Abstract:    Megaprojects provide basic public services and are of great significance to national and regional development. However, compared with general projects, the management efficiency of megaprojects is generally low. Previous studies have pointed out that the reason for the "performance paradox" of megaprojects is not solely due to the growth of construction scale and the improvement of engineering technology difficulty, but also managers′ lack of ability to master megaprojects. The definition of top management team (TMT) is rooted in upper echelons theory. Compared with individual leader, the TMT as a whole has a more significant impact on organizational operation, task promotion and management performance. Therefore, an in-depth study of the impact of the TMT on project performance plays an important role in improving project management efficiency. As the "mirror" of a TMT′s inherent characteristics, a TMT′s demographic characteristics will affect its decision-making judgment and behavior style and will further affect organizational performance. Among them, the effect with heterogeneity is more apparent. Unfortunately, studies on TMT heterogeneity are mostly concentrated in the field of business administration and take permanent groups as the object of discussion. There are few studies in the field of engineering. In addition, given the huge role in production and life and complexity of megaprojects, the government will give some support, but whether government support is conducive to the use of TMT capabilities and improve organizational effectiveness remains to be confirmed. Based on the actual context of megaprojects in China, this paper comprehensively analyzes the impact of megaproject TMT heterogeneity on project performance, and explores the institutional environment that exerts the characteristics of TMT heterogeneity. It not only enriches the research content of megaproject fields, but also provides empirical results for the government to build a more reasonable and efficient TMT in the construction of megaprojects.
    This paper aims to explore how TMT heterogeneity affects project performance through the moderating role of government support. Therefore, the hypotheses of the relationships among TMT heterogeneity, government support and project performance are proposed, and a theoretical model is built to analyze the impact of TMT heterogeneity on project performance. A regression analysis is employed to test these hypotheses with data collected from 89 mega infrastructure projects provided from the Mega Projects Case Study and Data Center. The results indicate that both age and education level heterogeneity have inverted U relationships with project performance; academic background heterogeneity and project performance show a positive relationship; and administrative level heterogeneity and project performance have no relationship. Furthermore, government support plays a moderating effect in the impacts of age, education level and academic background heterogeneity of TMT on project performance.
    This paper outlines the theoretical analysis and empirical research on the impact of TMT heterogeneity on project performance. At the same time, it discusses the moderating effect of government support in the influence of TMT heterogeneity on project performance. The study obtained the following four conclusions.
    Firstly, TMT age and education level heterogeneity have an inverted U relationship with project performance. This conclusion breaks through the previous linear correlation research framework that "the higher the heterogeneity of TMT, the better the team performance" and more truly depicts the particularity of TMT in megaprojects. When TMT age and education level heterogeneity are low, the quality of joint decision-making is decreased due to the lack of collision of ideas among members. When age and education level heterogeneity are high, endless conflicts and confrontations are inevitable. In particular, the TMT is a temporary organization whose members represent different stakeholder groups and lack the basis for cooperation with each other. When conflicts arise, the negative effects are exacerbated.
    Secondly, academic background heterogeneity has a significant positive impact on project performance. TMT members from different academic backgrounds not only provide more complete professional knowledge for constructing megaprojects, but they also broaden the whole team′s thinking and avoid thought limitations.
Thirdly, there is no significant relationship between administrative level heterogeneity and project performance. The power within TMTs with high administrative level heterogeneity is more concentrated. TMT members with lower administrative levels lack discursive power when the TMT makes joint decisions. The greater the difference in administrative level, the more difficult the cooperation between TMT members of different levels. TMT members of low administrative level tend obey, while the members of the high administrative level tend to be dictatorial. The advantages of high administrative level heterogeneity are therefore offset.
    Finally, the moderating effect of government support is partially proved. Government support significantly strengthens the influence of TMT age, academic background and education level heterogeneity on project performance. With the increasing level of government attention to TMT, the rights and obligations of TMT members will be clearer, and the decisions made by TMT members will be more rational. Members prefer to give full play to their heterogeneity within the framework of the procedure.
    The role of TMT members is not only to provide support and coordinate public affairs for megaprojects but also to make correct decisions. When establishing the megaproject TMT, the age and education level heterogeneity should be "moderate". In terms of academic background, TMT members with different backgrounds should be selected. For government, the institutional improvement brought by government support and the regulation of TMT member behavior will be more conducive to project construction and reduce the risk. Therefore, the government can make dynamic adjustments based on actual conditions to play a positive role in team heterogeneity.
     The contributions of this paper are as follows: (1) In theory, this paper introduces the upper echelons theory into the field of engineering management, which broadens the scope of the theory′s applications. At the same time, the research in this paper also indicates that exploring TMT heterogeneity from a single perspective does not reflect its deep role in project performance. (2) In practice, this paper reveals, for the first time, the relationship between TMT features and project performance and provides guidance for TMT building. (3) The conclusion of this paper can be generalized to a certain extent. It is also applicable to some other temporary organizations, such as the leading group formed to deal with emergencies and the temporary scientific research group formed to overcome technical difficulties.

Key words: mega infrastructure projects, top management team, heterogeneity, project performance