## 2 Theoretical Background and Hypothesis Development
### Competence-Related Traits
#### 2.2.2 Outer Traits  
The outer traits included six easily observed physical traits: age, eye gaze variation, glasses, length-to-width ratio, vocal energy, and vocal variation (Table 2).  
(e) Age. Age represents the estimated age of an individual based on his appearance (rather than his exact age). People hold a series of positive stereotypes about older staff [58], since age is often associated with working experience [59]. For this reason, age has been recognized as a dimension in which individual skills and capabilities may systematically grow over time [60]. For instance, older staff often master higher physical [61] and organizational social skills [59], and they are perceived to be more professional than younger staff [62]. Therefore, age is anticipated to be positively correlated with competence.  
(f) Glasses. Glasses refer to whether an individual is wearing glasses. In general, people believe that glasses are worn by those who are more intelligent, knowledgeable, and professional; thus, wearing glasses is an indicator of competence [63; 64; 65]. For instance, wearing glasses increases the perceived competence of candidates in political elections, thus leading to electoral success, and this effect is especially stronger in situations where intelligence is desirable [66]. For these reasons, wearing glasses is anticipated to be positively correlated with competence.  
(g) Eye-gaze variation. Eye-gaze variation is a measure of the extent to which an individual shifts his gaze during a speech. Previous studies have validated the negative impact of gaze variation on individuals' social perceptions, such as attractiveness [67], dominance [68], and confidence [69]. In video-mediated communication, gaze is defined as directly looking at a camera; thus, eye gaze variation means that an individual fails to maintain constant eye contact with the camera. However, even in online communication, eye gaze is very important. For instance, in clinical situations, physicians with a constant gaze in videoconferencing teletherapy are perceived to be more reliable [70; 71]. In online education situations, instructors with limited gaze variation are deemed to be more confident and have higher educational performance [72]. Therefore, gaze variation is anticipated to be negatively correlated with competence.  
(h) Length-to-width ratio. The length-to-width ratio relates to the shape of one's face, and it is calculated by dividing the face length (measured from the top of the eyelids to the upper lip) by the width. Previous studies have examined whether the length-to-width ratio is positively related to an individual's persuasiveness [73; 74]. For women, a higher length-to-width ratio is associated with social capability, emotional capability, and attractiveness [75]. For men, a higher length-to-width ratio indicates less aggressiveness and is more appealing [76; 77]. Therefore, the length-to-width ratio is anticipated to be positively correlated with competence.  
(i) Vocal energy. Vocal energy is the energy of a human voice and is usually quantified by voice amplitude. A wealth of research has proven that voice energy is positively related to self-confidence [78; 79; 80] and competence [81]. It also provides information associated with an individual's perceived dominance ([PERSON], [PERSON], and [PERSON], 1998 [82]) and increases persuasiveness ([PERSON] and [PERSON], 2020 [83]). Therefore, vocal energy is anticipated to be positively correlated with competence.  
(j) Vocal variation. Vocal variation refers to shifts in one's voice amplitude during speech. Vocal variation also carries rich, socially relevant information [84]. For instance, speakers who lack confidence and background knowledge about a topic tend to shift their voice during speech [85]. Previous studies have also suggested that larger vocal variations have a negative effect on speakers' perceptual performance [86]. Therefore, while vocal energy is considered a positive indicator of competence, vocal variation appears to be considered the opposite.