Our systematic searches of the economics, marketing, neuroscience, psychology, and sociology literatures yielded 2560 publications, resulting in 104 separate studies on celebrity influence. From these studies as well as studies retrieved from additional targeted searches, we identified 14 potential mechanisms through which celebrities may influence people’s health decisions (see Table 2).
When celebrities endorse a product or idea, they differentiate it from others. According to signaling theory, “signals” are tools that convey key information about an object or individual, and are interpreted by the receiver to aid in making a decision . Consumers of health information are often overwhelmed by contradicting advice from health professionals, friends, family and online resources. Given the array of recommended health products and behaviors, choosing among these options and making informed health decisions is difficult. To help in this task, people naturally look for “signals” that indicate one option as being more credible and effective than others . Due to the vaulted status of celebrities in society, their endorsements act as signals of superiority that distinguish the endorsed item from competitors, nudging people to change their health behaviors accordingly.
Herd behavior (economics)
The influence of celebrities’ medical advice is strengthened by people’s natural tendency to make decisions based on what others have done in similar situations . There are various reasons for this inclination to imitate, including the safety of numbers, comfort in adopting accepted opinions, and desire to acquire what others have . Viewed as trendsetters, celebrities are often early leaders of herd behaviors, whether involving new diets, exercise routines or medical procedures . Wanting to follow in their favorite celebrities’ footsteps, many will ignore their personal information and imitate the celebrity health choices they observe . This behavior initiates an informational cascade: the celebrity’s decisions are passed to others who make the same choices . As the number of followers increases, the herding effect lengthens and strengthens, spreading from person-to-person and changing health behaviors along the way . For instance, actor Angelina Jolie’s announcement that she had undergone a preventive double mastectomy after testing positive for the BRCA1 gene mutation led to explosive interest in genetic testing . However, due to the low prevalence of BRCA mutations, a recent systematic review only recommends testing in women who have family members with BRCA-associated cancers and who have had appropriate genetic screening and counselling . Even though routine testing is not recommended for women without a family history of BRCA mutations, Jolie’s announcement may have catalyzed a herd of women seeking the test, including many for whom it is neither appropriate nor cost-effective.
Meaning transfer (marketing)
Celebrities may be successful medical advisors because consumers see in them attributes they respect and want to emulate. This desire stems from a process marketing researchers call meaning transfer. For many people, celebrities represent important social or cultural meanings that become associated with ideas or products they endorse . People will in turn consume these items in hopes of acquiring the celebrities’ traits . The tobacco industry has a history of using this process to sell their products. Through fostering close relationships with movie studios and prominently featuring stars in advertisements [33, 34], companies try to transfer the attractive and sophisticated image of celebrities to their cigarettes. The strategy works: smoking in movies has been found to alter perceptions of and susceptibilities toward smoking among adolescents . In turn, such youths are more likely to initiate cigarette use and continue smoking behavior into adulthood [36–39]. Similarly, food and beverage companies have recruited sports celebrities to endorse their often unhealthy products, with one marketing analysis finding that such items were the second most frequently endorsed by athletes . Of these food products, 79% were classified as energy-dense and nutrient poor, while 93% of endorsed beverages derived 100% of their calories from additional sugars . The healthful impression people have toward professional athletes become transferred to the endorsed products, with one experimental study finding that parents perceive foods endorsed by athletes to be healthier even if they are not .
The same process occurs when celebrities attribute their good health, beauty and vitality to particular practices. Following a period of fatigue, anemia and vitamin D deficiency, actor Gwyneth Paltrow credited a three-week elimination diet for her recovery. By abstaining from various foods to which she was sensitive, Paltrow “cleansed” her body and came to lose weight, look better and feel more energetic. This diet, promoted in her latest cookbook , is not supported by research evidence; indeed, elimination diets are primarily studied to treat severe allergies and food intolerances [42, 43], and testing for food sensitivity is unfounded and expensive [44, 45]. Nevertheless, Paltrow’s slender physique, youthful appearance and healthful being, meanings transferred to her elimination diet, may convince people to adopt the diet without considering the potential risks or consulting their physician.
