The General Disease Factor (“ <i>d</i> ”): First Empirical Evidence From China

Dear Editor, Co-occurrence of disease is becoming an increasingly important area of research as the understanding of disease and clinical practice develops. Comorbidity occurs not only within mental disorders [1], but also between mental and physical diseases [2-4]. A general disease (d) factor has been proposed to explain the propensity of general comorbidity between mental and physical health [3]. The concept of d factor has been validated in UK adolescent, adult, and older groups [3], and is further supported by converging neural, genetic, and environmental evidence [4]. Thus, all current d factor studies were based on the Western population, meaning the predominantly white population. The prevalence and incidence of both mental and physical diseases in the Chinese population have significant differences from Western populations [5, 6]. In addition, recent Traditional Chinese Medicine (TCM) literature highlights a holistic mind–body framework [7] that conceptually aligns with transdiagnostic models such as the d factor, offering a culturally relevant context while acknowledging its differences from Western psychopathology. Therefore, the existence of the d factor needs to be further replicated across populations from different cultural backgrounds and ethnicities. Narcissism is a personality trait characterized by egocentric exceptionalism and social selfishness [8]. It has been linked to various aspects of individual well-being [9]. However, findings on the health consequences of narcissism in literature remain inconsistent. Some evidence showed an association between narcissism and better mental health and well-being [9]. Nevertheless, some studies suggest that narcissism is associated with heightened anxiety severity and acute anxiety episodes [10], as well as increased stress reactivity [11]. Moreover, narcissism has been positively associated with a range of unhealthy lifestyle behaviors, including alcohol consumption, marijuana use, and risky driving [12]. Given that the d factor reflects general vulnerability to physical and mental health conditions and is associated with unhealthy behaviors [3], and no research explores the relationship between narcissism and the d factor so far, we aim to test its existence in a Chinese population and explore its link with narcissism. We analyzed data from 1331 undergraduate medical students from Hunan Province, China (Mage = 19.15, SD = 2.44; 72.1% female) to test the existence of the d factor and its association with narcissism. Participants reported the lifetime presence of 29 specific health conditions using a binary format (0 = No, 1 = Yes). There are 7 mental health conditions, including depression, anxiety, phobia, obsessive-compulsive disorder (OCD), schizophrenia, speech problems, and sleep disorders. Physical health conditions include chronic fatigue syndrome (CFS), migraine, stroke, seizures, asthma, eczema, hay fever, arthritis, ulcer, Crohn's disease, irritable bowel syndrome (IBS), hearing problems, visual problems, tinnitus, overweight/obesity, diabetes, gallstones, kidney or bladder stones, heart problems, high blood pressure, back pain, and cancer (see details in Table S1). Narcissism was assessed using the 13-item Chinese version of the Narcissistic Personality Inventory [13], which evaluates three core dimensions: Leadership/Authority, Grandiose Exhibitionism, and Entitlement/Exploitativeness. A higher total score corresponded to greater narcissistic tendencies. The scale showed good reliability (Cronbach's α = 0.89). Following previous studies [3], we compared the model fits of three different factor analysis models (Figure 1): (1) a uni-factor model with only one common factor that all variables load into; (2) a correlated model with two specific health factors (mental and physical health factors); and (3) a bi-factor Exploratory Structural Equation Model (bi-factor ESEM), where health condition load on only onto two health factors but also an underlying dimension. We used the Weighted Least Squares Mean and Variance Adjusted (WLSMV) estimator, which is suitable for dichotomous indicators and provides less biased estimates and more accurate fit indices [14]. Comparative Fit Index (CFI) > 0.95, Tucker-Lewis Index (TLI) > 0.95, Root Mean Square Error of Approximation (RMSEA) < 0.06, and Standardized Root Mean Square Residual (SRMR) < 0.06 were used to indicate good model fits [15]. The d factor scores were extracted from the optimal model. Furthermore, we used Pearson's correlation to test the association between the d factor and narcissistic personality. The R (Version 4.2) with lavaan package and SPSS (version 27) were used to conduct data analysis. Three tested factor models. Our analysis revealed that, compared with the uni-factor model (CFI = 0.934, TFI = 0.929, RMSEA = 0.014, SRMR = 0.09) and correlated-factor model (CFI = 0.956, TFI = 0.952, RMSEA = 0.012, SRMR = 0.08), the bi-factor ESEM model showed the best model fit (CFI = 0.975, TFI = 0.971, RMSEA = 0.013, SRMR = 0.054). Both configural and scalar invariance models showed a good fit (Table S3) and strong invariance across gender (ΔCFI = 0.002, ΔRMSEA = −0.001). Except for asthma, Crohn's disease, and cancer (p > 0.05), all other health conditions (26/29) are significantly loaded onto a common factor. The item loading of all mental health considerations and half of the physical health conditions (12 out of 22) had factor loadings higher than 0.3. Furthermore, in addition to schizophrenia and speech problems, all other mental health conditions also loaded on a common mental factor. All physical health conditions positively and significantly loaded onto a common physical factor with half of the item loadings higher than 0.3. The detailed items loading is shown in Table 1. A significant correlation was found between the factor score and overall narcissism (r = 0.09, p = 0.001), indicating a higher level of narcissism was weakly associated with a stronger propensity for mental and physical multimorbidity. Specifically, the d factor was significantly associated with leadership/authority (r = 0.15, p < 0.001) and grandiose exhibitionism (r = 0.06, p < 0.001), but not with entitlement/exploitativeness (r = 0.03, p = 0.29). This study provides the first empirical evidence for the existence of a general disease factor (d factor), a common factor explaining mental and physical comorbidity, in a Chinese adult population. By applying the d factor framework to a non-Western sample, we provided initial evidence for its cross-cultural relevance and found that the shared vulnerability is best captured by a bi-factor ESEM model, consisting of a general factor alongside two specific dimensions for