problems. Future studies should explore its impact across the age range and in more diverse populations, look beyond depression, and explore the mechanisms involved with a view to better informing appropriate interventions.

What has changed in the experiences of people with mental health problems during the COVID-19 pandemic: a coproduced, qualitative interview study.

PURPOSE: We sought to understand how the experiences of people in the UK with pre-existing mental health conditions had developed during the course of the COVID-19 pandemic. METHODS: In September-October 2020, we interviewed adults with mental health conditions pre-dating the pandemic, whom we had previously interviewed 3 months earlier. Participants had been recruited through online advertising and voluntary sector community organisations. Semi-structured qualitative interviews were conducted by telephone or video-conference by researchers with lived experience of mental health difficulties, and, following principles of thematic analysis, were analysed to explore changes over time in people's experience of the pandemic. RESULTS: We interviewed 44 people, achieving diversity of demographic characteristics (73% female, 54% White British, aged 18-75) and a range of mental health conditions and service use among our sample. Three overarching themes were derived from interviews. The first theme "spectrum of adaptation" describes how participants reacted to reduced access to formal and informal support through personal coping responses or seeking new sources of help, with varying degrees of success. The second theme describes "accumulating pressures" from pandemic-related anxieties and sustained disruption to social contact and support, and to mental health treatment. The third theme "feeling overlooked" reflects participants' feeling of people with mental health conditions being ignored during the pandemic by policy-makers at all levels, which was compounded for people from ethnic minority communities or with physical health problems. CONCLUSION: In line with previous research, our study highlights the need to support marginalised groups who are at risk of increased inequalities, and to maintain crucial mental and physical healthcare and social care for people with existing mental health conditions, notwithstanding challenges of the pandemic.

The experience of loneliness among young people with depression: a qualitative meta-synthesis of the literature.

BACKGROUND: Young people have a higher prevalence of loneliness than other age groups, and they are also at risk of depression. Quantitative studies describe a bidirectional association between loneliness and depression, but there is limited understanding of how these influence each other. Little is known about the experience of loneliness among young people with depression. Qualitative approaches may help understand the relationship between loneliness and depression among young people, and how to intervene to improve outcomes. We aimed to conduct a meta-synthesis to understand the complex inter-relationship between loneliness and depression among young depressed people by synthesising evidence from a systematic review of qualitative studies. METHODS: We conducted a meta-synthesis of qualitative studies capturing experiences of loneliness among young people with depression. We systematically searched six electronic databases for selected search terms, critically appraised eligible studies, and analysed the data from included studies using the approach of thematic synthesis. We used feedback from an inter-disciplinary research workshop to improve reflexivity. RESULTS: Our inclusion criteria identified fourteen studies. Our analysis identified four themes: (1) social withdrawal due to poor mental health, (2) non-disclosure of depression contributing to social distance, (3) the desire to connect, and (4) paradoxes of loneliness and depression. These themes illustrated a range of pathways between depression and loneliness, and a sense of how these might be mutually reinforcing. Our findings suggest that where depressed individuals engage in certain behaviours (withdrawing; not confiding) for a range of reasons, this can lead to feelings of loneliness, an awareness of which worsens their mood, thus perpetuating their depression. CONCLUSIONS: Young people with depression experience loneliness as an insurmountable distance between themselves and others. Our findings identified non-disclosure of depression, and the debilitating nature of the depressive symptomatology, as factors perpetuating a vicious cycle of loneliness and depression. They suggest that approaches to tackling the problem might include helping young people communicate about their depression to trusted friends and educating their social networks in how to support them. The wider research literature suggests that cognitive interventions may have a role in shifting maladaptive cognitions about their social world.

Variation in the ß-endorphin, oxytocin, and dopamine receptor genes is associated with different dimensions of human sociality.

There is growing evidence that the number and quality of social relationships have substantial impacts on health, well-being, and longevity, and, at least in animals, on reproductive fitness. Although it is widely recognized that these outcomes are mediated by a number of neuropeptides, the roles these play remain debated. We suggest that an overemphasis on one neuropeptide (oxytocin), combined with a failure to distinguish between different social domains, has obscured the complexity involved. We use variation in 33 SNPs for the receptor genes for six well-known social neuropeptides in relation to three separate domains of sociality (social disposition, dyadic relationships, and social networks) to show that three neuropeptides (ß-endorphin, oxytocin, and dopamine) play particularly important roles, with each being associated predominantly with a different social domain. However, endorphins and dopamine have a much wider compass than oxytocin (whose effects are confined to romantic/reproductive relationships and often do not survive control for other neuropeptides). In contrast, vasopressin, serotonin, and testosterone play only limited roles.

