The prevalence of NPD in this general population sample was 6.2%, which falls in the middle of the broad range of estimates (0.0%–14.7%) found in previous epidemiologic surveys.25–38
The discrepancy in rates of NPD between this study and some others may be partly due to limitations of prior surveys with respect to geographically restricted and small sample sizes. Differences in diagnostic criteria, assessment instruments, and survey designs and methodologies may also have contributed to the discrepancies.
At variance with 1 epidemiologic study39
that found no sex differences in prevalence of NPD, this study found higher rates of NPD among men than among women. No clinical or epidemiologic studies have examined the relationship between race-ethnicity and NPD. The absence of such data is striking,79
given the substantial extent to which culture is intertwined with personality. New findings from the NESARC showed that rates of NPD were higher among Black men and women and Hispanic men. Why these minority groups were found to have differential risk of NPD raises important questions regarding the influence of cultural experiences, including acculturation, on expressions of personality psychopathology. Whether culturally specific experiences may protect against or increase vulnerability to NPD, or whether DSM-IV PD categories may be culturally uninformed, are important questions for future clinical and epidemiologic research. Cross-cultural research on NPD is also needed to understand how differences in religious and socio-cultural value systems contribute to the development of NPD.
NPD was inversely related to age, with the greatest decline occurring after age 29 years. NPD may be more prevalent among young adults due to developmental challenges in the transition from adolescence to adulthood. Taken together, these results suggest that NPD in adolescence and early adulthood may not always develop into adult NPD through possible mechanisms associated with developmental life experiences. These findings are consistent with the only prospective follow-up80
of patients with NPD. Although small (n=20), this 3-year follow-up found that about 50% of the 22- to 45-year-old subjects with DSM-IV NPD at baseline did not qualify for the diagnosis 3 years later. These results also suggest that NPD may not meet current DSM-IV diagnostic criteria that reflect enduring personality traits. However, the decline in NPD rates from the 30- to 44-year-old age group to the group of respondents 45-to-64 years-old was small, suggesting an enduring, severe PD, at least in some individuals. Further longitudinal work is warranted to differentiate individuals who may have shorter-term, context-dependent, versus enduring forms of NPD, and to identify family history, comorbidity, and developmental and intervening life experiences (e.g., employment, marriage) that influence their course. Further, the consistency of the observed age gradient in the present study with the outcome of the clinical prospective study suggests that age differences observed in this study may in part be real, and cannot be attributed solely to artifacts such as longer duration of illness, selective mortality, cohort effects, or recall or other biases. Further prospective epidemiologic research is needed to address this issue more definitively.
This study also identified sociodemographic characteristics associated with increased odds of NPD that were not generally reported in previous clinical and epidemiologic research due to limitations in sample size. The rates of NPD were generally greater among individuals who were separated, divorced, or widowed, results that did not vary by sex. These findings are consistent with prior studies that have shown that NPD relative to other PDs was uniquely related to causing significant others pain an duress3
and that the NPD is largely associated with costs experienced by others.81, 82
Whether being separated, divorced, or widowed, or never married represent true risk factors for NPD or vice versa are questions best addressed within a longitudinal framework.
In general, co-occurrence rates of other psychiatric disorders among individuals with NPD were much greater than the co-occurrence rates of NPD among individuals with other psychiatric disorders. Mirroring the distribution of psychiatric disorders in the general population, men with NPD had significantly higher rates of most substance use disorders and antisocial PD, whereas women with NPD generally had higher rates of MDD and most anxiety disorders. These results suggest more vigilance in the assessment of substance use and specific mood, anxiety, and other PDs among individuals with NPD, with due consideration for sex differences in the co-occurrence rates observed in this study. In contrast to distributions observed in the general population, rates of NPD among individuals with most substance, mood and anxiety disorders were greater among men than women. Further, the co-occurrence rates of NPD among individuals with other psychiatric disorders were lower than the corresponding rates of other psychiatric disorders among individuals with NPD, but were far from trivial. Taken together, these findings suggest increased vigilance in assessing NPD among individuals with substance use, mood, anxiety, and borderline and histrionic PDs, especially among men, whose rates of these disorders consistently exceeded those of women.
New findings from the NESARC highlight the importance of controlling for additional psychiatric disorders43,44
that are highly comorbid with each other when examining associations between NPD and other specific disorders. Regardless of whether 12-month or lifetime associations were examined, the majority of the odds ratios for associations between NPD and most other substance use, mood, anxiety, and other PDs were strong and significant with control only for sociodemographic characteristics. To understand further the unique contribution of other disorders to NPD, we additionally controlled for all other disorders assessed in the NESARC. Twelve-month and lifetime associations of NPD with bipolar I and PTSD remained significant, but were reduced, among men and women. Twelvemonth associations between NPD and GAD remained significant for both sexes, whereas the corresponding lifetime association was only significant among women. For both time periods, NPD was significantly associated with bipolar II among women, and the 12-month association between NPD and specific phobia remained significant, but reduced, only among women. Further, 12-month associations of NPD with alcohol abuse, alcohol dependence, and drug dependence remained significant among men, whereas lifetime associations remained significant only for drug abuse among men. The drop in magnitude of these associations when other comorbidity was controlled for is analogous to that in twin and genetic study designs and suggests that much of the association of NPD with these disorders appears due to factors common to these disorders. The present results highlight the importance of future research on common and specific genetic or environmental factors that underlie the comorbidity of NPD and these disorders.
