Cheerleader effect influenced by group of humans
Origin
The phrase was coined by the fictional character Barney Stinson (Neil Patrick Harris) in "Not a Father's Day", an episode of the television series How I Met Your Mother, first aired in November 2008. Barney points out to his friends a group of women that initially seem attractive, but who are all unattractive when examined individually. This point is made again by two other characters, Ted Mosby (Josh Radnor) and Robin Scherbatsky (Cobie Smulders), later in the episode, who note that some of Barney's friends also only seem attractive in a group. This occurrence may be explained by the processing of Barney's visual system, in which his brain automatically calculated a combined beauty level of that group of ladies. This overall impression then impacts his assessment of the specific female within that group, leading him to believe she is similar to the previously established better average attractiveness.Conditions for the effect to occur
Bias in recalling. The cheerleader effect occurred only when participants were asked to rate the attractiveness after images were removed from their vision, suggesting that an initial perceptual encoding with the existence of photos while rated could not lead to the effect.
Contrast effect. It was found that the cheerleader effect occurred when the target face was the most attractive face compared to other members in the group but not when it was the least attractive so that the comparison among faces is required.Studies and proposed explanations
First study
In 2014, the first research was reported by Drew Walker and Edward Vul. Across five studies, participants rated the attractiveness of male and female faces when shown in a group photo, and an individual photo, with the order of the photographs randomised. The cheerleader effect was quantified as the difference between the attractiveness ratings assigned in the experimental condition (in a group photo) and the control (in an isolated image) condition. It was found that participants consistently rated the person as more appealing in the group photograph compared to the individual picture.
This effect occurs with male-only, female-only and mixed gender groups, and both small and large groups. In addition, the effect occurs to the same extent with various group sizes of four and 16 people. Participants in studies looked more at the attractive people than the unattractive people in the group. 
Drew Walker and Edward Vul proposed that this effect arises due to the interplay of three cognitive phenomena:
The human visual system takes "ensemble representations" of faces in a group. This explanation was backed up by Timothy F. Brady and George A. Alvarez's findings in 2011.In the study, participants were displayed with 30 sets of circles, and circles of various sizes surrounded a tested circle. When asked to determine the tested circle's size, observers' memory of its size is biased by the mean size of all circles shown to them to estimate, showing that people do not encode visual images in memory independently.
Perception of individuals is biased towards this average. People's visual systems subconsciously and automatically calculate the average facial impression so that any extreme is ruled out.
"Attractive faces are only average." Results showed that composited faces were rated as more attractive and typical without extreme features. Humans develop this preference for "prototype" face from early life since they are easily identified, and individuals could extract social information from these most facelike stimuli to aid social interaction.When all three of these phenomena are taken together, researchers proposed that the cheerleader effect results from a "hierarchical encoding" and that the hierarchical structure of visual working memory makes observers summarize the group into an ensemble average. Specifically, the individual faces will seem more attractive in a group, as they appear more similar to the average group face, which is more attractive than members' faces.