## 3 Theoretical Framework  
Existing studies offer a wide range of concepts and definitions related to sustainable developments in the context of procurement and purchasing [24], such as green public procurement [25; 26; 27], sustainable public procurement [9; 28; 29; 30; 31], green purchasing [32; 33], sustainable supply chain management [34; 35; 24; 36], environmentally friendly supply chain management [37], environmentally responsible public procurement [38], green procurement behaviour [39] or sustainable procurement behaviour [9; 11]. These concepts sometimes describe the same developments (e.g., green public procurement and environmentally friendly procurement) and sometimes different phenomena (e.g., sustainable public procurement and sustainable supply chain management). In this study we examine SPP, which can be viewed as part of the broader concept of sustainable supply chain management (SSCM) [24], since several parts of the supply chain can be made more sustainable via procurement. SPP encompasses the integration of a concern for social and economic impacts in the procurement by public organisations [16; 40]. [PERSON] and [PERSON] [13] describe SPP as \"the acquisition of goods and services in a way that ensures that there is the least impact on society and the environment throughout the full life cycle of the product\". This is much in line with the European Commission's [41] definition of SPP as \"a process by which public authorities seek to achieve the appropriate balance between the three pillars of sustainable development--economic, social and environmental--when procuring goods, services or works at all stages of the project\". In line with these definitions [13; 16; 40; 41], SPP can be viewed as an umbrella concept, encompassing policies on both social and environmental procurement. SPP thus encompasses green public procurement (GPP) but is not synonymous with GPP. Prior research into GPP and SPP often focuses on the identification of factors that influence their implementation [8; 10; 42; 43; 44; 45]. Many studies identified structural factors that act as drivers or barriers, such as rules and regulations, budgets or the financial viability of sustainable alternatives [8; 37; 42; 46]. However, studies also show that the characteristics of the people involved in the procurement project,as well the organisation they are working in, affect the implementation of sustainable procurement. [PERSON] [10] found that individual level factors, such as commitment or ability, had the strongest impact on the implementation of GPP. Being willing and able to procure in a sustainable or environmentally friendly manner has been found to increase the implementation of both SPP and GPP. For example, [PERSON], [PERSON], [PERSON], and [PERSON] [47], found knowledge and expertise to be important in SPP; procurers have to be aware of sustainable procurement practices and have the knowledge to apply relevant rules and regulations. Furthermore, [PERSON] and [PERSON] [16] found that organisational incentives play a role in the implementation of SPP. [PERSON], [PERSON], [PERSON] and [PERSON] [39] found that the stronger the ability to control their own resources was, the more willing they were to implement green procurement. [PERSON] [10; 43] concluded that affective commitment to change increases the implementation of GPP. If a public procurer truly identified with green public procurement and it was in line with their own personal vision, the implementation of GPP increased. The more people are motivated to procure in a green manner, the more they are likely to actually do so (and thus perform better with regard to sustainable procurement). In the procurement and purchasing literature, relatively little is known about the role that these individual level factors play in procurers' performance in implementing new procurement initiatives, such as GPP and SPP. In the HRM literature, however, the relationship between employee behaviour and performance has been examined extensively and systematically.