Personality and Disease Susceptibility
The influence of an individual’s characteristics on disease incidence.
Personality and behavior are strongly interlinked and collectively shape how individuals navigate their lives, experience their surroundings, and are perceived by other members of society. Given the pervasive nature of personality, it is not surprising that the study of personality traits and their impact on health has been heavily researched. Despite the seemingly simplistic and intuitive nature of such a relationship, obtaining sound, empirical evidence has proven to be a challenge. One major barrier to research in this field has been the temporal constraints imposed in a research setting. Personality can change during one’s lifetime in response to environmental pressures, cognitive development, lived experiences, and the development of disease (Loosman et al. 2018). Studies assessing the relationship between personality and disease susceptibility should therefore measure personality traits on several occasions throughout an individual’s life, though this can be a very cumbersome approach. Cross-sectional studies comparing the personality traits of a group of healthy controls to a sample of individuals with established disease is an efficient way of acquiring data. However, given that the development of disease can have important effects on personality traits, it cannot be determined from cross-sectional studies whether a given personality trait is a disease risk factor, or a consequence of the disease. Longitudinal prospective studies overcome some of these challenges by following the same individuals over many years while routinely measuring personality traits and assessing for the presence of specific diseases. Although this methodology may seem straightforward, it presents with several logistical challenges, including participant attrition and consistent follow-up. Even when personality traits have been identified as independent risk factors for disease, the potential contribution of a third confounding variable is difficult to rule out.
When assessing the effect of personality on disease susceptibility, another important point to consider is the method that is used to measure personality. There is a plethora of measures that aim to reliably assess elements of personality, and there are also several definitions used to describe personality traits and states. Unfortunately, there is no gold-standard personality measure that is used throughout the literature to study the relationship between personality and disease susceptibility. Most empirical research has resorted to the Big Five personality traits, including openness to experience, conscientiousness, extraversion, agreeableness, and neuroticism, because of the high reliability and validity of this measure. Studies that have measured other personality traits are still important to consider because they offer additional information to address this complex research question.
The discussion below will address the relationship between personality and the risk of developing cancer, cardiovascular disease, diabetes mellitus, dementia, and mental illness. These diseases were chosen for discussion, because there is a large body of empirical research addressing the role of personality traits in each of these conditions. In order to provide readers with a better understanding of the strength of the available evidence, the methodology and personality measures used in the cited studies will be explicitly stated throughout the discussion below. Emphasis has been placed on studies using large sample sizes and rigorous methodologies. Several meta-analyses have also been included in the discussion below to help synthesize the evidence available for each topic.
There has been considerable interest in the relationship between personality and cancer. Personality traits are known to influence an individual’s lifestyle choices, including their exposure to carcinogens and participation in health promoting practices, and may therefore impact their susceptibility to cancer. Traits such as pessimism and neuroticism have been shown to strongly influence an individual’s mortality risk (see Personality and Mortality), and cancer is a common cause of death in modern society. Despite these logical arguments in favor of personality influencing cancer risk, research in this area has generally failed to identify such a relationship.
A meta-analysis was conducted using pooled prospective research studies from three different geographical regions in the world (Australia, Britain, and the United States of America) and included 2,156 incident cancer cases (lung, colon, breast, prostate, skin, and leukemia/lymphoma) among 42,843 adults. No association was found between any of the Big Five personality traits and incident cancer or cancer-related mortality (Jokela et al. 2014a, b). Interestingly, however, neuroticism and low extraversion (i.e., introversion) significantly impacted the clinical progression of prostate cancer. Men with high neuroticism and introversion were shown to have a worse prognosis, characterized by more severe disease characteristics and the presence of metastases (Perry et al. 2018).
In the GAZEL cohort (N = 13,768) established in France in 1993, the personalities of participants were measured at the onset of the study and the development of cancer was then tracked over a period of 16 years. The personality traits of participants that received a diagnosis of cancer (N = 1,139) were compared to the remainder of the cohort (Lemogne et al. 2013). A Type 1 personality – characterized by the tendency to suppress emotional expression – as assessed by the Personality-Stress Inventory, was associated with a decreased risk of breast cancer, while Type 5 personality – characterized by rational/anti-emotional tendencies – was not associated with any of the most common cancers, including prostate, breast, colorectal, or smoking related (i.e., oral cavity and pharynx, esophagus, larynx, trachea, bronchi/lungs, and bladder), even though it was originally hypothesized to be strongly associated with incident cancer (Lemogne et al. 2013). Type A personality within the GAZEL cohort, as measured by Bortner Type A Rating Scale, was also not associated with incident cancer or cancer progression (Lemogne et al. 2013). Further, a cross-sectional study used the stress inventory to assess the risk of colorectal cancer in Japan by surveying 497 newly diagnosed patients and 908 randomly selected controls (Nagano et al. 2008). Personalities characterized by emotional suppression or hopelessness were not associated with incident colorectal cancer. However, forming ambivalent connections (i.e., weak interpersonal relationships) and egocentricity were noted as potential protective factors (Nagano et al. 2008).