Source credibility (marketing)
The most successful celebrity medical advisors are those perceived to have high credibility. According to the source credibility model, credibility depends on trustworthiness and expertise . Trustworthiness refers to how honest and believable the endorser is in giving an opinion on the product , while expertise is the extent to which an endorser is thought to be a valid source of information . Credibility also creates congruence between celebrities and the items they promote, with studies finding that advertisements are more successful when the endorser’s image matches the pertinent attributes of the product [49, 50]. In acting as medical advisors, celebrities have, or portray themselves to have, an authentic connection . Many promote health behaviors for conditions they have personally suffered, such as former basketball player Magic Johnson’s endorsement of the home HIV test OraQuick  and actor Brooke Shields’ promotion of Paxil to treat postpartum depression . Others endorse products they credit for achieving their admired or respected traits. Numerous celebrities, including actor-singers Katy Perry, Jessica Simpson and Justin Bieber, have shared their troubles with acne and credited Proactiv Solution for their now clear complexions . After singer Carnie Wilson live-broadcast her gastric bypass surgery, and later made the media rounds to tout the procedure for her new frame, the number of individuals undergoing the surgery in the US increased from 19,000 to 100,000 per year . By sharing their past experiences and the sources of their health achievements, these celebrities are perceived as credible, enticing people to follow their advice.
Halo effect (marketing)
However, even celebrities without a genuine connection have been perceived as credible health advisors. This false medical credibility may stem from difficulty in separating it from a more generalized impression of the celebrity. In the halo effect, the predominant trait of an individual biases how all his or her other traits are judged . People exceptional in one way are assumed to similarly excel in other areas . The public often associates the perceived success of celebrities with generalized trustworthiness and wide-reaching expertise that extends beyond the celebrities’ industry and skill set. Celebrities are in turn perceived to have greater credibility than their non-celebrity counterparts, like physicians, despite having less medical knowledge and experience. Actor Lauren Bacall’s endorsement of Visudyne was founded on an unnamed friend’s experience with macular degeneration . Food personality Paula Deen sponsored the diabetes medication Victoza despite how her reputation as the “Butter Queen” would put her trustworthiness into question . Even though celebrities are often compensated for their endorsements, their riches and achievements pull consumers to view them as credible and follow their health recommendations.
Neural mechanisms of meaning transfer (neuroscience)
The influence of celebrity health endorsements may involve distinct cognitive processes. One study involving 26 women provides evidence for a mechanism similar to meaning transfer. When participants were presented with shoes paired with a celebrity’s face – paralleling an advertisement – functional magnetic resonance imaging (fMRI) scans found their medial orbitofrontal cortex was activated . The orbitofrontal cortex is involved in connecting neutral and reinforcing stimuli, with the medial portion specifically encoding positive associations . Therefore, activation of this region may indicate transfer of positive attributes from the celebrity (the reinforcing stimulus) to the object (the neutral stimulus) . Brain regions associated with explicit memories were also activated, suggesting that the transferred meanings stem from conscious recollections of past facts and episodes associated with the celebrity, rather than unconscious implicit memories . When people view celebrity health endorsements, they retrieve explicit memories related to the celebrity. If the memories are positive, they are transferred to the product or idea being endorsed and promote its adoption.
Neuropsychology of credibility (neuroscience)
There may also be a neural basis for credibility. It relates to the effect of celebrities’ perceived expertise – a component of the source credibility model from marketing literature – on the persuasiveness of endorsements. One study found consumers’ purchase intention and product recognition were increased for products accompanied by “expert celebrities” as compared to “non-expert celebrities” . This indicated that celebrity expertise has long-term positive effects on consumers’ attitude toward and ability to recall brands and products. fMRI scans revealed that seeing expert celebrities activated the caudate nucleus, a subcortical region involved in promoting trustful behavior and processing risks and rewards. Celebrities with high expertise may thus promote favorable attitudes toward the endorsed item by inducing trust and leading people to reassess the item’s value. In addition, memory formation was shaped at the medial temporal lobe, which is involved in memory encoding. Areas associated with understanding concepts and meanings were also activated. This suggests that increased processing of already-learned celebrity and product information causes the medial temporal lobe to create a favorable and lasting memory of the endorsed item, thereby encouraging its consumption . Although activation of brain regions on fMRI is only a surrogate marker of underlying neural mechanisms and the unique functions of individual structures are difficult to separate, these studies indicate that innate biological processes may exist that cultivate people’s trust in celebrities as medical advisors.
