Abstract:     In 2015, social security and entrepreneurship as two main forces for economic development were proposed at the NPC and CPPCC. However, inconsistency arises among researchers regarding whether social security can promote regional economy. Some scholars argue that social security can promote regional economy by stabilizing social development and improving human capital investment. Other scholars present that social security may dampen regional economy as increased government spending on social security has a crowding-out effect on local firms′ innovation support. The contradictory findings have contributed to the "mystery of social security" that perplexes policymakers and academics, and a new solution is urgently needed.
    The entrepreneurship provides a new perspective to open the "black box" of social security and regional economic growth. Entrepreneurship as another important driver of economic development is closely related to social security. Some studies find that social security can promote entrepreneurship, while others point out that social security has a negative impact on entrepreneurship. According to existing studies, these contradictory views between social security and entrepreneurship may be due to a lack of distinction between different types of entrepreneurial activities. Therefore, based on entrepreneurial motivation, this paper distinguishes two types of entrepreneurial activities—opportunity entrepreneurship and necessity entrepreneurship. Opportunity entrepreneurship refers to activities that individuals actively engage in when they perceive business opportunities. Necessity entrepreneurship refers to a type of activity forced by the lack of basic resources and security. 
    Drawing on the theory of resource dependence and the theory of the hierarchy of needs, we consider that social security has different effects on opportunity entrepreneurship and necessity entrepreneurship. On the one hand, with the increase of social security level, the pressure of unemployment, medical treatment, and pension that potential entrepreneurs face will be eased. In this case, the possibility that potential entrepreneurs devote themselves to necessity entrepreneurship to obtain resources and security for living will be decreased. It implies that social security may have a negative impact on necessity entrepreneurship. On the other hand, the rise of social security level creates a "safety net" for entrepreneurs, which improves the demand level and self-realization demand of entrepreneurs. It implies that social security may have a positive impact on opportunity entrepreneurship.
    Different entrepreneurial activities have significant different influence on regional economic growth. The small scale and weak innovation capability of the necessity entrepreneurship result in a low degree of product and service novelty. Therefore, the contribution of necessity entrepreneurship in the number of jobs, product innovation, and the market expansion is relatively limited. Therefore, necessity entrepreneurship may have a negative influence on regional economic growth. Opportunity entrepreneurship is a kind of initiative behavior that aims to transform business opportunities into marketization. It may promote the development of a product, business model innovation, market expansion, and industrial upgrading, and thus have a positive influence on regional economic growth.
   Based on the above arguments, social security may affect different types of entrepreneurial activities, in turn, different types of entrepreneurial activities may have different effects on regional economic growth. Therefore, we consider the question that whether opportunity entrepreneurship and necessity entrepreneurship can reveal the internal mechanism of social security affecting regional economic growth. In order to explore the internal mechanism, this paper divides entrepreneurial activities into opportunity and necessity and discusses their different mediating role between social security and regional economic growth respectively. The regional technology and capital level can affect the choice of different types of entrepreneurships because the vitality, demand and ability of potential entrepreneurs are different at various levels. Therefore, this paper divides four regions according to the level of technology and capital to explore the mediating role of opportunity and necessity entrepreneurship between social security and regional economic growth in different regions.
   This paper uses provincial dynamic panel data from 2009 to 2016 to explore the mentioned problems. The results show that: opportunity entrepreneurship and necessity entrepreneurship play a significant partial mediating role in the relationship between social security and regional economic growth, and in regions from low to high technological and capital level, the impact of social security on regional economy changes from simplification to diversification. In short, by introducing opportunity entrepreneurship and necessity entrepreneurship into our theoretical framework, this paper reveals the "black box" of the impact of social security on regional economy and solves the "mystery of social security" to a certain extent. 
   Our study offers important implications for policy makers. First, the government should accelerate the transformation from necessity entrepreneurship to opportunity entrepreneurship to promote the positive interaction between social security and economic development. Specifically, to reduce the negative impact of necessity entrepreneurship on regional economic growth, the government should increase their spending on unemployment security. Meanwhile, the government should enhance the positive role of opportunity entrepreneurship in promoting regional economic growth. What′s more, the government expenditure on pension security should be cautious, which needs to be weighed in many aspects to ensure the transformation of entrepreneurial activities from type of necessity to opportunity. Finally, in different regions, the government should make efforts to create a favorable market environment that is conducive to social security to play a promoting role and avoid restraining role. In regions with high level of technology and capital, the government should provide more learning and exchange opportunities for entrepreneurs, and stimulate more potential entrepreneurs to choose opportunity entrepreneurs. In regions with low level of technology and capital, the government should improve the living conditions of potential entrepreneurs, and encourage them to pursue a higher level of self-realization.
   This paper makes contributions in three ways. First, we introduce entrepreneurship into the framework of social security and regional economic growth, revealing the mechanisms of the two types of entrepreneurial activities through which social security affects regional economic growth. Second, we find that social security has opposing effects on different types of entrepreneurial activities. This provides significant implications for policymakers to accelerate the transformation from necessity entrepreneurship to opportunity entrepreneurship in the context of economic transition. Finally, we identify the rule of social security impact on regional economy based on the analysis of different technology and capital regions, which provides a possible perspective for the follow-up study to unravel the mystery of social security, and extends research on the relationship between social security and regional economy.

Key words: social security, necessity entrepreneurship, opportunity entrepreneurship, regional economic growth