My research focuses on the dynamics of population health and aging, and encompasses two interconnected areas: (1) social structure and inequality in health; and (2) population process of aging and mortality. In the first area, I have investigated health consequences of various social structures, institutions, and policies (e.g., income inequality, medicine, marriage, marriage market, and one-child policy); the role of selection vs. causation in the health production process (e.g., marriage and health, education and health); and the trends of socioeconomic and demographic disparities in health. In the second area, I have investigated population heterogeneity (at geographic level, e.g., country, and group level, e.g., race) and temporal trends (age, period, cohort) in mortality and life span; the role of cohort evolution and mortality selection in the cohort trend of demographic and biological aging; the origins and pathways of health and mortality over life course; and heterogeneity in the life course of obesity and its effect on mortality. 

Methodologically, I am interested in developing and evaluating statistical and demographic methods to investigate the interplay of temporal changes, population dynamics, and population heterogeneity on the trends of health / mortality disparities and life expectancy gap.

​Social Structure and Inequality in Health
I study a broad range of social determinants of health from macro conditions and process (e.g., income inequality, medical expansion, one-child policy), meso environment (e.g., work environment, marriage market), to individual-level sociodemographic status (e.g., marriage, education). I am particularly interested in the group heterogeneity, lifelong consequences, and temporal changes of these effects. The first line of this work seeks to understand the health consequence of income inequality, education, and occupation. I studied the gender-specific effect of income inequality on health (SSM 2009) and brought the temporal perspective to bear on the lagged effects of income inequality on mortality, which explained the mixed findings in this controversial topic (SSM 2012). I also studied the macro social and economic causes of widening health disparities over time in the U.S. with co-author Linda George (Duke University) (SSM 2012) and the widening obesity disparities among Chinese children with co-authors Wei He, Sherman James and Givonna Merli (Duke University) (AJPH 2014). Other work unveils the contribution of selection factors to the education - health link (SSR 2017) and lifelong health consequence of job dissatisfaction (SSM 2017).

The second line investigates the effects of marriage, household structure, and marriage market on health. With Patricia Thomas (Purdue University), I examine whether the married tend to overestimate their health and whether marriage is universally beneficial for people located in different positions of health spectrum (JHSB 2013). We find the protective effect of marriage against mortality decreases with deteriorating health and the married tend to overestimate their health status. These two phenomena (diminishing protection and overestimation) contribute to, but do not completely explain, each other. My recent work co-authored with Dmitry Tumin (Ohio State University) investigates the heterogeneity of the marriage effect across the propensity to marry using heterogeneous treatment effect model which then is able to mitigate both pre-marriage selection bias and marriage effect heterogeneity bias under the assumption of ignorability (JMF 2018). Other work investigates the long lasting effect of sex ratio at sexual maturity on male mortality with Emma Zang (Duke University) (SSM 2018) and the impact of one-child policy and sibling composition on children's obesity risk with Wei He (Duke University) (APS 2017).

The third line tackles a prominent dynamic in today's societies: medical expansion. As the biomedical model becomes increasingly dominant in the explanation of health, illness, and other human problems and behavior, medicine has dramatically expanded in the last several decades in the developed countries. But how this expansion process has impacted population health is not sufficiently studied. With Linda George (Duke University), I have investigated the process of medical expansion in the OECD countries in the last several decades, which is represented by expansions in three major components of the health care system: rising medical investment, medical professionalization/specialization, and expanded pharmaceutical industry. I study how medical expansion may affect population health and how this effect may be contingent on the level of socioeconomic development and the stage of epidemiological transition (JHSB 2018); the lay public's confidence in medicine in the era of medical expansion (SSR 2015); and the role of consumers in this expansion process (SSR 2015).