Classical conditioning (psychology)
Human psychology may also explain the substantial influence celebrities have on health decisions. Classical conditioning is a process by which people learn to associate two stimuli such that exposure to either achieves similar responses . Celebrities, in this case, are unconditional stimuli that elicit positive unconditional responses. Through repeat pairings over time, the things celebrities endorse come to elicit a positive conditional response given their association with the celebrity. These items become conditional stimuli whereby they elicit the same positive sentiments even without the celebrity. One recent study found that coupling an attractive and trustworthy celebrity with a product as an unconditional-conditional stimulus pairing led to significantly higher product ratings, indicating a positive conditional response .
In addition, according to the concept of belongingness in conditioning research, a conditional stimulus that is closely matched with an unconditional stimulus more easily evokes the conditional response . High congruency between celebrities and their medical advice should thus lead to more intense sentiments generated toward the message. In one study, pairing a product with a highly congruent celebrity as compared to a poorly congruent one led to stronger conditioning, in the form of a more positive attitude . Therefore, celebrity medical advice may be conditioned to evoke consumers’ positive perceptions of celebrities, an effect that is strengthened when the advice matches the celebrity’s image.
The psychology literature also suggests that advice from celebrities who match people’s self-conception have greater influence. Self-conception includes the thoughts and attitudes people have of their actual self, those they would like for their ideal self, and those they use to present their social self . Just as how products or brands are marketed, celebrities create an image for themselves they hope resonates with their fans . Since people frequently use these images to define their self-conception, congruent advice can be highly effective [64, 65]. For celebrities viewed as inspirational and personally relevant, their advice will be compatible with people’s ideal self such that the self-esteem motive – to elevate one’s actual self toward one’s ideal self  – pushes people to follow the advice. One study found that compatibility between a celebrity endorser’s image and a person’s ideal self was associated with higher advertisement ratings and greater purchase intention . Conversely, for celebrities who portray themselves as similar to their admirers, their advice will be compatible with people’s actual self such that the self-consistency motive – to maintain one’s actual self  – is the motivating factor.
Cognitive dissonance (psychology)
The desire to maintain mental consistency may account for why people follow celebrity medical advice. According to cognitive dissonance theory from social psychology, people experience psychological discomfort, or dissonance, when there is conflict between the decisions they make, the behaviors they choose, the information they hear, and/or the beliefs, opinions, values and ideas they hold . This discomfort will naturally motivate people to reduce or avoid dissonance . Cognitive dissonance has been used to explain how people rationalize difficult decisions . For example, when celebrities offer medical advice, ardent fans may experience dissonance if they do not follow it: the action conflicts with their celebrity adoration. However, following the advice can also create discomfort since endorsed behaviors are often difficult, expensive and unconventional. To reduce dissonance, followers unconsciously modify their cognitions, such as believing the celebrity advice is more credible than alternatives . They also adopt new beliefs or commit to actions that diminish inconsistencies, including seeking information supporting the celebrity advice . Lastly, they trivialize dissonant cognitions to make the conflict seem less important, such as minimizing the advice’s costs and harms . In this way, people justify their decisions to follow celebrities’ medical advice while strengthening their celebrity attachments in the process.
People are prone to celebrity influence if they have strong feelings of attachment toward a celebrity. Previous research has demonstrated that intense attachments can result if celebrities are responsive to people’s needs for autonomy, relatedness and competence. Autonomy is the need to believe that one’s actions are self-determined without constraint or coercion. Relatedness is the need to feel intimate with and cared for by others. Competence is the need to feel effective and capable in one’s activities . Celebrities can provide inputs to fulfill these needs, thereby fostering strong attachments. For example, media queen Oprah Winfrey has built a legion of ardent fans, in part through such inputs. Through her numerous outlets, she urges viewers to take control of their own lives, shares personal details and emotions, and encourages people to feel confident and worthy. Given their strong attachments, many of Oprah’s followers faithfully adopt her sometimes-dubious medical advice. When Oprah promoted the herbal cold remedy Airborne on her television show, sales of the supplement soared despite a lack of supporting research evidence. Airborne’s manufacturers claimed that a double-blind, placebo-controlled study supported the supplement’s effectiveness; however, the company that conducted the study, GNG Pharmaceutical Services, turned out to be a two-man enterprise with no scientists or doctors, and created just to perform the Airborne “study” of questionable validity . The U.S. Federal Trade Commission even charged Airborne’s manufacturers in 2008 for falsifying claims of efficacy, eventually reaching a $30 million USD settlement . However, Oprah’s followers and others continue to buy and use the product .