A prospective Japanese study examining 209 breast cancer cases among 29,098 women utilized a culturally sensitive personality questionnaire that measured having “ikigai” – a Japanese word for something that make’s life worth living – decisiveness, ease of anger arousal, and perceived stress (Sawada et al. 2016). Convergent with previously mentioned studies, evidence showed no relationship between personality and incident breast cancer (Sawada et al. 2016). Therefore, the evidence largely indicates that there is no relationship between personality and susceptibility to cancer. However, some evidence points towards the possible association between personality and cancer progression (see Personality and Mortality).
Evidence suggests that personality traits may serve as important risk factors for cardiovascular disease. A Japanese cohort study (N = 1,322) examined the relationship between personality, as measured by the Eysench Personality Questionnaire – a 48-item survey that assess four personality traits including extraversion, neuroticism, psychoticism, and lie – and metabolic syndrome, which is a significant risk factor for cardiovascular disease and Type II diabetes mellitus. Extraversion was found to be associated with higher overall metabolic syndrome scores as well as individual metabolic syndrome factors, including elevated waist circumference, elevated triglycerides, elevated blood pressure, and elevated fasting blood glucose (Ohseto et al. 2018). Interestingly, low conscientiousness – characterized by increased impulsivity and lack of discipline – is associated with elevated serum leptin levels. Leptin is a hormone that suppresses appetite but is elevated in obesity due to increased leptin resistance. Importantly, high levels of leptin have been associated with the development of cardiovascular disease (Sutin et al. 2013).
A recent study employed the Hunter-Wolf A/B personality score to examine the relationship between a Type A personality – marked by leadership, high ambition, and determination – and the risk of cardiovascular disease (N = 3,396, Bogalusa Heart Study) (Pollock et al. 2017). Tracking adolescents into early adulthood (median follow-up time = 11 years) revealed a positive association between Type A personality traits and cardiovascular risk factors, including body mass index and fasting blood glucose. Importantly, Type A personality traits were found to increase 10 year cardiovascular risk as evidenced by elevated Framingham scores (Pollock et al. 2017). In a large prospective cohort study (N = 24,543), high extraversion was associated with an increased risk of stroke, high neuroticism was associated with an increased risk of coronary artery disease, and low conscientiousness was associated with higher mortality risk from both coronary artery disease and stroke (Markus Jokela et al. 2013). Further, a meta-analysis including 25 studies showed evidence for the predictive value of anger and hostility in coronary artery disease, and their effect in worsening disease prognosis (Chida and Steptoe 2009). Negative affect as described by high neuroticism was also shown to predict coronary artery disease (Suls and Bunde 2005). These studies highlight the complex relationship between personality and cardiovascular disease. Given these findings, it is possible that personality traits may become important targets of early interventions to halt incident cardiovascular disease and slow disease progression.
Socially negative personality traits (e.g., hostility and angry temperament) have been implicated in the development and progression of diabetes mellitus using a biopsychosocial model. This model describes the interconnectedness of biological factors (i.e., biomarkers that influence health outcomes), psychological factors (i.e., low self-esteem and personality changes), and social factors (i.e., social isolation or low socioeconomic status), and their impact on diabetes mellitus (Temoshok 1990). A review paper discussed evidence of personality traits that are linked with the development of diabetes mellitus and other psychosocial factors that are implicated in disease susceptibility and progression. High hostility marked by a propensity to hold a cynical, angry, and aggressive attitude toward others was prospectively correlated with increased fasting blood glucose and cross-sectionally associated with increased insulin resistance, HBA1c – a blood marker of glucose control – and prevalence of diabetes mellitus (Hackett and Steptoe 2016). Further, in two cohort studies (N1 = 11,615 and N2 = 5,598), an angry temperament was found to be associated with an increased incidence of diabetes mellitus while controlling for potential confounding variables including demographic factors, exercise levels, diet, alcohol consumption, and smoking (Hackett and Steptoe 2016). In a meta-analysis that included five prospective cohort studies (N = 34,913), low conscientiousness was associated with an increased incidence of diabetes mellitus and diabetes related mortality (Jokela et al. 2014a, b). Because low conscientiousness is characterized by increased impulsivity, the worse health outcomes associated with higher levels of this personality trait may result from poor health management and reduced participation in health promoting behaviors, such as exercising and weight management (Jokela et al. 2014a, b).
