Distribution-Based Causal Inference : A Review and Practical Guidance for Epidemiologists

Peer reviewe

Temporal and population dynamics of depressive symptoms : empirical and modeling approaches

Depression has been estimated to be the second largest cause of years lived with a disability, and much research on depressive symptoms exists. Despite this, basic research has not found natural taxa that would correspond to clinical diagnoses for depression. It is often assumed that a one-dimensional latent continuum underlies depressive symptoms, but empirical evidence does not support this idea either. Therefore, it has been suggested that depressive symptoms are part of a complex causal network that has not yet been adequately understood. This thesis aims to understand statistical variation and joint variation of individual depressive symptoms over time, the causal relationships between these symptoms, and their potentially adaptive evolutionary origins. The research material consists of the prospective Young Finns study that began in 1980 and included 3596 participants, the 10317-participant Wisconsin Longitudinal study, and mathematical models for the evolution of cooperation. First, temporal trajectories of a depressive-symptom sum was modeled with a stochastic differential equation model which results in a more empirically justified approach than typical latent-trait models, allowing causal roles for individual symptoms rather than viewing them as passive reflections. Regarding individual symptoms, it was shown that body-image dissatisfaction was the most temporally stable symptom, and strongly associated with chronically elevated dysphoria over a 16 year follow-up. In contrast, symptoms related to sleep and tiredness were the least stable, and novel methods based on non-Gaussian distributions suggested that sleep problems cause other depressive symptoms. Finally, combining the bargaining models of depression with mathematical models for the evolution of cooperation showed that, in theory, evolution should favor the emergence of depressive symptoms in natural populations, as they promote fitness-enhancing cooperation by rendering defection from joint enterprises less tempting. Overall, instead of a single disorder, depressive symptoms may reflect multiple processes, some of them being adaptive instead of dysfunctional.Masennus on arvioitu toisiksi suurimmaksi toimintakyvyttömänä elettyjen elinvuosien aiheuttajaksi maailmassa, ja masennusoireita on tutkittu paljon. Tästä huolimatta perustutkimus ei ole löytänyt kliinistä diagnostiikkaa vastaavaa luonnollista rajaa masentuneiden ja ei-masentuneiden välille. Yksiulotteinen jatkumo usein oletetaan masennusoireiden taustavaikuttajaksi, mutta tätäkään ajatusta uusin tutkimus ei tue. Oireiden ajatellaan olevan osa huonosti tunnettua syy- ja seuraussuhteiden verkostoa. Tässä väitöskirjassa pyritään ymmärtämään yksittäisten masennusoireiden tilastollista vaihtelua ja yhteisvaihtelua ajassa, oireiden kausaaliyhteyksiä, ja niiden mahdollisia evoluution kannalta adaptiivisia ominaisuuksia. Tutkimusmateriaalina toimivat 1980-luvulta asti seurattu 3596:n suomalaisen havaintoaineisto, Lasten Sepelvaltimotaudin Riskitekijät -tutkimus, 10317 amerikkalaisen havaintoaineisto, Wisconsinin pitkättäistutkimus, sekä yhteistyön evoluution matemaattiset mallit. Usein tutkittua masennusoiresummaa tarkasteltiin stokastisen differentiaaliyhtälömallin avulla. Se tuottaa tyypillisiä latentin taustatekijän malleja realistisemman tilastollisen aikasarjakuvauksen, ja mahdollistaa yksittäisten oireiden toimimisen kausaalisesti aktiivisina tekijöinä yksiulotteisen taustatekijän heijastusten sijaan. Näistä yksittäisistä oireista todettiin, että tyytymättömyys omaan ruumiinkuvaan oli sekä ajassa poikkeuksellisen pysyvää että vahvasti yhteydessä 16 vuoden ajan suhteellisen korkeana pysyneeseen masennuspisteeseen. Sen sijaan uneen ja väsymykseen liittyvä oireilu oli lyhytkestoisempaa, ja uudenlaisen epä-Gaussisiin jakaumiin perustuvan päättelyn mukaan aiheutti muita masennusoireita. Yhdistämällä masennuksen neuvottelustrategia-malli yhteistyön evoluution matemaattisiin malleihin osoitettiin että teoriassa evoluution tulisi suosia masennuspiirteiden kehittymistä populaatiotasolla. Ne voivat edesauttaa yhteistyön syntymistä vähentämällä vapaamatkustamisen yksilökohtaisia hyötyjä. Tulokset vihjaavat että yksittäisen häiriön sijaan, masennusoireet heijastelevat useita prosesseja, joista osa voi olla ennemmin adaptiivisia kuin toimintahäiriöitä

A Two-Week Vacation in the Tropics and Psychological Well-Being—An Observational Follow-Up Study

Despite the vast annual number of international visitors to the tropics, surprisingly little data are available on the psychological well-being associated with the travels or with travelers’ diarrhoea (TD). We herein recruited participants of a vaccination trial, OEV-123, before their 12-day holiday in Benin, West Africa. We assessed the travelers’ psychological distress with a general health questionnaire (GHQ-12) and retrieved data on TD from the trial database. The GHQ-12 was completed before (wave 0), at return (wave 1), and 1-month after (wave 2) the trip. Of the 174 participants, 73% were women, with a mean age 40 years. Moreover, 24% reported psychological distress before traveling, 10% immediately after, and 16% 1-month after the trip (GHQ-12, 3 or more; 0–12 scoring). The findings showed that psychological well-being increased after the tropical holiday. The GHQ-12 middle wave sum score differed from the wave 0 (p < 0.001) and wave 2 (p = 0.008) sum scores, with travelers reporting highest levels of well-being on their return, with evidence of a lasting improvement. TD was experienced by 71%, and it had a negative impact on psychological well-being only if experienced after travel

