Natural Experiments Research Articles

A guide for planning triangulation studies to investigate complex causal questions in behavioural and psychiatric research.

At the basis of many important research questions is causality - does X causally impact Y? For behavioural and psychiatric traits, answering such questions can be particularly challenging, as they are highly complex and multifactorial. 'Triangulation' refers to prospectively choosing, conducting and integrating several methods to investigate a specific causal question. If different methods, with different sources of bias, all indicate a causal effect, the finding is much less likely to be spurious. While triangulation can be a powerful approach, its interpretation differs across (sub)fields and there are no formal guidelines. Here, we aim to provide clarity and guidance around the process of triangulation for behavioural and psychiatric epidemiology, so that results of existing triangulation studies can be better interpreted, and new triangulation studies better designed. We first introduce the concept of triangulation and how it is applied in epidemiological investigations of behavioural and psychiatric traits. Next, we put forth a systematic step-by-step guide, that can be used to design a triangulation study (accompanied by a worked example). Finally, we provide important general recommendations for future studies. While the literature contains varying interpretations, triangulation generally refers to an investigation that assesses the robustness of a potential causal finding by explicitly combining different approaches. This may include multiple types of statistical methods, the same method applied in multiple samples, or multiple different measurements of the variable(s) of interest. In behavioural and psychiatric epidemiology, triangulation commonly includes prospective cohort studies, natural experiments and/or genetically informative designs (including the increasingly popular method of Mendelian randomization). The guide that we propose aids the planning and interpreting of triangulation by prompting crucial considerations. Broadly, its steps are as follows: determine your causal question, draw a directed acyclic graph, identify available resources and samples, identify suitable methodological approaches, further specify the causal question for each method, explicate the effects of potential biases and, pre-specify expected results. We illustrated the guide's use by considering the question: 'Does maternal tobacco smoking during pregnancy cause offspring depression?'. In the current era of big data, and with increasing (public) availability of large-scale datasets, triangulation will become increasingly relevant in identifying robust risk factors for adverse mental health outcomes. Our hope is that this review and guide will provide clarity and direction, as well as stimulate more researchers to apply triangulation to causal questions around behavioural and psychiatric traits.

Insights on Arc Magmatic Systems Drawn from Natural Melt Inclusions and Crystallization Experiments at PH2O =800 MPa under Oxidizing Conditions

Abstract Whole rock compositions at Buldir Volcano, western Aleutian arc, record a strong, continuous trend of iron depletion with decreasing MgO, classically interpreted as a calc-alkaline liquid line of descent. In contrast, olivine-hosted melt inclusions have higher total iron (FeO*) than whole rocks and show little change in FeO* with decreasing MgO. To investigate this discrepancy and determine the conditions required for strong iron depletion, we conducted oxygen fugacity (ƒO2) buffered, water-saturated crystallization experiments at 800 MPa and ƒO2 = QFM+1.6 ± 0.4 (1$\sigma$) (where QFM refers to the quartz-fayalite-magnetite buffer) on a high-Al, basaltic starting material modeled after a Buldir lava. Experimental conditions were informed by olivine-hosted melt inclusions that record minimum entrapment pressures as high as 570 MPa, >6 wt.% H2O, and ƒO2 of QFM+1.4 (±0.2), making Buldir one of the most oxidized and wettest arc volcanoes documented globally. The experiments produce melts with Si-enrichment and Fe-depletion signatures characteristic of evolved, calc-alkaline magmas at the lowest MgO, although FeO* remains roughly constant over most of the experimental temperature range. Experiments saturate CrAl-spinel and olivine at 1160oC, followed by clinopyroxene and Al-spinel at 1085oC, hornblende at 1060oC, and, finally, plagioclase and magnetite between 1040oC and 960oC. Hornblende crystallization, not magnetite, generates the largest increase in SiO2 and largest decrease in FeO* in coexisting melts. Compositions of melt inclusions are consistent with experimental melts and reflect crystallization of a basaltic parent magma at high PH2O. In contrast, the whole rock compositional trends are influenced by magma mixing and phenocryst redistribution and accumulation. The crystallization experiments and natural liquids (melt inclusions and groundmass glass) from Buldir suggest that for an oxidized, hydrous primary basalt starting composition, significant Fe-depletion from the melt will not occur until intermediate to late stages of magma crystallization (< ~4.5 wt.% MgO). We conclude that the Buldir whole rock trend cannot be reproduced by crystallization at arc-relevant oxygen fugacities and is not a true liquid line of descent, warranting caution when interpreting volcanic trends globally.