A growing body of evidence suggests that personality traits also play a role in the development of dementia. Out of the Big Five personality traits, neuroticism, conscientiousness, and agreeableness appear have the strongest association with this condition. Individuals with mild cognitive impairment and established dementia exhibit higher levels of neuroticism and lower levels of conscientiousness, extraversion, openness and agreeableness when compared to healthy controls (Antonia Terracciano et al. 2017). In one longitudinal study, the Big Five personality traits were measured using the NEO personality inventory in a sample of 1,671 adults in the United States. Participants were followed for a mean of 12 years and assessed for the presence of Alzheimer’s disease. High neuroticism and low conscientiousness were found to increase the risk of Alzheimer’s disease. For every one standard deviation increase in neuroticism, or decrease in conscientiousness, the risk of developing Alzheimer’s disease increased by approximately 30%. Individuals that fell within the highest quartile for neuroticism were shown to have three times the risk of developing Alzheimer’s disease compared to individuals that scored within the lowest quartile. Similarly, the risk of developing Alzheimer’s disease was three times higher in people that scored in the lowest quartile for conscientiousness compared to the highest quartile (Antonio Terracciano et al. 2014). Many of these findings were recently reproduced in a sample of over 10,000 adults in the United States that were followed for a mean of 6 years. In this study, high neuroticism, low conscientiousness, and low agreeableness were shown to independently increase the risk of developing dementia, including Alzheimer’s disease. A 20% increased risk of developing dementia was seen with each standard deviation increase in neuroticism, and each standard deviation decrease in conscientiousness or agreeableness. High neuroticism and low conscientiousness also increased the risk of developing mild cognitive impairment. Importantly, lower levels of conscientiousness were predictive of the transition from mild cognitive impairment to dementia (Antonia Terracciano et al. 2017). Recent research has focused on characterizing changes in personality traits that occur before mild cognitive impairment and dementia become clinically detectable. Neuroticism was found to linearly increase over time in individuals that later went on to develop dementia or mild cognitive impairment, whereas the sample of healthy controls exhibited stable levels of this personality trait (Yoneda et al. 2018). This study highlights the possibility of using personality measures to detect dementia before it becomes clinically apparent.
Perhaps the greatest evidence in support of a relationship between personality and the development of disease exists in the context of mental illness. Several empirical studies have assessed the relationship between the Big Five personality traits and the development of specific mental disorders. In general, patients with established mental illness exhibit higher levels of neuroticism and lower levels of conscientiousness than other members of the population. Neuroticism has also been identified as the most important risk factor for the development of several mental disorders. Furthermore, higher levels of neuroticism are associated with a greater number of comorbidities and increased utilization of mental health services (Lahey 2009).
The relationship between the Big Five personality traits and overall mental health was assessed in a meta-synthesis of 36 meta-analyses containing a total of over 500,000 participants. Mental health, as defined by cognitive, emotional, and social well-being, and the presence of psychopathology, was strongly associated with personality when all of the Big Five personality traits were assessed simultaneously. When individual personality traits were analyzed, neuroticism, conscientiousness, and agreeableness were found to have the strongest associations with overall mental health (Strickhouser et al. 2017). In one large-scale meta-analysis conducted on 175 studies with over 75,000 collective participants, neuroticism was found to be the most important personality trait associated with common mental disorders. High levels of neuroticism and low levels of conscientiousness were associated with all forms of mental illness screened for in this body of literature, including major depressive disorder, unipolar depression, dysthymic disorder, generalized anxiety disorder, posttraumatic stress disorder, panic disorder, agoraphobia, social phobia, specific phobia, obsessive compulsive disorder, and substance use disorder. In addition, higher levels of introversion were seen in individuals with dysthymic disorder and social phobia, and increased disinhibition was common in patients with substance use disorder, dysthymic disorder, and obsessive-compulsive disorder (Kotov et al. 2010).
Associations between personality traits and personality disorders (PDs) have also been explored through meta-analysis of the literature. Evidence suggests that individuals with paranoid PD, schizoid PD, schizotypal PD, borderline PD, dependent PD, and avoidant PD exhibit higher levels of neuroticism than healthy controls. Lower levels of conscientiousness were observed in individuals with antisocial PD, borderline PD, and dependent PD. In addition, introversion was associated with paranoid PD, schizoid PD, schizotypal PD, and avoidant PD, while low levels of agreeableness were associated with paranoid PD, antisocial PD, borderline PD, and narcissistic PD (Samuel and Widiger 2008). Furthermore, meta-analysis of studies investigating the relationship between personality traits and schizophrenia identified higher levels of neuroticism and lower levels of conscientiousness, agreeableness, openness, and extroversion in patients with schizophrenia compared to the general population (Ohi et al. 2016). These meta-analyses identified important associations between personality traits and established mental illness across large bodies of research and diverse study populations. However, given that many of the studies that were included in each meta-analysis were cross-sectional in design, it is unclear from this body of research whether personality traits are risk factors for mental illness or whether established mental illness selects for specific personality traits.
