Epidemiology and Disparities

Epidemiology studies the patterns, causes, and effects of health and disease in populations, using a specific measurement vocabulary. Incidence is the number of new cases of an illness per population at risk, in a given amount of time. Prevalence is the number of cases of an illness overall — whether new or chronic — per population, in a given amount of time. Morbidity is the burden or degree of illness associated with a given disease. Mortality refers to deaths caused by a given disease. This vocabulary underlies how sociologists and public health researchers describe one of the clearest patterns in health research: health and healthcare outcomes track social class.

Key Takeaways

• Incidence (new cases per population at risk, over time) differs from prevalence (all cases, new or existing, per population, over time); morbidity (illness burden) differs from mortality (deaths caused by disease).

• Health tracks social class: egalitarian societies have the best outcomes, and the UK's Black Report (1980, Sir Douglas Black) found that class-based health gaps persisted — and widened — despite the welfare state.

• The Second Sickness (Howard Waitzkin, 1983) describes health outcomes worsened by social injustice, illustrated by the professional-vs-working-class life expectancy gap.

• Low-income groups face worse self-rated health, more life-shortening chronic disease, higher suicide and homicide rates, and higher infant mortality.

• The U.S. lacks centralized healthcare; the Affordable Care Act (2010) targets cost and coverage, while Medicare (age/condition-based: 65+, end-stage renal disease, ALS) and Medicaid (need-based) are the main public insurance programs — though provider avoidance of both limits access.

• Disparities in care stem from multiple independent mechanisms: access, nutrition, sense of control, smoking/obesity/inactivity, provider bias (including weight bias), language/cultural barriers, and discrimination against women and LGBT patients.

MCAT Callout — Incidence vs. prevalence: incidence counts only new cases arising in a time window — it tracks how fast a disease is spreading. Prevalence counts all cases, new and existing, at a given time — it tracks how much disease burden exists overall. A vignette describing newly diagnosed cases over a study period is testing incidence; one describing the total number of people currently living with a condition is testing prevalence.

MCAT Callout — Morbidity vs. mortality: morbidity describes the burden of illness — how sick a population is, whether or not anyone dies. Mortality describes deaths caused by a disease. A vignette describing symptom severity, disability, or quality-of-life impact is testing morbidity; one describing death rates is testing mortality.

Inequities in Health

It's been clearly shown that poor environmental and social factors negatively impact health. Health is dependent on geography, social factors, and economic factors, and socioeconomic improvements lead to increased general health. The best health outcomes are seen in egalitarian societies — those with the smallest gaps between social classes.

Modern welfare states attempt to eliminate social differences in health, but the UK's Black Report (1980), chaired by Sir Douglas Black, found that class differences in health persisted despite the National Health Service and the broader welfare state — the mortality gap between the country's highest and lowest social classes had actually widened, not narrowed. Consistent with this finding, professional groups have longer life expectancy than the working class.

Medical sociologist Howard Waitzkin named this pattern the Second Sickness in his 1983 book The Second Sickness: Contradictions of Capitalist Health Care — the exacerbation of health outcomes caused by social injustice. The class-based gap in life expectancy between professional and working-class groups is a direct illustration of the Second Sickness: a "second," socially produced layer of illness stacked on top of whatever biological disease a person already has.

Low-income groups are more likely to have poorer health, be uninsured, and die younger than middle- or high-class adults. Poverty, in combination with a culture of inequality, leads to worse health outcomes, and these effects run across age, gender, and racial or ethnic boundaries. Members of the lower class are roughly four times more likely to view themselves as being in worse health compared to members of the affluent class, and they're much more likely to develop life-shortening diseases such as lung cancer, diabetes, and heart disease. Lower-class individuals are also more likely to commit suicide and to die from homicide. Infant mortality rate is likewise much higher in some populations.

Inequities in Healthcare

Quality healthcare tends to favor those in higher social classes. America is one of the few industrialized countries that doesn't run healthcare through a central government. The Affordable Care Act, enacted in 2010, attempts to address this problem — it reduces the overall cost of healthcare and increases the coverage and affordability of insurance.

Medicare covers patients over 65, along with those who have end-stage renal disease or ALS. Medicaid covers patients who are in significant financial need. Some doctors won't accept patients covered by these two public insurance programs, and many doctors won't open practices in low-income neighborhoods. As a result, individuals in the lower class are less likely to seek medical attention, and more likely to rely on overcrowded clinics when they do.

MCAT Callout — Medicare vs. Medicaid: Medicare is age- and condition-based — it covers people 65 and older, plus people with end-stage renal disease or ALS, regardless of income. Medicaid is need-based — it covers people in significant financial need, regardless of age or diagnosis. A vignette describing a 70-year-old patient is pointing toward Medicare; one describing a low-income patient of any age is pointing toward Medicaid.