Associations between neurochemical receptor genes, 2D:4D, impulsivity and relationship quality.

The ratio between the second and fourth digits (2D:4D) has been widely used as a proxy for fetal exposure to androgens and has been linked to a number of sociosexual traits in humans. However, the role of genes in this equation remains unknown. Here (N = 474), we test, firstly, for associations between 2D:4D and single-nucleotide polymorphisms (SNPs) in nine neurochemical receptor genes (AR, OXTR, AVPR1A, OPRM1, DRD1/2, ANKK1, 5HTR1A/2A), and secondly, whether digit ratios mediate the relationship between genetic variation and sociosexuality. We demonstrate significant associations between AR, OPRM1 and AVPR1A and 2D:4D. Moreover, mediation analysis indicates that, in women, AR and OPRM1 variation drives digit ratios, which are related positively to impulsivity and, for OPRM1, negatively to romantic relationship quality. Although these findings are subject to multiple testing issues, this study provides preliminary evidence that in women genetic factors may affect both impulsivity and perceived relationship quality through influencing factors indexed by digit ratios.

Group music performance causes elevated pain thresholds and social bonding in small and large groups of singers.

Over our evolutionary history, humans have faced the problem of how to create and maintain social bonds in progressively larger groups compared to those of our primate ancestors. Evidence from historical and anthropological records suggests that group music-making might act as a mechanism by which this large-scale social bonding could occur. While previous research has shown effects of music making on social bonds in small group contexts, the question of whether this effect 'scales up' to larger groups is particularly important when considering the potential role of music for large-scale social bonding. The current study recruited individuals from a community choir that met in both small (n = 20 - 80) and large (a 'megachoir' combining individuals from the smaller subchoirs n = 232) group contexts. Participants gave self-report measures (via a survey) of social bonding and had pain threshold measurements taken (as a proxy for endorphin release) before and after 90 minutes of singing. Results showed that feelings of inclusion, connectivity, positive affect, and measures of endorphin release all increased across singing rehearsals and that the influence of group singing was comparable for pain thresholds in the large versus small group context. Levels of social closeness were found to be greater at pre- and post-levels for the small choir condition. However, the large choir condition experienced a greater change in social closeness as compared to the small condition. The finding that singing together fosters social closeness - even in large contexts where individuals are not known to each other - is consistent with evolutionary accounts that emphasize the role of music in social bonding, particularly in the context of creating larger cohesive groups than other primates are able to manage.

New insights into differences in brain organization between Neanderthals and anatomically modern humans.

Previous research has identified morphological differences between the brains of Neanderthals and anatomically modern humans (AMHs). However, studies using endocasts or the cranium itself are limited to investigating external surface features and the overall size and shape of the brain. A complementary approach uses comparative primate data to estimate the size of internal brain areas. Previous attempts to do this have generally assumed that identical total brain volumes imply identical internal organization. Here, we argue that, in the case of Neanderthals and AMHs, differences in the size of the body and visual system imply differences in organization between the same-sized brains of these two taxa. We show that Neanderthals had significantly larger visual systems than contemporary AMHs (indexed by orbital volume) and that when this, along with their greater body mass, is taken into account, Neanderthals have significantly smaller adjusted endocranial capacities than contemporary AMHs. We discuss possible implications of differing brain organization in terms of social cognition, and consider these in the context of differing abilities to cope with fluctuating resources and cultural maintenance.

Is orbital volume associated with eyeball and visual cortex volume in humans?

BACKGROUND: In humans orbital volume increases linearly with absolute latitude. Scaling across mammals between visual system components suggests that these larger orbits should translate into larger eyes and visual cortices in high latitude humans. Larger eyes at high latitudes may be required to maintain adequate visual acuity and enhance visual sensitivity under lower light levels. AIM: To test the assumption that orbital volume can accurately index eyeball and visual cortex volumes specifically in humans. SUBJECTS AND METHODS: Structural Magnetic Resonance Imaging (MRI) techniques are employed to measure eye and orbit (n = 88) and brain and visual cortex (n = 99) volumes in living humans. Facial dimensions and foramen magnum area (a proxy for body mass) were also measured. RESULTS: A significant positive linear relationship was found between (i) orbital and eyeball volumes, (ii) eyeball and visual cortex grey matter volumes and (iii) different visual cortical areas, independently of overall brain volume. CONCLUSION: In humans the components of the visual system scale from orbit to eye to visual cortex volume independently of overall brain size. These findings indicate that orbit volume can index eye and visual cortex volume in humans, suggesting that larger high latitude orbits do translate into larger visual cortices.

