## 2 Literature Review
### Research on the Impact of Aging on Productivity  
As the issue of aging becomes increasingly prominent, the impact of aging on productivity has aroused interest from researchers. However, conclusions have not reached a consensus on the relationship between aging and productivity [36]; studies confirmed both negative and positive impacts. From a negative research perspective, aging is seen to inhibit productivity growth mainly through human capital. Specifically, population aging has a depressive effect on human capital, since aging workers are subject to gradual reductions in their cognitive ability and other productivity-reducing effects of aging [37]. An increase in the aging population implies a lower labor supply and higher labor force costs that weaken economic growth and productivity [38]. Moreover, as people age, physical strength declines, leading to a decrease in productive capacity and impaired labor productivity [39]. Productivity also declines with age due to skill obsolescence, that is, the atrophy of skills associated with aging negatively impacts human capital; this is also known as technical skill obsolescence [40]. However, it is argued that these negative impacts can be offset to some extend by introducing non-discriminatory employment policies, improving worksite condition and retraining [41]. From a macro perspective, as the share of older workers increases and due to the widespread opinion that older workers are less productive than younger workers, the aging population is likely to have a negative impact on overall productivity [42]. However, from a positive research perspective, aging is seen to raise total factor productivity (TFP) because of investment in human capital and knowledge accumulation. Specifically, the ever-growing aging population promotes opportunities for an increase in human capital investment to reduce the dampening effect of aging on productivity, i.e., population aging promotes productive capacity [43]. Compared to younger workers, older workers usually have more experience in production and can make fuller use of their experience to increase productivity. For example, [PERSON] et al. [39] investigated the determinants of agricultural TFP in China and found that the elderly population had a significant and positive impact on technical efficiency improvement. Similarly, [PERSON] et al. [43] found that population aging impacted positively agricultural GTFP in China. From a micro perspective, a case study using microdata of the assembly plant of truck manufacturing by [PERSON] and [PERSON] [42] showed that the average age-productivity profile of workers increased until age 65. Although older workers were slightly more likely to make errors, they seldom made severe errors. In other words, the implication here is that older workers can grasp difficult situations and then focus on the important tasks. It is also noteworthy that the increase in labor costs caused by aging may also have a positive effect on TFP. For example, using panel data of 22 emerging countries, [PERSON] et al. [44] explored three mediating effects of labor costs on TFP and found that TFP turned out to be the main channel in which labor costs affect energy intensity. In another study using a panel data set of 30 provinces from 2010 to 2019, [PERSON] and [PERSON] [45] found that the passive increase in labor costs caused by aging significantly promoted GTFP in China. Some researchers have concluded that the relationship between aging and productivity is nonlinear. For example, [PERSON] and [PERSON] [46] found an inverted U-shaped curve between population aging and TFP in China's manufacturing industry. Although various modeling approaches and different datasets have been applied, the abovementioned research shows that conclusions on the impact of population aging on productivity have not yet reached a consensus. A reasonable explanation may be because we assume that observations are independent.