These results lend support to multifactorial discrimination as a fundamental cause of mental health inequities (Tables 2, 3, 4 and 5), given that discrimination meets all four criteria proposed by Link and Phelan [8]. To our knowledge, this is the first study to consider multifactorial discrimination as a fundamental cause of mental health inequities. Our results offer a possible explanation as to why multiple minority identities experience 1) multiple adverse psychological health outcomes, 2) higher exposure to risk factors, 3) less access to protective factors, and 4) poor mental health trajectories [14, 15, 34,35,36].

Our finding that multifactorial discrimination influences multiple psychological outcomes (Table 2) is consistent with other studies. Among Black/African-American LGBQ adolescents, racist and antigay discrimination was found to be associated with suicidal ideation and depressive symptoms [37]. Discrimination, including experiences of homophobia and racism, significantly predicted symptoms of psychological distress in a population of gay, Latino men [14]. Compared to White sexual minorities, more Black/African-American and Latino/Hispanic LGBQ subjects reported a history of serious suicide attempts [38]. Meanwhile, compared to heterosexual women of color and white sexual minority women, sexual minority women of color had greater risk of self-reported lifetime substance use [39].

Similarly, the results indicating that multifactorial discrimination affects multiple stress measures (Table 3) are substantiated by recent studies on multiple minority populations. One study showed that young Black/African-American men who have sex with men experience more distal stressors due to racism and homophobia [40]. Furthermore, in relation to advantaged groups, Black sexual minority women experience greater stress due to multifactorial discrimination on the basis of their triply disadvantaged social status in terms of sex, race, and sexual orientation [41, 42]. Research suggests that stress is a mediator of the relationship between discrimination and health in multiple minorities [14, 43, 44].

Moreover, the finding that multifactorial discrimination influences access to protective psychosocial resources is supported by studies that show lower levels of mastery and social support among diverse racial/ethnic LGBQs [45, 46], and studies that show these psychosocial resources buffer experiences of minority stress [6, 47, 48]. Interestingly, our results show that discrimination specifically affects access to individual-level protective factors such as mastery and self-esteem as opposed to interpersonal-level factors such as social support and collective esteem (Table 4).

Finally, our results highlight multifactorial discrimination as a continuous contributor to psychiatric morbidity even after accounting for stress-related risk factors (Table 5). These findings coincide with research examining the different pathways through which discrimination is capable of perpetuating health inequalities more broadly. While the alternative mechanisms through which discrimination affects mental health over time are unidentifiable in our analysis, research shows that multifactorial discrimination encountered by multiple minorities is associated with increased health-risk behaviors, which may be linked to various poor health outcomes [49,50,51]. Additionally, multifactorial discrimination may be responsible for widening health disparities by decreasing access to quality health care among marginalized communities. A nationwide survey found that non-White sexual minorities experienced higher rates of discrimination in health care settings relative to their White counterparts [52]. When compared to White sexual minorities, non-White sexual minorities were more likely to report fears and concerns about obtaining necessary health services due to past experiences of discrimination and substandard care [53]. In another study, African American sexual minority women who attributed their negative experiences in a healthcare setting to multifactorial discrimination decreased their health care utilization following the negative experience [54]. Among minority populations, discrimination during health care is significantly associated with delayed care and stopped treatment [55, 56]. Disengagement with the health care system due to negative patient-provider interactions, resulting from discrimination, may partially explain poor health outcomes across marginalized communities [57,58,59]. Given that discrimination can affect disease outcomes, through its effect on health care access, the relationship between discrimination and poor health is thought to be partly mediated by non-stress-related risk factors. Even if stress caused by multifactorial discrimination could be eliminated, research indicates that discrimination would continue to maintain and reproduce health inequalities through other intervening mechanisms.

Although multifactorial discrimination has not previously been identified as a fundamental cause of mental health inequities, recent research proposes stigma as a fundamental cause of health inequalities [33]. Hatzenbuehler, Link, and Phelan argue that stigma is an all-encompassing concept that includes various stigmatized statuses, including, but not limited to HIV status, obesity, sexual orientation, disability, and race/ethnicity. They also state that discrimination is a “constitutive feature of stigma” because “the overall stigma process incorporates several other elements, such as labeling and stereotyping”, making the concept broader than discrimination [33]. The Project STRIDE dataset did incorporate a stigma measure, which was moderately correlated with discrimination. However, we used the discrimination measure in our analysis not only for its validity and reliability [60], but also to address a specific theoretical framework that centers on discrimination as it relates to intersecting minority identities, considering individual, intersectional experiences and behavior. While stigma typically refers to beliefs and attitudes that lead people to fear, avoid, or reject those they perceive as different, discrimination is the behavioral manifestation of stigma [61]. Arguably, this quality makes discrimination easier to measure than stigma. Therefore, just as both racism and residential segregation have been established as fundamental causes despite being linked [9, 62], we believe our analysis provides strong evidence to include multifactorial discrimination as a fundamental cause in its own right.

Limitations and future research directions

An important limitation concerning our analysis is that heterosexual Black/African-American and Latino/Hispanic individuals were not included in the Project STRIDE study sample. While we could not quantitatively distinguish between the adversity experienced by non-White LGBs and non-White heterosexuals, we were able to examine differences within sexual minorities. We structured our analysis such that we compared non-White LGBs to White LGBs, which allowed for an intersectional interpretation of discrimination among the multiple minorities, meaning that the discrimination experienced by non-White LGBs was related to their multiple, intersecting minority identities. Our analysis cannot prove that stress exposure, access to protective psychosocial resources, and the prevalence of mental health outcomes observed for non-White LGBs are any different than they would have been for Black/African-American and Latino/Hispanic heterosexuals. However, such differences are thoroughly supported in previous research [63, 64], and we are confident in the assumption that racial/ethnic sexual minorities experience additional and interacting burdens compared to heterosexual racial/ethnic minorities. Further research is needed to parse the association between multifactorial discrimination and mental health inequities in more diverse, heterogeneous populations. Such studies would allow for a better understanding of how social identities mutually interact with and simultaneously influence each other within varied contexts [65].

We were also limited in our evaluation of the fourth criterion of the fundamental cause theory, which speaks to the persisting nature of a fundamental cause in its ability to reproduce health inequalities through alternate, intervening mechanisms. Our source data included a 1-year follow-up of study participants, which is not long enough to demonstrate that discrimination causes health inequalities to persist over time. Notably, the fourth criterion of the fundamental cause theory describes a longitudinal, population-level phenomenon that cannot be effectively captured by the individual-level data from Project STRIDE. However, our limited analysis does establish discrimination as a significant predictor of depression after adjusting for relevant risk factors, and the association between discrimination and psychiatric morbidity endures even among those with low depression. This finding implicates unmeasured mechanisms through which discrimination reinforces and generates mental health inequities.

The inability to assess institutional discrimination was another limitation of our research. We were unable to analyze how institutions, such as the health care system, the education system, and the criminal justice system, may discriminate against multiple minorities, such as those in Project STRIDE. Since our measure of multifactorial discrimination only truly captures interpersonal discrimination, our understanding of multifactorial discrimination as a fundamental cause is limited to the interpersonal level in the context of this study. As a result, this analysis likely underestimates the pervasiveness of multifactorial discrimination as a fundamental cause of mental health inequities. Future research should consider institutional discrimination when exploring the effects of multifactorial discrimination on the health of multiple minorities.

Lastly, our focus on mental health inequities limited our ability to empirically test whether discrimination acts a fundamental cause of all health inequalities. However, other studies have shown that discrimination contributes to poor physical health outcomes [66,67,68,69,70]. Moreover, stress physiology research reveals that Black/African-American sexual minority males have a flatter diurnal cortisol curve compared to White sexual minority males [71], which has been extensively linked to poor mental health and physical health outcomes in the literature [72,73,74,75,76]. Future studies should incorporate physical health outcomes in their evaluation of multifactorial discrimination as a fundamental cause of health inequalities.