Complex Organics Surrounding the FU Ori–type Object V1057 Cyg Indicative of Sublimated Ices

FU Ori– and EX Lup–type objects present natural experiments for understanding a critical stage in the star and planet formation process. These objects offer insight into the diversity of molecules available to forming planetary systems due to a sudden increase in accretion, and central luminosity causes the disk and surrounding material to increase in temperature. This allows for volatiles to sublimate off of grains and exist in the gas phase for tens to hundreds of years after initial outburst. While this dynamic stage may be common for solar-type protostars, observations of the chemical impact of these bursts are rare. In this article, we present observations from the NOrthern Extended Millimeter Array of five young stellar objects (YSOs) that have undergone outbursts within the past 100 yr and catalog the volatile chemistry found within ∼1000 au of the YSO. Only one source clearly shows a line-rich spectra with >11 molecules detected, including complex organics and water, as is expected for a spectra signature for a postoutburst source. This source is V1057 Cyg, and we present it as the northern analog to the well-studied and molecule-rich FU Ori source, V883 Ori. Our conclusions on the chemical inventory of the other four sources in our sample are sensitivity limited, as V1057 Cyg contains the highest disk/envelope gas mass.

From Roe to Dobbs: 50 Years of Cause and Effect of US State Abortion Regulations.

The Roe era was hardly a monolith. For more than 50 years-beginning with abortion reforms in the 1960s and continuing through the Dobbs decision in 2022-state regulations of abortion were neither uniform nor consistent. States reformed and repealed abortion bans leading up to the Roe decision in 1973. Following Roe, they enacted both demand-side regulations of people seeking abortions and supply-side regulations of people providing abortions. The resulting laboratory of state policies affords natural experiments that have yielded evidence on the effects of abortion regulations on demographic, health, economic, and other social outcomes. I present a brief history of state policy variation from 1967 through 2016 and review the empirical scholarship studying its effects. This literature demonstrates that the liberalization of abortion access in the 1960s and 1970s allowed women greater control over their fertility, resulting in increased educational attainment and earnings. Subsequent state restrictions in the 1980s through 2010s had the opposite effect, particularly when they increased the financial and logistical costs of obtaining an abortion. I conclude with a discussion of implications for the post-Dobbs era, considering to what extent evidence from the past foretells the future.

Two tiers, not one: Different sources of extrinsic mortality have opposing effects on life history traits.

Guided by concepts from life history (LH) theory, a large human research literature has tested the hypothesis that exposures to extrinsic mortality (EM) promote the development of faster LH strategies (e.g., earlier/faster reproduction, higher offspring number). A competing model proposes that, because EM in the past was intimately linked to energetic constraints, such exposures specifically led to the development of slower LH strategies. We empirically address this debate by examining (1) LH variation among small-scale societies under different environmental conditions; (2) country-, regional- and community-level correlations between ecological conditions, mortality, maturational timing, and fertility; (3) individual-level correlations between this same set of factors; and (4) natural experiments leveraging the impact of externally-caused changes in mortality on LH traits. Partially supporting each model, we found that harsh conditions encompassing energetic stress and ambient cues to EM (external cues received through sensory systems) have countervailing effects on the development of LH strategies, both delaying pubertal maturation and promoting an accelerated pace of reproduction and higher offspring number. We conclude that, although energetics are fundamental to many developmental processes, providing a first tier of environmental influence, this first tier alone cannot explain these countervailing effects. An important second tier of environmental influence is afforded by ambient cues to EM. We advance a 2-tiered model that delineates this second tier and its central role in regulating development of LH strategies. Consideration of the first and second tier together is necessary to account for the observed countervailing shifts toward both slower and faster LH traits.