Population Process of Aging and Mortality  
I attempt to understand the heterogeneous and changing speed of aging and consequent morbidity and mortality patterns across individuals, birth cohorts, countries, and over time from the perspectives of mathematical / statistical demography and biodemography of aging. The first line of this work seeks to model population heterogeneity (at geographic level, e.g., country, and group level, e.g., race) and temporal trends (age, period, cohort) in mortality and life span. With Yang Claire Yang and Kenneth Land, I substantially extend the classical Strehler-Mildvan (SM) general theory of mortality and aging by synthesizing it with ecological demography. I study heterogeneity in the SM model and the ecological causes and implications of this heterogeneity on expected maximum survival age based on historical mortality data of 42 countries (Demography 2011). I have also investigated the evolving mortality trend and mortality variation among developed countries over age, period and cohort in the context of cohort evolution, mortality selection, epidemiological transition and population aging (PRPR 2016). Recent work with Emma Zang (Duke) uncovers the racial disparities in cohort patterns of mortality in the United States (IJE 2018). Ongoing work with Scott Lynch (Duke) uses an innovative approach to delineate the cohort and period effects in mortality.

The second line examines the cohort forces in the life course pattern of health, aging, and mortality. I have systematically investigated the trend of the rate of demographic aging and the rate of biological aging (i.e., senescence) across birth cohorts born since mid-18th century among 7 developed countries. I study the effects of cohort evolution and mortality selection on this trend and the implications for the study of limits on the human lifespan and evolution of aging (Demography 2014). The joint effect of cohort evolution and mortality selection on cohort life-course mortality pattern leads me to further investigate its consequence on epidemiological transition in less developed countries and the implications on the high level of chronic diseases burden in these countries during the age of degenerative and man-made diseases. My work with Siwei Cheng (NYU) uses simulation technique to generate cohort level aggregate mortality pattern from individual level mortality curve which enables me to test several fundamental hypotheses in biodemography of aging (Biodemography and Social Biology 2019). Recent work with Emma Zang (Duke) investigates the early and middle adulthood mortality pattern across birth cohorts in the U.S. (IJE 2018). My ongoing work studies the role of cohort forces in the origins of health and mortality.

The third line addresses to what extent will the rising obesity epidemic affect future increases in life expectancy. Different from most studies in this area, I bring population heterogeneity and life course perspectives to bear on the study of obesity impact on mortality and life expectancy. The effect of obesity is contingent on people’s health endowment and socioeconomic resources, which causes everyone to be unequally harmed by obesity (SSM 2012). Furthermore, the timing of the transition to obesity status and duration of obesity also matter for the death outcome. My work with Dmitry Tumin and Zhenchao Qian (Brown University) captures heterogeneity in the body mass index (BMI) trajectories among older populations and suggests that BMI trajectories are more predictive of mortality risk than static BMI status (AJE 2013). With Dmitry Tumin, I further study the origins of adult obesity in early life (e.g., birth region and family background) (BMC-Public Health 2015). My recent work with Jonathan Dirlam (Ohio State University) investigates how selection biases (mortality selection bias, healthy participant effect and reverse causality) may affect the evolving impact of obesity on mortality risk over the life course (Plos One 2016).  


Modeling Trends in Health Disparities and Life Expectancy Gap
​I bring demography and population dynamics to the study of trend in health disparities within the context of social, technological and epidemiological changes. Any historical trend in inequality of social outcomes, e.g., health, is confounded with another two related temporal factors: life course and cohort. With Yang Claire Yang (UNC-Chapel Hill) and Kenneth Land (Duke University), I have developed an integrated model to filter the contributions of the above confounded factors from the temporal trend in health disparities, but this model should be broadly applicable to the study of social inequality in many different substantive contexts (ASR 2011; PRPR 2016; Handbook of Causal Analysis for Social Research 2013). Moreover, the temporal trend in inequality (e.g., health disparities) can increase or decline as a result of two distinct mechanisms: the gradient or set of differential relationships of social statuses to social outcomes can change or the composition of the population with respect to social statuses can change (SSR 2012). I currently focus on evaluating the implications of changing population compositions on the cohort trends of health disparities. I am developing a semi-parametric method to mitigate lagged selection bias in trend studies. This project is funded by NCHS R03 grant (Research and Methods in Health Statistics RFA-SH-16-001)​. I am also currently developing a counterfactual simulation method to evaluate mortality selection effect in the trends of health disparities and gap in life expectancy.