mental and physical health, reflecting the common tendency underlying comorbidities across these domains. In addition, we also identified, for the first time, a link between narcissism and the vulnerability of both mental and physical health. Our study provides a unique perspective to understand the general comorbidity between mental and physical illnesses and further explores the important implications of the concept of the d factor. The findings of our study are consistent with previous d factor studies [3], indicating that there might be an underlying domain to explain the general comorbidities across mental and physical illnesses. The previous meta-evidence has shown a range of mental and physical comorbidities. For instance, depressive symptoms have been found associated with several physical illnesses, such as epilepsy, fibromyalgia, and migraine [16]. Studies focusing on the Chinese population have shown similar findings, for example, a study based on a national community survey presented a wide range of comorbidity between mental disorders and chronic back or neck pain [17]. Furthermore, patterns and molecular correlations (e.g., shared genes) have been observed among some disease comorbidities [18]. Thus, our study further provided empirical evidence supporting the concept of the d factor, reinforcing its robustness and generalizability in diverse contexts. The relationship between narcissism and the d factor was first identified in our study. The d factor, conceptualized as a general disease factor, has been proposed to account for the shared variance underlying both mental and physical health disorders [3]. Narcissism has likewise been linked to a range of maladaptive health behaviors and health-related outcomes. For instance, Hill [12] found that narcissistic traits are positively correlated with substance use, risky driving, and impulsive health choices, although some narcissistic individuals may also engage in healthy behaviors when these align with their self-image goals. These overlapping lifestyle patterns suggest a potential behavioral pathway through which narcissism may influence the d factor. Beyond health behaviors, narcissistic individuals frequently experience poor emotion regulation, heightened stress reactivity, and interpersonal difficulties [9]—all of which are recognized risk factors for both mental and physical health problems. Chronic stress, in particular, is a major contributor to general disease vulnerability, including conditions such as depression, cardiovascular disease, and immune dysfunction [19]. Taken together, these findings suggest that narcissism may contribute to elevated d factor scores through both direct mechanisms—such as engagement in unhealthy behaviors, and indirect mechanisms involving emotional dysregulation, stress, and social strain. Thus, narcissistic traits may represent a psychological risk factor for an individual's general susceptibility to disease. From a clinical perspective, the d factor suggests that the presence of one condition may elevate vulnerability to additional mental or physical health problems. Its conceptual parallels with TCM may offer a culturally relevant lens for interpreting such comorbidity. Recent TCM literature highlights mind–body unity and regulatory effects on both physical and emotional functioning [7, 20], which conceptually echo transdiagnostic models such as the d factor. While these parallels remain preliminary, they suggest that traditional medical frameworks may inform considerations of the d factor's generalizability in non-Western contexts. Our findings have important implications for Chinese healthcare and public health systems. The identification of a general disease factor suggests that mental and physical health conditions share common vulnerabilities, which could be leveraged to design more integrated medical services. For example, healthcare providers could implement screening programs that assess both mental and physical health simultaneously, enabling early detection and intervention. At the policy level, recognizing shared risk factors may guide resource allocation and support the development of integrated treatment programs, ultimately promoting a more holistic approach to population health in China. These implications underscore the potential value of translating the d factor concept into practical strategies for improving health outcomes in non-Western healthcare contexts. The key strengths of the current study are validating the presence of a general disease factor in a Chinese young adult sample and exploring the association between the d factor and narcissism. Although the correlation between the d factor and narcissism was statistically significant, it was very small (r = 0.09), indicating limited practical impact and warranting cautious interpretation. Several limitations should be considered. First, the university medical student sample lacks representativeness, which may limit the generalizability of our findings to other Chinese populations. While the high education level of this sample may reduce misunderstandings of mental health constructs, the limited diversity constrains the breadth of inference, highlighting the need for replication in larger, more diverse, and multi-age samples. Second, all health conditions were self-reported, which may introduce recall bias and social desirability effects, potentially affecting data validity and reliability. Nevertheless, the reliance on self-reported data may influence the observed d factor structure, and future studies using clinically validated assessments are warranted to further confirm these findings. The concept of a general disease factor accounting for mental and physical comorbidity was first tested in a Chinese sample. Although our findings provide a more comprehensive perspective on general comorbidity and highlight the shared risk factors and underlying mechanisms linking mental and physical health conditions, they should be interpreted with caution given the limited representativeness of our sample. Nevertheless, the results tentatively suggest the value of adopting holistic approaches to population health in both clinical practice and research. In addition, our study was the first to identify a possible association between narcissism and the d factor, indicating that narcissistic traits may play a role in general disease vulnerability; however, this relationship also warrants further validation in more diverse and representative populations. The authors have nothing to report. Ethical approval was obtained by the Medical Ethics Committee of Hunan Polytechnic of Environment and Biology. Data are available from the corresponding author upon reasonable request. 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