## 3 Influencing Factors and Control Variables
### Control Variables: Education and Income  
Because education embodies a lot of information on economic and social progress such as scientific and technological progress and human capital accumulation, it is an important and essential control variable. Education may reduce ecological consumption by stimulating ecological awareness and increasing pro-ecological practices. Higher education levels would enable individuals to have more access to various scientific information and knowledge to understand complicated environmental issues and identify causes and consequences of ecological crisis [29]. Furthermore, higher levels of education would increase individual willingness to live an ecologically sustainable life such as installing more renewable energy equipment and participating more in recycling activities [23].  
The negative impacts of education may be insignificant because of the attitude-behavior gap [40]. In practice, higher education levels and enough information and comprehension of the ecological crisis may not be transformed into ecologically friendly lifestyles and consumption habits. Moreover, individuals with higher levels of education tend to have high levels of living standards, which often demand higher levels of ecological consumption. Therefore, it is unclear whether education would reduce ecological consumption or not.  
Because the EKC hypothesis is quite well known and the EF is a consumption-side proxy of ecological consumption, income is the most important control variable. The EKC hypothesis implies that there is an inversed U-shaped relationship between ecological consumption and income. At low levels of income, levels of ecological consumption and income tend to increase simultaneously. When income reaches a threshold point, levels of ecological consumption would decrease along with further increases in income levels.  
Based on a panel dataset of 16 Central and Eastern European Countries during 1991-2014, [PERSON] et al. validated an N-shaped relationship between income and the EF [28]. Based on a panel dataset of 26 EU countries for the time period 1990-2013 and employing the sub-footprints of the EF as the indicators of ecological consumption, [PERSON] et al. revealed that the EKC hypothesis is not valid [17]. However, based on a panel dataset of 16 EU countries for the time period 1997-2014, [PERSON] et al. found that a 1% increase in real GDP would reduce total EF by 0.81%, which supports the EKC [27]. The above literature shows that the estimations of the EKC hypothesis for developed countries are inconsistent.  
By employing the EF to measure ecological consumption, estimations of the EKC hypothesis for developing countries are also inconclusive. Based on time series datasets, [PERSON] and [PERSON] confirmed the EKC in India for the time period 1971-2014 [23], and [PERSON] et al. confirmed the EKC in each of the MINT countries during 1971-2013 [24]. Based on panel datasets, [PERSON] et al. validated the EKC in the MINT countries for the time period 1990-2013 [25], but [PERSON] and [PERSON] indicated that economic growth and ecological consumption were positively related in 14 sub-Saharan Africa countries for the time period 1990-2014 [21]. Based on three panel datasets during 1975-2007, [PERSON] and [PERSON] showed that the EKC hypothesis was valid in 15 MENA countries and eight oil-exporting MENA countries, and that the relationship between economic growth and ecological consumption was U-shaped in seven non-oil-exporting MENA countries [26].