General psychopathology factor and borderline personality disorder : Evidence for substantial overlap from two nationally representative surveys of U.S. adults

A general psychopathology factor reflects an underlying liability for a wide range of mental disorders. There is suggestive evidence that borderline personality disorder (BPD) may be strongly associated with the general psychopathology factor, but there are no detailed data on the degree of overlap between the general psychopathology factor and BPD or its individual symptoms. This study examined the overlap between the general psychopathology factor and BPD using cross-sectional survey data from two nationally representative samples of U.S. adults, the National Comorbidity Survey Replication (N = 5,692) and the National Comorbidity Survey follow-up (N = 5,001). Structural equation modeling was used to fit a bifactor general psychopathology model and to examine the general psychopathology factor's associations with (a) a series of Clusters A, B, and C personality disorder symptoms including BPD symptoms and (b) a latent BPD. Results showed that the shared variance between the general psychopathology factor and a latent BPD was 56% in the National Comorbidity Survey Replication and 71% in the National Comorbidity Survey follow-up. The correlation between the general factor and BPD could be set to unity without worsening model fit, suggesting that BPD closely reflects a general liability to psychopathology. The affective features of BPD were particularly strongly associated with the general psychopathology factor. Findings are discussed with respect to the nosology of BPD and the treatment of mental disorders.Peer reviewe

A Two-Week Vacation in the Tropics and Psychological Well-Being—An Observational Follow-Up Study

Despite the vast annual number of international visitors to the tropics, surprisingly little data are available on the psychological well-being associated with the travels or with travelers’ diarrhoea (TD). We herein recruited participants of a vaccination trial, OEV-123, before their 12-day holiday in Benin, West Africa. We assessed the travelers’ psychological distress with a general health questionnaire (GHQ-12) and retrieved data on TD from the trial database. The GHQ-12 was completed before (wave 0), at return (wave 1), and 1-month after (wave 2) the trip. Of the 174 participants, 73% were women, with a mean age 40 years. Moreover, 24% reported psychological distress before traveling, 10% immediately after, and 16% 1-month after the trip (GHQ-12, 3 or more; 0–12 scoring). The findings showed that psychological well-being increased after the tropical holiday. The GHQ-12 middle wave sum score differed from the wave 0 (p < 0.001) and wave 2 (p = 0.008) sum scores, with travelers reporting highest levels of well-being on their return, with evidence of a lasting improvement. TD was experienced by 71%, and it had a negative impact on psychological well-being only if experienced after travel

Strength is negatively associated with depression and accounts for some of the sex difference A replication and extension

Background Depression occurs about twice as often in women as in men, a disparity that remains poorly understood. In a previous publication, Hagen and Rosenstrom predicted and found that grip strength, a highly sexually dimorphic index of physical formidability, mediated much of the effect of sex on depression. Striking results like this are more likely to be published than null results, potentially biasing the scientific record. It is therefore critical to replicate and extend them. Methodology Using new data from the 2013-14 cycle of the National Health and Nutrition Examination Survey, a nationally representative sample of US households (n = 3650), we replicated models of the effect of sex and grip strength on depression reported in Hagen and Rosenstrom, along with additional potential confounds and a new detailed symptom-level exploration. Results Overall, the effects from the original paper were reproduced although with smaller effect sizes. Grip strength mediated 38% of the effect of sex on depression, compared to 63% in Hagen and Rosenstrom. These results were extended with findings that grip strength had a stronger association with some depression symptoms, like suicidality, low interest and low mood than with other symptoms, like appetite changes. Conclusions Grip strength is negatively associated with depression, especially its cognitive-affective symptoms, controlling for numerous possible confounds. Although many factors influence depression, few of these reliably occur cross-culturally in a sex-stratified manner and so are unlikely to explain the well-established, cross-cultural sex difference in depression. The sex difference in upper body strength occurs in all populations and is therefore a candidate evolutionary explanation for some of the sex difference in depression. Lay summary: Why are women at twice the risk of developing depression as men? Depression typically occurs during social conflicts, such as physical or sexual abuse. Physically strong individuals can often single-handedly resolve conflicts in their favor, whereas physically weaker individuals often need help from others. We argue that depression is a credible cry for help. Because men generally have greater strength than women, we argue that men may be more likely to resolve conflicts using physical formidability and women to signal others for help. We find that higher grip strength is associated with lower depression, particularly symptoms like feeling down or thoughts of suicide and that strength accounts for part of the sex difference in rates of depression.Peer reviewe

The General Psychopathology Factor: Structural Stability and Generalizability to Within-Individual Changes

Objectives: Although cross-sectional investigations have found a bifactor structure of psychiatric comorbidity that includes a general psychopathology factor plus more specific factors, prospective evidence supporting the bifactor structure is still limited. We evaluated the structural stability (i.e., longitudinal invariance) of the bifactor model in comparison to an alternative structure, a correlated factors model without a general psychopathology factor. We also investigated the models’ generalizability to change processes in psychopathology. Methods: The analyses were conducted on 10-year follow-up data from 5,001 respondents in the US National Comorbidity Survey. Invariance was evaluated through a series of nested invariance tests using confirmatory factor analysis, and the models’ generalizability to change processes was investigated using change scores of disorder status. Results: The bifactor model and the correlated factors model exhibited an equal degree of strong structural stability over time. Only the bifactor model satisfactorily characterized the structure of temporal changes in psychopathology. Conclusions: The bifactor structure with a general psychopathology factor is stable over time and describes temporal changes in psychopathology. The findings support the notion that the general psychopathology factor describes a transdiagnostic etiology and may therefore provide a useful target for intervention and treatment.Peer reviewe