While life expectancy is a key indicator in public health and demography, it may obscure unequal experiences of different population groups. By contrast, lifespan inequality captures variations in age at death, offering a broader view of health disparities. Focusing on lifespan inequality enables policymakers and researchers to identify vulnerable populations, design more equitable health and social policies, and evaluate progress toward reducing inequities. This thesis is comprised of three interrelated studies that examine life expectancy and lifespan inequality from different perspectives. The first study explores state-level variations in life expectancy and lifespan inequality in the US over a 55-year period, employing Theil’s entropy index to measure lifespan inequality and a Panel-Corrected Standard Errors (PCSE) model to assess associated socioeconomic and demographic factors. The analysis reveals a significant negative correlation between life expectancy and lifespan inequality across states. Educational attainment, health insurance coverage, and physician density show negative associations with lifespan inequality, whereas violent crime rates, smoking prevalence, and income inequality exhibit positive associations. The second study investigates the impact of homicide on life expectancy and lifespan inequality at the state level in the US from 1968 to 2020, focusing on demographic, socioeconomic, and policy determinants. Utilizing data from the US Mortality Database and cause-eliminated life tables and Panel-Corrected Standard Errors (PCSE) models, the study reveals that Southern states experience the most substantial impacts on both life expectancy and lifespan inequality, while the Northeast exhibits comparatively lower effects. Males consistently face greater impacts than females. Key findings underscore systemic inequities: a higher percentage of Black populations and individuals aged 25–34 correlate with greater homicide-related reductions in life expectancy and increased lifespan inequality. Educational attainment, particularly the high school graduation rate, helps mitigate these effects. Corrections and judicial expenditures affect both life expectancy and lifespan inequality. In contrast, police and health spending are associated with decreases in lifespan inequality but no change in life expectancy, whereas welfare expenditures correlate with increased lifespan inequality but are correlated with life expectancy only for the total population. The final study conducts a comparative analysis of the demographic impacts of three major pandemics: the 1918 influenza pandemic, the HIV/AIDS epidemic, and the COVID-19 pandemic on global life expectancy and lifespan inequality. The study finds that the 1918 influenza pandemic caused the most pronounced reductions in life expectancy, especially among young adults, thereby leading to substantial lifespan inequality. By contrast, HIV/AIDS had a gradual yet enduring impact, predominantly affecting young and middle-aged adults and exacerbating health disparities in regions with limited healthcare resources. The COVID-19 pandemic primarily affected older populations, producing smaller reductions in life expectancy but uniquely decreasing lifespan inequality due to the concentration of mortality among the elderly. Gender-specific effects varied: while the 1918 influenza and COVID-19 pandemics showed relatively uniform gender impacts, HIV/AIDS disproportionately affected women. Collectively, the findings across these studies underscore the need to address the interconnected issues of life expectancy, lifespan inequality, and systemic disparities. Targeted gender-specific policies and public expenditures (e.g., increased judicial and healthcare funding) have the potential to reshape mortality patterns and mitigate disparities. Pandemic responses should be tailored to protect vulnerable age and demographic groups by ensuring equitable access to healthcare and vaccines.