Luck in high-stakes firm lottery allocation: A natural experimental test of Coasean efficiency and invariance in a prominent sport finance setting

The NBA Player-Draft is a prominent sport-finance setting, with large rents allocated to teams. Several properties make this setting ideal to study the Coase Theorem in a cartelized-firm setting, where the Theorem’s empirical credence and overall relevance to real-world financial settings continue to be questioned. Identification of a high-stakes, salient setting featuring exogenously-assigned property rights has proven elusive. While several studies examine the Coasean implications of professional sports league drafts, we find draft rights are endogenously-assigned. We cultivate, verify, specify, and examine an exogenously-assigned draft right—draft lottery luck—for the 2000–2015 NBA Drafts. Given its revealed exogeneity, we utilize variation in annual lottery luck as a natural experiment to study the Coase Theorem and find robust evidence for “instantaneous” Coasean efficiency (as in efficiency of the efficient market hypothesis) but also evidence that Coasean invariance, with respect to firm profits, fails. Secondary analysis reveals respective mechanisms and sport-financial implications.

A comparative analysis of cross-sectional study and natural experiment in rail transit-travel behavior research: A case study in Wuhan, China

There has been a global increase in investment in rail transit, driven by its potential to enhance transportation efficiency, reduce air pollution, and stimulate economic growth. Both cross-sectional studies and natural experiments have contributed to the growing body of evidence supporting these claims. While natural experiments are commonly preferred for evaluating the impact of rail transit, cross-sectional studies remain popular due to their ease of data collection. However, there is a scarcity of studies that compare these two approaches using the same dataset to assess the robustness of cross-sectional studies. Using a two-wave panel dataset from Wuhan, China, this study used both cross-sectional and natural experimental analyses to examine the relationship between urban rail transit and travel behavior. The study attempted to enhance the credibility of the cross-sectional analysis by controlling for confounding variables and by combining it with the propensity score matching (PSM) method, respectively. The results revealed that the cross-sectional analyses could produce similar results, when setting a more stringent significance level. The findings suggested that well-designed cross-sectional studies can be reliable and represent a cost-effective alternative to resource-intensive natural experiments.

Testing Cloud Adjustment Hypotheses for the Maintenance of Earth's Hemispheric Albedo Symmetry With Natural Experiments

AbstractEarth's Northern and Southern Hemispheres reflect essentially equal amounts of sunlight. How—and whether—this hemispheric albedo symmetry is maintained remains a mystery. We decompose Earth's hemispheric albedo symmetry into components associated with the surface, clear‐sky atmosphere, and different cloud types as defined by cloud effective pressure and optical thickness. Climatologically, greater reflection by the surface, aerosols, and high clouds in the Northern Hemisphere is balanced by greater low and mid‐level cloudiness in the Southern Hemisphere. Both hemispheres have darkened at similar rates over the past two decades; whether the darkening from more rapidly declining aerosol in the Northern Hemisphere is causing a departure from all‐sky symmetry remains uncertain. Natural experiments including long‐term trends, sea ice loss, and volcanic eruptions provide strong evidence against the hypothesis that extratropical low clouds compensate changing clear‐sky asymmetries on annual‐to‐decadal timescales but some evidence that tropical high cloud shifts may do so.

Construction of a mathematical model of turbulent heat and mass transfer processes for the case of electron beam melting of titanium alloy casts

