## 2 Materials and Methods
### Statement of Interest and Research Questions  
Taking into consideration that most of the previous works on BS focused on various physical and behavioral biometric techniques, while in users' acceptance--a key element for BS success [15]--less attention has been given, it is of great importance for this aspect to be further examined. Thus, acceptance is a complicated notion since it includes social and organizational factors within it [34]. Previous research on users' acceptance of new technologies, including biometrics, has used several models for further understanding of this aspect. The technology acceptance model [35] is one of the most known, focusing on users' perceived usability (easiness and usefulness). The technology acceptance model has unfolded to be a principal one for comprehending predictors of human behavior toward the likelihood of acceptance or rejection of each technology. It presumes a mediating role of the two above-mentioned variables in a complex interrelation among system characteristics and potential system usage. The theory of planned behavior supports that users' behavior is determined by behavioral intentions and reasoned actions, including perceived behavioral control. It concerns an attitude-intention-behavior model, which posits that intention is determined by attitude, subjective norms, and perceived behavioral control, predicting the individual acceptance and usage of various technological features [36]. Thus, the theory of reasoned action puts an emphasis on consciously intended behaviors, and especially as far as BSs are concerned, users seem to be willing to adopt them, provided that they understand their benefits. The unified theory of acceptance and use of technology identifies four key factors, namely performance expectancy, effort expectancy, social influence and facilitating conditions, as well as four moderators, the gender, age, experience and voluntariness so that users accept new technologies, such as BSs. The performance expectancy, effort expectancy, and social influence were theorized, affecting behavioral intention to use a technology, while behavioral intention and facilitating conditions determine technology use. Moreover, various combinations of the four moderators were found to moderate various relationships, focusing on the importance of technology acceptance and use [37]. The information system success model measures systems quality and technical success, including the concepts of information quality, the semantic success via the concepts of use, user satisfaction, and individual impacts, as well as the organizational impacts via the concept of effectiveness success. These six dimensions of success are considered to be interrelated rather than independent, aiming at aiding in the understanding of the possible causal interrelationships among the dimensions of success and to provide a deeper exposition of the relationships [38]. The motivation model focuses on users' extrinsic motivation and intrinsic motivation to examine to what depth they are willing to accept the technology, considering users' three hierarchical stages of generality, namely, personality, life domain and state. These allow to understand intrinsic (engaging in an activity out of pleasure and satisfaction) and extrinsic motivations (engaging in an activity in order to obtain something outside the activity) more precisely [39]. Social cognitive theory supports that for interpreting users' acceptance, behavioral, personal and environmental factors are determinants of each other [40]. However, it is important to note that the above theories/models consider either a specific limited parameter for examination, failing to pay attention to the users' socio-demographic profile or ignore the usability facilitating factors deriving from the technology itself. Ref. [22] indicates that social and cultural differences, as well as gender, should be included as variables in models dealing with technology acceptance.