Having a poor sense of self-identity and/or low self-esteem also makes people more susceptible to celebrity attachment . In three studies on parasocial interactions – unidirectional connections fans make with media personalities – people with low self-esteem used celebrity relationships to move closer to their ideal self, a benefit that people with high self-esteem derive from real relationship partners . Two models can explain this tendency. In the absorption-addiction model, people lacking a clear sense of self become absorbed with celebrities to attain a more complete identity. This parasocial relationship strengthens over time, as individuals become addicted to their absorption and seek greater and more personal links . According to the empty-self model, autonomous persons who value self-containment and self-sufficiency often have to sacrifice interpersonal relationships. These individuals, referred to as ‘empty selves’, have a consistent emotional need that celebrity attachment can fulfill . Therefore, both traits of celebrities and their fans foster strong parasocial relationships, which enhance the former’s influence. Particularly in people suffering from poor mental health, attachments can progress to a borderline-pathological level of celebrity worship, in which parasocial relationships irrationally substitute for real life .
Social networks (sociology)
The widespread uptake of celebrity medical advice can also be explained as a social contagion that diffuses through social networks. Social networks are systems of people linked through personal connections, such as family, marital and friendship ties . Observational studies have found these interconnections to have significant effects on people’s health, including smoking , obesity , sexual activities  and happiness . One person’s health decisions create externalities, by which connected individuals experience indirect consequences . Within this interrelated system, clusters of people sharing common health behaviors form. Although celebrities only have loose social ties to most people, their newsworthiness and star quality, and the intense parasocial relationships some individuals have allow them to feature prominently within social networks. Thus, celebrities have great influence as medical advisors: as with Angelina Jolie’s announcement of her double mastectomy, celebrities’ messages reach many people simultaneously, and diffuse across social ties to affect diverse clusters. As social networks broaden with the development of social media technologies , celebrities can reach broader audiences faster and more intensely than ever before . Indeed, online networks have become a key tool for celebrities such as Gwyneth Paltrow  and Jessica Alba  to disseminate medical advice.
Commodification and social capital (sociology)
The ways in which people look to celebrities for health advice may stem from the broader context of consumerism. In our current capitalist society, celebrity culture appears to be one of the many entities that have been commoditized. Celebrities themselves have become products that can be bought and sold. As old celebrities fade into obscurity and breakout stars gain newfound fame, there is a consistently refreshed stock of articles for trade . But celebrities are not just inanimate objects for sale: consumers also “purchase” celebrities by acquiring their endorsed products, mimicking their lifestyles, and heeding their medical advice.
In sociology, these parasocial relationships have been conceptualized as a means of acquiring celebrities’ social capital: the benefits and resources accrued through social relationships . Although acquiring the same coveted status of famous people is nearly impossible, doing what celebrities do and imitating their behaviors is a strategy for people seeking to raise their social status . Celebrities, in this sense, have become resources in forming consumers’ social identities, used to shape the ways people see themselves and want others to see them . Following celebrity medical advice may be a method for consumers to gain social capital and participate in the practices that make celebrities ‘special’, thereby elevating them to the upper echelons of society.
Social construction of reality (sociology)
The ways in which people evaluate, interpret and perceive health information can also be influenced by celebrities. According to social constructivism, reality is a cultural product, formed by people’s interactions with each other and their environments . The ways individuals create and learn knowledge is determined by the social activities in which they engage. This applies both to the social construction of health information as well as the social reconstruction of this information by each and every person in different ways depending on their unique social environment. This mechanism of celebrity influence is supported by a recent interview-based study’s finding that people assess information about vaccination differently: acceptors wholly accept social norms, reliers follow the norms of their social networks, and searchers independently seek whatever information they need . Celebrities modify the ways all three types of people evaluate, interpret and perceive health information and consequently influence their health behaviors, albeit in different ways.
For acceptors, celebrities shape social norms that are internalized and acted upon. Much of the interest in detox cleansing and fasting, for example, can be attributed to celebrities like Salma Hayek and Ashton Kutcher who have made such behaviors socially acceptable and popular for weight loss and reducing gastrointestinal malaises. For reliers, many of their friends, family members and colleagues may follow or discuss celebrity medical advice, which indirectly encourages them to act similarly. For searchers, the information they gather may knowingly or unknowingly include advice from celebrities, especially as the internet burgeons with the health information they share. This means that all types of people, not just gossipmongers or people with low self-esteem, can be affected by the ways celebrities shape the social construction and reconstruction of health information.