The cheerleader effect, also known as the group attractiveness effect or the friend effect, is a proposed cognitive bias which causes people to perceive individuals as 1.5–2.0% more attractive in a group than when seen alone. The first paper to report this effect was written by Drew Walker and Edward Vul, in 2013.

Physical attractiveness implies individuals' preferences in a sexual selection based on the evolutionary psychology. In 1979, Donald Symons first proposed this evolutionary explanation, suggesting that the evolving physical attractiveness results from mate assessment favoring partners who exhibited signs of good health and fertility, including face averageness. This preference was proved to be shared across cultures. Two parts constitute physical attractiveness, and most former studies investigated underlying mechanisms leading to cheerleader effect specifically in its subset, facial attractiveness. Nevertheless, a study has recognized this effect in another physical appearance indicator, human body perceptions.

The effect size of the cheerleader effect is not modulated by the presentation time, the number of individuals surrounding the target, spatial arrangement of the faces in the group. However, another study argued that the arrangement of faces in the group might influence this effect since people's central viewing tendency might affect observers to focus more on the perceived attractiveness of the middle face in the group.

Findings of this effect are interdisciplinary in applications. Based on them, mate choice, marketing, and social media tactics are designed to increase the attractiveness of a target individual or item via the help of the group.

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 on November 10, 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.

In 2013, 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: