Socioeconomic status (SES) is defined as the social standing of an individual measured by a combination of income, education, and occupation1. The examination of SES often demonstrates inequities in access to healthcare and resources, which can result in an increase mortality and morbidity. An example of such is the effect of SES on depression. Depression is a mood disorder that is characterized by severe symptoms that affect how an individual thinks, feels, and acts2. Depression is one of the most common mental disorders in the United States and in 2015 approximately 16 million adults had at least one major depression episode in the past year3. SES and depression have an inverse relationship because individuals with lower SES tend to have higher rates of depression.
This relationship was examined in a community-based longitudinal study that was conducted in Alameda County, California. Everson and his colleagues discovered a positive relationship between socioeconomic status (measured by education or income) and prevalent and incident depression4. Similar results were also found when Lorant conducted a meta-analysis on socioeconomic inequalities in depression. He discovered that low SES slightly increases the risk of depression and moderately increases the risk of continuing depression5. Gilman and his colleagues took a different approach with regards to SES by examining childhood SES, which was measured by parental occupation at the time of the participants’ birth and when they turned seven years old. The researchers wanted to test the hypothesis that low childhood SES predicts an increased risk of adult depression. The results of the study found that participants with lower childhood SES had twice the risk for major depression in comparison to participants with higher childhood SES6. Even from a life course perspective the same relationship held true; individuals with higher education were more successful in postponing depression than their counterparts7.
Even though there is consensus in the literature about the relationship between SES and depression, the relationship is particularly strong among females in comparison to males. In the general population, the higher prevalence of depression among females than males have been consistently observed8. In a study conducted by Goodwin and Gotlib, data were drawn from the Midlife Development in the United Stats Survey and multivariate logistic regression analyses were used to examine the relationship between gender and depression. The results from their study was that female gender was associated with increased odds of experiencing depression8. In another study, Susan Nolen-Hoeksema found that women are more vulnerable to develop depression in response to stress due to multiple factors such as gender intensification, biological responses, and copying styles. Women are also twice as likely as men to experience depression from early adolescence through adulthood9.
The mediators in the conceptual model above that link SES and depression are physical activity and locus of control. Physical activity, according to the World Health Organization (WHO), is defined as “any bodily movement produced by skeletal muscles that require energy expenditure”10. In one study conducted by Strawbridge and his colleagues, physical activity was measured by a physical activity scale based on four questions: usual frequency of physical exercise, taking part in active sports, taking long walks, and swimming. The response set for each question was never, sometimes, and often, and each response received zero points, one point, and two points respectively11. In another study conducted by Miech and Shanahan, physical activity was measured by reporting the degree of difficulty (no difficulty, some difficulty, and a great deal of difficulty) for the following tasks: going up and down the stairs, kneeling or stopping, lifting or carrying objects less than ten pounds, using hands or fingers, and walking7.
The second mediator, locus of control, is the extent to which an individual believes he or she has power over rewards, punishments, or other events that occur in his or her life12. Locus of control was measured in one of the studies by having participants rate eight statements using a point scale that ranged from “strongly agree” (five points) to “strongly disagree” (one point). Some examples of the rated statements were: “I am responsible for my own success”, “I can do just about anything I really set my mind to”, and “my misfortunes are the result of mistakes I have made”7.
Both physical activity and locus of control mediate the effects of SES on depression. In the conceptual model above, SES is determined by income and education. Income and education influence an individual’s built environment and level of stress. Furthermore, built environment affects physical activity and stress affects locus of control. The mediators then influence depression through physical health, self-esteem, and self-efficacy.
Literature Findings of the Mediating Pathways
The first connection on the SES and depression pathway is SES (measured by income and education) and its effect on the built environment. Lower SES individuals are more likely to reside in impoverished areas with poor built environments in comparison to individuals with higher SES12. Individuals with lower SES were also more likely to live in areas with poor access to facilities whereas their counterparts had a significantly greater relative odds of having 1 or more facilities13. Furthermore, the reduced access to facilities among the lower SES individuals resulted in a decrease in physical activity13, which demonstrates the next connection which is the effect of the built environment on physical activity.
In a study conducted by Susan and her colleagues, the researchers found that the built environment can enhance the feasibility and appeal of physical activity through street connectivity and good design14. In another study, it was discovered that residents of built environments with high walkability had seventy more minutes of physical activity than residents with poor built environments with low walkability15. Both research studies demonstrate the importance of built environment and the effect it has on physical activity. Continuing down the first mediating pathway, physical activity then affects both physical health and self esteem.
It is well noted in the literature that an increase in physical activity results in an improvement in overall physical health. Thus individuals that engage in regular physical activity demonstrate better health outcomes across a plethora of physical conditions16. Regular physical activity also contributes to a better quality of life and mood states, an example being self-esteem16. In a randomized control study McAuley and his colleagues studied the extent to which changes in physical activity was related to changes in self-esteem. The researchers found that an increase in the frequency of physical activity resulted in an increase in self-esteem in strength, physical condition, and overall appearance17.