This paper describes a mathematical model built for turbulent heat and mass transfer processes in the case of electron beam melting of titanium alloy ingots. The object of research is the conditions that ensure the quality of ingots. The model makes it possible to calculate the distribution of hydrodynamic flows in the liquid metal and temperature fields in the ingot, to determine the profile of the metal crystallization front, taking into account the interphase transition zones. The model solves the problem of finding the necessary melting regimes of ingots by calculation, in contrast to high-cost natural experiments. The thermal and hydrodynamic processes during the melting of a cylindrical ingot with a diameter of 110 mm of the newest titanium alloy Ti-6Al-7Nb for medical use were calculated and its melting parameters were determined. The small diameter of the ingot significantly facilitates its further machining. The geometry of the two-phase zone of the liquidus-solidus transition, which determines the crystallization front of the metal, was calculated. The position and geometry of this front greatly affects the quality of ingot formation and the concentration of the distribution of alloying elements and the homogeneity of the metal across its volume. A sufficiently flat crystallization front has been obtained, under which the given conditions are ensured. It was found that heat transfer in the liquid phase of the metal is mainly caused by heat and mass transfer due to its movement, and heat and mass transfer significantly depends on the power of the electron beam and its distribution on the surface of the bath. According to the calculated regimes, at the Institute of Electric Welding named after E. O. Paton, the National Academy of Sciences of Ukraine, high-quality ingots for the needs of the medical industry were smelted. The castings are used for the manufacture of light and ultra-strong endoprostheses and implants, which are chemically neutral and biologically and biomechanically compatible with the human body and do not cause rejection.

The evolution of thermal performance curves in fungi farmed by attine ant mutualists in aboveground or belowground microclimates.

Fungi are abundant and ecologically important at a global scale, but little is known about whether their thermal adaptations are shaped by biochemical constraints (i.e. the Hotter is Better Model, HBM) or evolutionary tradeoffs (i.e., the Specialist Generalist Model, SGM). We tested these hypotheses by generating thermal performance curves (TPCs) of fungal cultivars farmed by six species of Panamanian fungus-farming 'attine' ants. These fungi represent evolutionary transitions in farming strategies as four cultivars are farmed by ants belowground at stable temperatures near 25°C and two cultivars are farmed aboveground at variable temperatures. We generated TPCs using a common garden experiment confining fungal isolates to different temperatures and then used a Bayesian hierarchical modeling approach to compare competing temperature sensitivity models. Some thermal performance traits differed consistently across farming strategies, with aboveground cultivars having: 1) higher tolerance to low temperatures (CTLmin) and 2) higher maximum growth rate at the optimal temperature (rmax). However, two core assumptions shared by the HBM or SGM were not supported as aboveground cultivars did not show systematic increases in either their optimal temperature (Topt) or thermal tolerance breadth. These results harness ant farming systems as long-term natural experiments to decouple the effects of environmental thermal variation and innate physiological temperature sensitivity on fungal thermal evolution. The results have clear implications for predicting climate warming induced breaking points in animal-microbe mutualisms.

Genome-Wide Molecular Adaptation in Algal Primary Productivity Induced by Prolonged Exposure to Environmentally Realistic Concentration of Nanoplastics.

Little information is known about the long-term effects of nanoplastics (NPs) in aquatic environments, especially under environmental-related scenarios. Herein, three differently charged NPs (nPS, nPS-NH2, and nPS-COOH) were exposed at an environmentally realistic concentration (10 μg/L) for 100 days to explore the variation of primary productivity (i.e., algae) in aquatic ecosystems. Our results demonstrated that the algae adapted to all three types of NPs by enhancing the algal number (by 10.34-16.52%), chlorophyll a (by 11.28-17.65%), and carbon-fixing enzyme activity (by 49.19-68.33%), which were further confirmed by the exposure results from natural water culturing experiments. Based on the algal chloroplast number and biovolume at the individual level, only nPS caused algal differentiation into two heterogeneous subpopulations (54.92 vs 45.08%), while nPS-NH2 and nPS-COOH did not cause the differentiation of the algal population. Moreover, the molecular adaptation mechanisms of algae to NPs were unraveled by integrating epigenomics and transcriptomics. Mean methylation rates of algae on exposure to nPS, nPS-NH2, and nPS-COOH were significantly elevated. In addition, the direction of gene expression regulation via differentially methylated regions associated with genes when exposed to nPS-COOH was distinct from those of nPS and nPS-NH2. Our results highlight the importance of assessing the long-term ecotoxicity of NPs and provide useful information for understanding the effect of NPs on aquatic ecosystems.