There are several primary reasons low-income groups have higher mortality rates: poor access to quality medical care, poor nutrition, and feeling less in control of life circumstances. The poor are also more likely to smoke, be overweight, and less likely to engage in physical activity. Minorities and low-income groups tend to face greater barriers to care and poorer quality of care when they do receive it, and culture and non-native-language barriers act as additional obstacles to diagnosis and treatment.

Other patient characteristics can also affect how patients are treated. Obese patients are less likely to be recommended an effective weight-loss program, since providers may inaccurately assume they simply lack the willpower to lose weight; obese patients are also less likely to keep a consistent doctor due to strained relationships, and less likely to receive quality preventative care and screenings.

Women tend to be favored by the healthcare system in some respects — they're more likely to be insured, they utilize services more than men, and they receive more services per visit than men do, likely reflecting women's higher morbidity rates. However, women are also more likely to be delayed or unable to obtain necessary medical care, dental care, and prescription medicines. LGBT men and women, meanwhile, are still heavily discriminated against in healthcare settings due to remaining prejudices and homophobia.

Why Epidemiology and Disparities Matters for the MCAT

MCAT questions on epidemiology and health disparities typically ask you to apply the right measurement term or identify the mechanism behind a described disparity:

• Choose the right epidemiological measure — incidence vs. prevalence, or morbidity vs. mortality — based on whether a passage describes new cases, total cases, illness burden, or deaths.

• Identify Medicare vs. Medicaid eligibility from a patient's age, diagnosis, or income described in a vignette.

• Recognize the Second Sickness when a passage links a health outcome to social injustice rather than biology alone.

• Distinguish mechanisms behind a health disparity — access to care, nutrition, psychological sense of control, provider bias, language/cultural barriers, or discrimination — rather than assuming disparities are only about insurance status or cost.

Common MCAT Mistakes

• Treating incidence and prevalence as interchangeable. Incidence counts only new cases arising in a given time window; prevalence counts all cases, new and existing, at a given time. A passage describing a study's newly diagnosed cases is testing incidence, not prevalence.

• Mixing up morbidity and mortality. Morbidity is the burden of illness — how sick a population is; mortality is deaths caused by a disease. A passage describing disability or quality-of-life impact, with no mention of death, is testing morbidity.

• Assuming Medicare is need-based like Medicaid. Medicare eligibility is age- and condition-based (65+, end-stage renal disease, ALS) regardless of income; Medicaid eligibility is need-based regardless of age or diagnosis. These are two independent eligibility tracks, not two tiers of the same program.

• Attributing health disparities to a single cause, such as insurance status alone. The article describes multiple independent mechanisms — access to care, nutrition, sense of control, provider bias (including weight bias), language/cultural barriers, and discrimination — any of which can drive a disparity even when insurance coverage isn't the issue described in the vignette.

MCAT-Style Concept Check

Question: A researcher publishes two figures from the same population in the same year: one reporting the number of people newly diagnosed with a condition during the year, and another reporting the total number of people living with that condition, including those diagnosed in prior years. These two figures respectively represent:

• A) Incidence and prevalence

• B) Prevalence and incidence

• C) Morbidity and mortality

• D) Mortality and morbidity

Answer: A

Explanation: The first figure — new diagnoses arising within the year — is incidence, which counts only new cases per population at risk in a given time window. The second figure — everyone currently living with the condition, new and prior diagnoses combined — is prevalence, which counts all cases, new or existing, per population. Morbidity and mortality are a different pair of measures entirely: morbidity describes illness burden and mortality describes deaths, neither of which is what's being counted here (case counts, not severity or death).

FAQ

What's the difference between incidence and prevalence?

Incidence is the number of new cases of an illness per population at risk, in a given amount of time — it tracks how fast a disease is spreading. Prevalence is the number of cases overall, new or chronic, per population, in a given amount of time — it tracks the total disease burden that exists.

What's the difference between morbidity and mortality?

Morbidity is the burden or degree of illness associated with a disease — how sick a population is. Mortality refers to deaths caused by that disease. A passage about symptom severity or disability is testing morbidity; a passage about death rates is testing mortality.

Who is covered by Medicare versus Medicaid?

Medicare covers patients over 65, along with those who have end-stage renal disease or ALS, regardless of income. Medicaid covers patients who are in significant financial need, regardless of age or diagnosis. Some doctors decline to accept patients covered by either program, which can limit access for the populations these programs serve.

What is the Second Sickness?

The Second Sickness is a term coined by medical sociologist Howard Waitzkin in his 1983 book The Second Sickness: Contradictions of Capitalist Health Care, describing the exacerbation of health outcomes caused by social injustice — a socially produced layer of illness added on top of whatever biological disease a person already has, illustrated by the persistent life-expectancy gap between professional and working-class groups.