The findings on comorbidity in the present study are consistent with most early clinical research4
that demonstrated the strongest relationships between NPD and bipolar I and substance use disorders, but at variance with most previous clinical research that has found no associations between NPD and specific anxiety disorders.4,9,11,13,15
Although no prior study has examined sex differences in NPD comorbidity, the present study found that some substance use disorders were associated with NPD among men, whereas GAD, specific phobia, and bipolar II disorder were consistently associated with NPD among women. Further, NPD was highly comorbid with borderline and schizotypal PDs among both men and women, consistent with some,18,20,21,83
but not all,22–24
prior clinical research.22–24
The strong relationship between NPD and histrionic PD found in early studies18–21,83
was observed in the present study only among men. The association with obsessive-compulsive PD has been observed in some prior clinical studies,20,21
but was found only among men in the present study.
Examination of sex-specific and non sex-specific patterns of comorbidity observed in this study can provide a starting point for future research that seeks to identify common and unique factors underlying disorder-specific associations. As described above, some of the associations between NPD and other psychiatric disorders were sex-specific. Interestingly, in those cases when associations remained significant among both men and women, there were no differences in the magnitude of the associations. Taken together, these findings generate more focused hypotheses about factors underlying NPD comorbidity. For example, when sex differences exist for associations between NPD and other psychiatric disorders, is there something about one’s gender that would increase vulnerability to that comorbidity? Further, when no sex differences were observed in the magnitude of NPD associations, future research should focus on potential shared environmental and/or genetic factors underlying these disease-specific associations that might explain this pattern of comorbidity. Taken together, these results underscore the importance of future research to address sex differences in NPD comorbidity.
Interestingly, a significant negative lifetime association between NPD and dysthymia among both men and women was observed in this study. That men with NPD were significantly less likely to have dysthymia compared with men without NPD could be viewed within the context of the increased rates of some substance use disorders among men, but not women, with NPD. Substance abuse and dependence may reflect attempts on the part of men with NPD not only to reestablish or maintain grandiosity, but also to defend against the negative affect accompanying dysthymia that often accompanies aging and life’s inevitable limitations. Taken together, these results suggest a propensity of men with NPD to self-medicate to maintain a sense of omnipotence and grandiosity, to protect a very fragile self esteem,1
and to ameliorate feelings of depression, guilt, and worthlessness associated with dysthymia.84
Why NPD was negatively associated with dysthymia among women is less clear. Future research is needed to elucidate the nature and role of depressive states in the development, course, and severity of NPD among men and women.
That NPD was associated with social and role dysfunction is consistent with the definition of NPD in the DSM-IV as well as with findings from 1 clinical study.3
However, the present study found that disability was associated with NPD among men, but not among women, when other psychiatric disorders were controlled for in the analyses. These results strongly suggest that much of the disability associated with NPD among women may be attributed to its comorbidity with other disorders. That the relationship between NPD and disability among men remained significant with adjustments for comorbidity further suggests that NPD may have a more severe expression in men relative to women. These findings also underscore the need for further research to provide the evidence necessary to strengthen arguments for the inclusion of NPD in the DSM-IV on the basis of impairment. Further longitudinal research that builds upon a growing body of recent research in this area is also needed to understand the impact of disability on the course, outcome, and comorbidity of NPD.
Potential study limitations are noted. This study is based on data from the Wave 2 NESARC. We were unable to reinterview respondents to the Wave 1 interview who were deceased or unable or unwilling to participate. However, the Wave 2 response rate, much higher than that obtained in most national surveys to date, combined with statistical adjustments for nonresponse at both the person and household levels on numerous sociodemographic characteristics and the presence of any lifetime Wave 1 Axis I or II disorder, considerably minimized the impact of nonresponse bias on study findings. Although the NESARC included a group quarters sampling frame, some special populations were not included in the sample, e.g., individuals under age 18 years and those incarcerated or hospitalized during the interview periods.
In summary, the prevalence of NPD in this U.S. general population sample was significantly greater among men than among women and the disorder was associated with substantial mental disability among men. The present study has also identified population subgroups at risk for NPD that have rarely been reported in previous studies. Importantly, NPD was inversely related to age, suggesting that the disorder may be less chronic than previously recognized. This study has also highlighted the need for future epidemiologic, clinical, and genetically informed studies to identify unique and common factors underlying disorder-specific comorbidity with NPD found in the NESARC sample. Important sex differences in rates of and associations with NPD can inform more focused, hypothesis-driven investigations of those factors and provide important information for treatment planning.