To better understand the temporal relationship between personality and the onset of mental disorders, several studies have followed large groups of individuals with known personality traits over many years. Among the Big Five personality traits, neuroticism has been identified as the best predictor of future mental illness. In the largest longitudinal study performed on this topic to date, over one million Swedish men had their personality traits assessed between the ages of 18 and 19 and were screened for mental illness into adulthood. This study found that emotional instability, which is an indicator of high levels of neuroticism, is an independent risk factor for bipolar disorder, schizophrenia, and schizoaffective disorder (Hayes et al. 2017). In another longitudinal study conducted out of Sweden, over 20,000 adults from the Swedish Twin Registry were assessed for major depressive disorder 25 years after completing the Eysenck Personality Inventory for neuroticism and extroversion. The level of neuroticism exhibited by study participants was directly proportional to their lifetime risk of developing major depressive disorder. An inverse relationship was also observed between neuroticism and the duration of time before an individual experienced their first major depressive episode (Kendler et al. 2006). In one longitudinal study conducted in Switzerland, it was estimated that for every one standard deviation increase in neuroticism from the average, an individual’s absolute risk of developing major depressive disorder is increased by 6.4% (Hengartner et al. 2016). Importantly, genetic factors have been shown to play a prominent role in mediating the relationship between neuroticism and major depressive disorder (Kendler et al. 2006).
Neuroticism has also been identified as a risk factor for eating disorders, such as anorexia nervosa and bulimia nervosa. In one longitudinal study of over 2,500 adolescent females in Spain, individuals that scored within the top quartile for neuroticism had a threefold increased risk of developing an eating disorder within the following 18 months compared to those that scored within the bottom quartile (Cervera et al. 2003). In the context of substance use disorders, conscientiousness has been identified as the most important personality trait. Low conscientiousness and high neuroticism were shown to be risk factors for smoking cigarettes, problem drinking, illicit drug use, and prescription drug abuse through a longitudinal study of over 4,000 adults in the United States over 10 years. Interestingly, individuals with higher conscientiousness were able to overcome some of the detrimental effects of high neuroticism on substance use, suggesting that interactions between personality traits may influence the risk of developing certain forms of mental illness (Turiano et al. 2012).
Many of the relationships described above have been further supported by a large-scale meta-analysis conducted on 46 longitudinal studies investigating the link between neuroticism and the development of depression, anxiety, and substance use disorders. Neuroticism was found to predict the future development of each of these conditions, albeit to a lesser extent than that identified through meta-analysis of cross-sectional studies. The prospective associations between neuroticism and mental disorders were strongest for depression and anxiety, and these associations persisted after adjusting for baseline psychiatric symptoms (Ormel et al. 2013).
Several models have been developed to conceptualize the relationship between personality traits and mental illness. According to the spectrum model, personality traits and mental disorders fall on opposite ends of the same spectrum. Mental illness is therefore seen as an extreme manifestation of specific personality traits. In the scar model, personality traits are thought to be driven by established mental illness. The development of a mental disorder is proposed to leave a permanent scar in the form of persistent personality changes. The state model similarly posits that mental illness facilitates changes in personality, though these changes are only transiently seen during active episodes of mental illness. The common cause model proposes that personality and mental illness are not directly related. Instead, they are thought to share one or more common causes, which confounds empirical studies. Finally, the vulnerability model proposes that personality traits cause mental illness either through direct effects or through indirect effects on other risk factors, such as stressful life events (Ormel et al. 2013). Though the results of longitudinal and prospective studies make the vulnerability and common cause models enticing, there is empirical evidence to support each of the proposed models. Further research is therefore required to address the mechanism(s) linking personality traits and mental illness.
It is evident from the discussion above that personality traits have important effects on physical and mental health. Despite the challenges inherent to studying the relationship between personality and disease, a large body of rigorous empirical research has emerged on this topic. Of the Big Five personality traits, neuroticism and conscientiousness appear to be the most important risk factors for the development of disease. The impact of high neuroticism and low conscientiousness is greatest for mental illness, though physical diseases are also influenced by these personality traits (Strickhouser et al. 2017). Neuroticism alone has been estimated to cost 2.5 times more than mood disorders, anxiety disorders, and substance use disorders combined. Furthermore, individuals within the top quartile for neuroticism produce excess costs of approximately $1.4 billion per 1 million individuals (Cuijpers 2010). Several models have been developed to explain the relationship between personality traits and disease development. Longitudinal studies have provided strong support for the vulnerability and common cause models, though the precise mechanism mediating this relationship has not been firmly established. Recent research has begun to investigate how specific facets of the Big Five personality traits influence disease susceptibility. These studies are anticipated to further expand our knowledge of this complex relationship. Future research should address the clinical utility of personality measures as diagnostic and prognostic indicators for specific diseases. The potential for interventions that modify personality traits to influence disease risk and progression should also be explored.
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