Examining the reversal hypothesis on socioeconomic status and noncommunicable diseases, and gender disparities in China: a nationwide longitudinal analysis between 1991 and 2020.

Background: The term "reversal hypothesis" has been attached to the observation that the socio-economic status (SES) gradient of non-communicable diseases (NCDs) tends to reverse over time as country's economic and social development progresses. We examined this hypothesis in China between 1991 and 2020, to test whether a divergence in the experiences of men and women can be identified and to explore the possible contributors to any disparity. Methods: Using nine waves of China Health and Nutrition Survey (CHNS, 1991-2015) and six waves of the China Family Panel Studies (CFPS, 2010-2020), we investigated the SES-NCD gradient among participants aged 18 years and older. We examined the relationship between self-reported doctor diagnosed NCDs and SES proxied by formal educational years by gender using multivariable logistic regression and Blinder-Oaxaca decomposition models. Findings: Prevalence of a list of NCDs in the overall population increased from 3.52 % (1991) to 13.72 % (2020). The association between SES and NCDs has shifted from positive to negative over time, shifting earlier among women compared to men. For both men and women, the difference in prevalence between high and low SES groups was mostly explained by the socio-demographic and risk behaviour variables included in the models: age, being overweight/obesity, employment status, residency, and region. However, the contribution size of these factors and consistency was different by gender and changed over time. Conclusions: China has experienced a reversal in the SES-NCD gradient which occurred first among women. The explanations of SES differences are complex and differ by gender.