Interest rate liberalization and corporate innovation: Evidence from natural experiments in China

AbstractWe examine whether and how interest rate liberalization affects corporate innovation in China. Using the removal of the interest rate ceiling and floor as exogenous shocks, and the numbers of granted patents to proxy for corporate innovation, we find that corporate innovation increases after interest rate liberalization. Our finding holds through several robustness tests. Furthermore, we show that state‐owned enterprises (SOEs) and non‐SOEs benefit from interest rate liberalization differently: Non‐SOEs benefit more from interest rate ceiling removal, whereas SOEs benefit more from interest rate floor removal. Cross‐sectional analyses also show that the effect is more pronounced when bank competition is higher and when firms rely more on external finance. Exploring potential channels through which interest rate liberalization spurs innovation, we find that although ceiling removal mitigates firms’ financial constraints, floor removal lowers corporate loan costs. Our study provides evidence of how financial reform benefits the real economy.

Compression-based inference of network motif sets.

Physical and functional constraints on biological networks lead to complex topological patterns across multiple scales in their organization. A particular type of higher-order network feature that has received considerable interest is network motifs, defined as statistically regular subgraphs. These may implement fundamental logical and computational circuits and are referred to as "building blocks of complex networks". Their well-defined structures and small sizes also enable the testing of their functions in synthetic and natural biological experiments. Here, we develop a framework for motif mining based on lossless network compression using subgraph contractions. This provides an alternative definition of motif significance which allows us to compare different motifs and select the collectively most significant set of motifs as well as other prominent network features in terms of their combined compression of the network. Our approach inherently accounts for multiple testing and correlations between subgraphs and does not rely on a priori specification of an appropriate null model. It thus overcomes common problems in hypothesis testing-based motif analysis and guarantees robust statistical inference. We validate our methodology on numerical data and then apply it on synaptic-resolution biological neural networks, as a medium for comparative connectomics, by evaluating their respective compressibility and characterize their inferred circuit motifs.

Effects of simulated natural light brightness on visual perception in virtual reality forests: An eye-tracking study

Although natural light is essential for human health and well-being, the potential effects of different brightness levels of light on the visual perception of forests have not been discussed. Using controlled eye-tracking experiments, this study explored the impact of seven simulated natural brightness levels, from the darkest to the brightest, on the visual perception of 118 participants in a virtual reality (VR) forest environment. One of the most significant findings of this study was the marginal effect of natural light brightness levels on visual physiological stress in VR forests, with the pupillary unrest index decreasing significantly with the increase of natural light brightness; Secondly, the study analyzed the influence of natural light brightness levels in VR forests on the visual engagement behavior of eye tracking, that is, bright forest environments showed an overall trend associated with longer fixation duration, less saccade duration, and greater fixation direction ratio; Thirdly, in terms of visual perception evaluation, the study found an inverted U-shaped trend of mood, restorative perception, perceived safety, and natural light brightness level in VR forests. In addition, the study also found a significant correlation between visual physiological and psychological indicators in VR forest natural light visual perception experiments. These findings help us understand the visual perception effect of forest nature light exposure as a whole, and provided important clues and a basis for future research on improving the visual comfort of forest natural light.

Warming reduces trophic diversity in high-latitude food webs.

The physical effects of climate warming have been well documented, but the biological responses are far less well known, especially at the ecosystem level and at large (intercontinental) scales. Global warming over the next century is generally predicted to reduce food web complexity, but this is rarely tested empirically due to the dearth of studies isolating the effects of temperature on complex natural food webs. To overcome this obstacle, we used 'natural experiments' across 14 streams in Iceland and Russia, with natural warming of up to 20°C above the coldest stream in each high-latitude region, where anthropogenic warming is predicted to be especially rapid. Using biomass-weighted stable isotope data, we found that community isotopic divergence (a universal, taxon-free measure of trophic diversity) was consistently lower in warmer streams. We also found a clear shift towards greater assimilation of autochthonous carbon, which was driven by increasing dominance of herbivores but without a concomitant increase in algal stocks. Overall, our results support the prediction that higher temperatures will simplify high-latitude freshwater ecosystems and provide the first mechanistic glimpses of how warming alters energy transfer through food webs at intercontinental scales.