Exploring the experiences of loneliness in adults with mental health problems: A participatory qualitative interview study.

BACKGROUND: Loneliness is associated with many mental health conditions, as both a potential causal and an exacerbating factor. Richer evidence about how people with mental health problems experience loneliness, and about what makes it more or less severe, is needed to underpin research on strategies to help address loneliness. METHODS: Our aim was to explore experiences of loneliness, as well as what helps address it, among a diverse sample of adults living with mental health problems in the UK. We recruited purposively via online networks and community organisations, with most interviews conducted during the COVID-19 pandemic. Qualitative semi-structured interviews were conducted with 59 consenting participants face-to-face, by video call or telephone. Researchers with relevant lived experience were involved at all stages, including design, data collection, analysis and writing up of results. FINDINGS: Analysis led to identification of four overarching themes: 1. What the word "lonely" meant to participants, 2. Connections between loneliness and mental health, 3. Contributory factors to continuing loneliness, 4. Ways of reducing loneliness. Central aspects of loneliness were lack of meaningful connections with others and lack of a sense of belonging to valued groups and communities. Some drivers of loneliness, such as losses and transitions, were universal, but specific links were also made between living with mental health problems and being lonely. These included direct effects of mental health symptoms, the need to withdraw to cope with mental health problems, and impacts of stigma and poverty. CONCLUSIONS: The multiplicity of contributors to loneliness that we identified, and of potential strategies for reducing it, suggest that a variety of approaches are relevant to reducing loneliness among people with mental health problems, including peer support and supported self-help, psychological and social interventions, and strategies to facilitate change at community and societal levels. The views and experiences of adults living with mental health problems are a rich source for understanding why loneliness is frequent in this context and what may address it. Co-produced approaches to developing and testing approaches to loneliness interventions can draw on this experiential knowledge.

Social laughter is correlated with an elevated pain threshold.

Although laughter forms an important part of human non-verbal communication, it has received rather less attention than it deserves in both the experimental and the observational literatures. Relaxed social (Duchenne) laughter is associated with feelings of wellbeing and heightened affect, a proximate explanation for which might be the release of endorphins. We tested this hypothesis in a series of six experimental studies in both the laboratory (watching videos) and naturalistic contexts (watching stage performances), using change in pain threshold as an assay for endorphin release. The results show that pain thresholds are significantly higher after laughter than in the control condition. This pain-tolerance effect is due to laughter itself and not simply due to a change in positive affect. We suggest that laughter, through an endorphin-mediated opiate effect, may play a crucial role in social bonding.

Fission-fusion and the evolution of hominin social systems.

The course of hominin evolution has involved successive migrations towards higher absolute latitudes over the past three million years. Poorer habitat quality further from the equator has led to the necessity for groups occupying higher latitudes to live at lower population densities. Coupled with a trend towards increasing group size over this time period, this tendency towards expansion has led to exponential increases in the area requirements of hominin groups, and a concomitant need to adjust foraging patterns. The current analyses suggest that the development of increasingly complex, multi-level fission-fusion social systems could have freed hominins of the foraging constraints imposed by large group sizes and low population densities. Analyses of the fossil record suggest latitudinally-driven differences in area requirements of the australopithecines from East and South Africa, and African and Asian Homo erectus. In contrast, chronologically-driven differences appear between H. erectus as a whole and Homo heidelbergensis, and between H. heidelbergensis and the Neanderthals. These results are discussed in relation to studies of the foraging patterns of primates and hunter-gatherers.

Latitudinal variation in light levels drives human visual system size.

Ambient light levels influence visual system size in birds and primates. Here, we argue that the same is true for humans. Light levels, in terms of both the amount of light hitting the Earth's surface and day length, decrease with increasing latitude. We demonstrate a significant positive relationship between absolute latitude and human orbital volume, an index of eyeball size. Owing to tight scaling between visual system components, this will translate into enlarged visual cortices at higher latitudes. We also show that visual acuity measured under full-daylight conditions is constant across latitudes, indicating that selection for larger visual systems has mitigated the effect of reduced ambient light levels. This provides, to our knowledge, the first support that light levels drive intraspecific variation in visual system size in the human population.