Introduction
The onset of depressive disorders inflicts significant suffering on individuals, with major depressive disorders (MDDs) being a typical category characterised by one or more episodes. MDD was the fifth top cause of years lived with disability in 2016.1 MDD also played a role in contributing to the burden associated with suicide and ischaemic heart disease.2 The prevalence rates of MDD vary significantly across nations, but the overall rate is approximately 6%.3 The lifetime risk of developing MDD is 15%–18%,4 indicating the common occurrence of MDD that severely limits psychosocial functioning and diminishes the quality of life; nearly one-fifth of individuals experience it at some point in their lives.5
Since 2000, the visibility of depressive disorder in China has steadily increased. The cumulative incidence rate of MDD in rural areas of Northeast China is 3.9%, with a significantly higher rate among females (5.3%) than males (2.9%).6 A study conducted between 2001 and 2005, based on epidemiological surveys in four Chinese provinces, reported a prevalence rate of depressive disorder at 6.1% (with MDD at 2.1% and dysthymia at 2.0%). Depressive disorder is more prevalent among females, rural residents and those aged 40 and above.7 The 2013 China Mental Health Survey revealed a depressive disorder prevalence of 6.8% (with MDD at 3.4% and dysthymia at 1.4%), with a lifetime prevalence of 3.6% (MDD at 2.1% and dysthymia at 1.0%).8 This research will focus on MDD, considering its more severe symptoms and higher prevalence rate.
Depressive disorder correlates with age, period and cohort factors. Adolescent depressive disorder raises the risk of depression and anxiety later in life.9 Depressive disorder can contribute to medical diseases and expedite biological ageing, while diseases can heighten the likelihood of depressive disorder in older adults.10 Historical events, especially in the context of globalisation, can profoundly affect individuals across borders. For instance, a study highlighted a period effect on depressive disorder prevalence among Canadians older adults, with peaks in 2001, 2008 and 2012, coinciding with the ‘September 11 terrorist attacks’, the global financial crisis and the volatility of the Canadian stock market.11
Besides, depressive disorder presents notable gender disparities. While its incidence generally rises from childhood to adolescence,12 females face double the risk of diagnosis compared with males by early adulthood.12 Women also experience heightened depressive disorder risk during pregnancy and postpartum.13 Conversely, some studies suggest when males experience depression, they may exhibit symptoms different from current diagnostic criteria, potentially nullifying gender differences in depressive disorder prevalence when alternative symptoms are combined with traditional symptoms.14 All the research underscores the importance of considering gender in MDD research for effective prevention and treatment strategies. Thus, this study will explore MDD trends by gender.
Decomposing the time trend of disease onset into age, period and cohort effects helps reflect the underlying societal, economic, demographic, health outcome and behavioural changes. Some research employing the age-period-cohort (APC) analysis tool, which is based on the decomposable and indecomposable parts of the APC model,15 revealed a widespread decrease in the age-standardised incidence rate of depressive disorder but an increase in the incidence rate among the older adults in China.16 Using a similar method, another study discovered an inverted U-shaped curve with the highest MDD risk observed in the 1951–1955 Chinese birth cohort.17 However, there is a deficiency in studying the variations in the onset of MDD within the cohort and a significant gap in exploring and synthesising the generational patterns of MDD incidence.
The leading challenge of APC model implementation lies in the identification problem. Scholars have approached this issue from different perspectives, attempting to address it by imposing explicit or latent constraints on model parameters based on theoretical foundations or external information.18 Some research pointed out the methodological and theoretical limitations of previous APC models, which solely focused on between-cohort differences and assumed that such differences remain fixed throughout the entire life course, overlooking the dynamic life course within cohorts.19 Building on this, the age-period-cohort-interaction (APC-I) model was proposed.
Employing the APC-I model, we aimed to analyse the temporal trends and cohort variations of MDD within the general population and separately for females and males, considering age, period and birth cohort dimensions. We will also discuss the generational patterns and shifts in MDD.