Call for papers: Generating stronger evidence to inform policy and practice: natural experiments on built environments, health behaviours and chronic diseases

Call for papers: Generating stronger evidence to inform policy and practice: natural experiments on built environments, health behaviours and chronic diseases

Elevational shifts in reproductive ecology indicate the climate response of a model chasmophyte, Rainer's bellflower (Campanula raineri).

Elevation gradients provide 'natural experiments' for investigating plant climate change responses, advantageous for the study of protected species and life forms for which transplantation experiments are illegal or unfeasible, such as chasmophytes with perennial rhizomes pervading rock fissures. Elevational climatic differences impact mountain plant reproductive traits (pollen and seed quality, sexual vs. vegetative investment) and pollinator community composition; we investigated the reproductive ecology of a model chasmophyte, Campanula raineri Perp. (Campanulaceae), throughout its current elevational/climatic range to understand where sub-optimal conditions jeopardise survival. We hypothesised that: 1) reproductive fitness measures are positively correlated with elevation, indicative of the relationship between fitness and climate; 2) C. raineri, like other campanulas, is pollinated mainly by Hymenoptera; 3) potential pollinators shift with elevation. We measured pollen and seed quality, seed production, the relative investment in sexual vs. vegetative structures and vegetative (Grime's CSR) strategies at different elevations. Potential pollinators were assessed by combining molecular and morphological identification. Whereas CSR strategies were not linked to elevation, pollen and seed quality were positively correlated, as was seed production per fruit (Hypothesis 1 is supported). The main pollinators of C. raineri were Apidae, Andrenidae, Halictidae (Hymenoptera) and Syrphidae (Diptera), probably complemented by a range of occasional pollinators and visitors (Hypothesis 2 partially supported). Potential pollinator communities showed a taxonomic shift towards Diptera with elevation (particularly Anthomyiidae and Muscidae) and away from Hymenoptera (Hypothesis 3 was supported). Pollinator availability is maintained at all elevations by taxon replacement. However, reduced pollen quality and seed production at lower elevations suggest an impact of climate change on reproduction (especially <1200 m a.s.l., where seed germination was limited). Aside from guiding targeted conservation actions for C. raineri, our results highlight problems that may be common to mountain chasmophytes worldwide.

State Schooling Policies and Cognitive Performance Trajectories: A Natural Experiment in a National US Cohort of Black and White Adults.

Education is strongly associated with cognitive outcomes at older ages, yet the extent to which these associations reflect causal effects remains uncertain due to potential confounding. Leveraging changes in historical measures of state-level education policies as natural experiments, we estimated the effects of educational attainment on cognitive performance over 10 years in 20,248 non-Hispanic Black and non-Hispanic White participants, aged 45+ in the REasons for Geographic and Racial Disparities in Stroke (REGARDS) cohort (2003-2020) by (1) using state- and year- specific compulsory schooling laws, school-term length, attendance rate, and student-teacher ratio policies to predict educational attainment for US Census microsample data from 1980 and 1990, and (2) applying policy-predicted years of education (PPYEd) to predict memory, verbal fluency, and a cognitive composite. We estimated overall and race- and sex-specific effects of PPYEd on level and change in each cognitive outcome using random intercept and slope models, adjusting for age, year of first cognitive assessment, and indicators for state of residence at age 6. Each year of PPYEd was associated with higher baseline cognition (0.11 standard deviation [SD] increase in composite measure for each year of PPYEd, 95% confidence interval [CI]: 0.07, 0.15). Subanalyses focusing on individual cognitive domains estimate the largest effects of PPYEd on memory. PPYEd was not associated with rate of change in cognitive scores. Estimates were similar across Black and White participants and across sex. Historical policies shaping educational attainment are associated with better later life memory, a major determinant of dementia risk.

When market access gains do not spur urban growth

Abstract We revisit the natural experiments of division and unification of Germany. The negative shock to local market access following the division of Germany led to a fast and strong downward adjustment of the size of West German cities near the new border. In contrast, the positive shock of reunification did not lead to any change in their relative size. Even three decades after reunification, no convergence can be observed. We show that local subsidies to East Germany could have contributed to this asymmetry in time.

Terrorism, perpetrators and polarization: evidence from natural experiments

Terrorism, perpetrators and polarization: evidence from natural experiments