Studies have found that participants can accurately distinguish political affiliation based on photos of faces. One study found that these effects seem to have been driven by traits attributed to the faces; specifically, power (a composite of ratings of dominance and facial maturity) and warmth (a composite of ratings of likeability and trustworthiness). Republican faces were perceived as more powerful than Democrats and, to the extent that a face was perceived as powerful, it was more likely to be categorized as Republican. On the other hand, the warmer a face was perceived, the more likely it was to be categorized as a Democrat. Other studies have also found relationships between facial traits and perceived political ideology. For example, one study found that conservative politicians in Finland were more attractive than candidates on the political left. In yet another study, politicians judged to be conservative were more attractive, intelligent-looking, and of higher social class than those judged to be more liberal. These judgments can have electoral consequences.
Although the extant research has provided important information about factors that may underlie categorizations of faces according to political party affiliation, it may be limited in some critical ways. These studies focus primarily on the target in isolation from the perceiver. It is important to note that this research draws from ideas based in an ecological theory of perception adapted to theories of ecological social perception. These theories argue that faces signal certain things to perceivers about what the target may afford. A target may appear, for instance, more or less trustworthy or dominant, which may lead the perceiver to trust or fear that person, accordingly. However, it is likely that the accuracy of categorizing ambiguous targets is driven at least in part by the perceivers' identities, dispositions, or states. For example, it is known that perceptions of ambiguous group members may be influenced by perceivers' attitudes toward the groups or exposure to members of the group. One study found that heterosexual perceivers who had more experience with gay men were better at categorizing gay faces. Similarly, it is plausible that categorizations and trait ascriptions of Democrat and Republican faces may differ based on the political ideology of the perceiver/judge. There does exist some evidence that other perceiver identities and ideologies influence categorizations and judgments based on political affiliation. For instance, one study found that people categorized faces as political ingroup or outgroup members largely as a function of likeability. Another recent study found that perceiver gender influences ratings towards male and female politicians.
Target political affiliation may interact with perceiver identity in another important way. In addition to accurate perceptions of political ideology, one study found that participants were more likely to classify faces as outgroup members than as ingroup members. These findings are consistent with a more general ingroup overexclusion effect. The ingroup overexclusion effect is thought to be a result of motivated social cognitions related to social identity, such that people tend to be protective of the ingroup. As a result of this protectiveness, perceivers may show a default bias toward categorizing others as outgroup members. This may be the case especially when groups are perceptually ambiguous. For example, one group of scholars have observed an ingroup over-exclusion effect for racially ambiguous targets, finding that Northern Italians (who had strongly identified as such) were more likely to exclude ambiguous targets that had a mix of Northern and Southern Italian features.