The last connection in the first mediating pathway is physical health and self-esteem and their effects on depression. In one research study, it was discovered that individuals who reported poorer physical health experienced greater depression18. The research suggests that the above statement holds true because depression is often the result of declining health among older adults18. In another study, researchers found that individuals with higher education were more successful in postponing depression because of their better physical health7. Self-esteem also demonstrated a similar relationship with depression. Having low self-esteem and negative self-evaluations resulted in an increase in depressive episodes19. This is due to the fact that individuals with low self-esteem are more vulnerable and sensitive to challenges, thus resulting in an increase in depression19.
SES not only affects built environment but stress as well. In the second mediating pathway, lower SES is associated with both social and environmental conditions that contribute to chronic stress20. Some examples of stressors that contribute to chronic stress are crime, noise pollution, and discrimination. Lower SES is also associated with distress and poor health outcomes because individuals are not able to purchase good or services that can help alleviate stress20. The next connection in the second mediating pathway is stress and its effect on locus of control.
Individuals who are more likely to experience stressful events are usually less equipped to cope with the events, thus resulting in a decrease in locus of control6. The decrease in locus of control results in the loss of control of the situation, which then contributes to a decrease in self-efficacy. In one research study, it was found that individuals with more locus of control had higher self-efficacy21. This is because an individual’s sense of control is associated with their well being and if he or she perceives themselves as having more control, then they are more likely perform better on a task21.
The last connection on the second mediating pathway is the relationship between self-efficacy and depression. In a study conducted by Bandura, it was found that individuals with high self-efficacy often visualized scenarios that provided positive guidance and support for success22. On the contrary, individuals with low self-efficacy often visualized scenarios that resulted in their failure and made them dwell on everything that could go wrong22. This contributes to depression because individuals with low self-efficacy dwell on their coping deficiencies and tend to believe they cannot manage threats. Furthermore, low self-efficacy contributes to negative thought patterns which also has an effect on depression.
Quantitative Methods
SES can be measured either by income, education, or occupation. However, the most common measures are income and education. In one research study, education was measured by the highest year of school that a participant completed7. The strength of this quantitative method is that information regarding education level was easy to attain. However, the limitation of this method is that the quality of the education was not captured so it can vary. In another study, income was measured by having the participant disclose their total family income before taxes during an interview5. The strength is that unlike education income does not vary in its quality. The limitation is that since it is self-reported, participants may overestimate their family income.
In the literature, depression is usually measured by the Center for Epidemiologic Studies Depression Scale Revised (CESD-R). In one study the participants self-reported symptoms using the CES-D and those who scored 24 or higher were diagnosed as having depression7. The strength of this quantitative method is that the CESD-R is a widely used screening test for depression and it measures symptoms defined by the American Psychiatric Association Diagnostic and Statistical Manual (DSM-V). The limitation of this method is that since this is self-reported data, the results may be skewed.
The two mediators in the conceptual model above are physical activity and locus of control. Physical activity was measured using a physical activity scale based on four questions. The strength of this quantitative method is that this scale has been used in prior analyses to demonstrate a protective effect for physical activity on all-cause mortality for both men and women11. The limitation of this method is that since it is self-reported data, the results may be skewed. Locus of Control was measured by having participants rate statements using a point scale that ranged from “strongly agree” to “strongly disagree” and the data was drawn from a national probability telephone survey of United States households7. The strengths of this measure are that the statements focused on interpersonal and intrapersonal factors and the demographic characteristics of the survey were nationally representative7. Similar to physical activity, the limitation of this measure is that it was self-reported data so the results may also be skewed.
Limitations and Future Research
One of the limitations of studying the association between SES and depression is that in many research studies gender and age have huge implications in the SES and depression pathway5. Females tend to have higher rates of depression in comparison to men and as an individual gets older rates of depression also increase. Thus it is necessary to look at gender and age as separate groups when studying the effects of SES on depression. Another limitation is that SES is measured differently in each study. There are some researchers that use income to measure SES, and others use education and/or occupation. This may lead to inconsistent associations between SES and depression because the quantitative methods utilized are not standard throughout the literature. Lastly, many research studies measure SES and its effect on depression only in adulthood, which may also lead to inconsistencies in the findings because it does not capture SES throughout an individual’s lifetime. Furthermore, due to the rate of depression increasing with age, it is important to view SES from a life course perspective and not measure it at just one point in time. The focus of future research should be establishing a uniform quantitative method to measure SES and to capture both childhood and adult SES to help understand the effect it has on depression. Depression imposes a public health burden; thus it is essential to recognize the risk factors for this disorder.