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Homophily from Ancient Greek ὁμοῦ (homou, “together”) and Greek φιλία (philia, "friendship") is the tendency of individuals to associate and bond with similar others, as in the proverb "birds of a feather flock together". The presence of homophily has been discovered in a vast array of network studies. More than 100 studies that have observed homophily in some form or another and they establish that similarity breeds connection. These include age, gender, class, and organizational role.
Individuals in homophilic relationships share common characteristics (beliefs, values, education, etc.) that make communication and relationship formation easier. The opposite of homophily is heterophily or intermingling.
Homophily between mated pairs in animals has been extensively studied in the field of evolutionary biology in which it is known as assortative mating. Homophily between mated pairs is common within natural animal mating populations.
Types and dimensionsEdit
Baseline vs. inbreedingEdit
To test the relevance of homophily researchers have distinguished between baseline homophily and inbreeding homophily. The former is simply the amount of homophily that would be expected by chance given an existing uneven distribution of people with varying characteristics, and the second is the amount of homophily over and above this expected value, typically due to personal preferences and choices.
Status vs. valueEdit
In their original formulation of homophily, Lazarsfeld and Merton (1954) distinguished between status homophily and value homophily.
The authors find that individuals with similar social status characteristics are more likely to associate with each other than by chance. "Status" includes both ascribed characteristics like race, ethnicity, sex, and age; and acquired characteristics like religion and education. In contrast, value homophily involves association with others who think in similar ways, regardless of differences in these status characteristics. 
Race and ethnicityEdit
Social networks in United States today are strongly divided by race and ethnicity, which account for the greatest proportion of inbreeding homophily (though classification by these criteria can be problematic in sociology due to fuzzy boundaries and different definitions of race). Smaller groups have lower diversity simply due to the number of members, and this tends to give racial and ethnic minority groups a higher baseline homophily. Race and ethnicity also correlates with educational attainment and occupation, which increase baseline homophily further.
Sex and genderEdit
With regard to sex and gender, baseline homophily of networks is relatively low compared to race and ethnicity. Men and women frequently live together, and are both large and equally-sized populations. Most sex homophily is of the inbreeding type.
Most age homophily is of the baseline type. An interesting pattern of inbreeding age homophily for groups of different ages was found by Marsden (1988). It indicated a strong relationship between someone's age and the social distance to other people with regard to confiding in someone. For example, the larger age gap someone had, the smaller chances that they were confided by others with lower ages to "discuss important matters".
Family of birth accounts for considerable baseline homophily with respect to education, occupation, and social class.
Social Media and HomophilyEdit
As social networks are largely divide by race, social networking websites like Facebook are also fostering homophilic atmospheres. When a Facebook user likes or interacts with an article or post of a certain ideology, Facebook, continues to show that user posts of that similar ideology (that Facebook believes they will be drawn to). In their research paper titled, "Birds of a Feather: Homophily in Social Networks," McPherson, Smith-Lovin, and Cook write that homogenous personal networks result in limited "social worlds in a way that has powerful implications for the information they receive, the attitudes the form, and the interactions they experience." This homophily can foster divides and echo chambers on social networking sites, where people of similar ideologies only interact with each other.
Geography: Baseline homophily often arises when the people who are located nearby also have similar characteristics. People are more likely to have contact with those who are geographically closer than those who are distant. Technology such as the telephone, e-mail, and social networks have reduced by not eliminated this effect.
Family ties: Family relationships often produce relatively close, frequent contact among those who are at great geographic distance. These ties tend to decay slowly, but can be dramatically restructured when new marriages occur.
Organizations: School, work, and volunteer activities provide the great majority of non-family ties. Many friendships, confiding relations, and social support ties are formed within voluntary groups. The social homogeneity of most organizations creates a strong baseline homophily in networks that are formed there.
Isomorphic sources: The connections between people who occupy equivalent roles will induce homophily in the system of network ties. This is common in three domains: workplace (for example all heads of HR departments will tend to associate with other HR heads), family (for example mothers tend to associate with other mothers), and informal networks.
Cognitive processes: People who have demographic similarity tend to own shared knowledge, and therefore they have a greater ease of communication and share cultural tastes, which can also generate homophily.
Effect of homophilyEdit
One study reported that perception of interpersonal similarity improves coordination and increase the expected payoff of interactions, above and beyond the effect of merely "liking others". Another study claimed that homophily produces tolerance and cooperation in social spaces. Other studies have claimed that homophily helps people to access information, diffuse innovations and behaviors, and form opinions and social norms. Homophily influences diffusion patterns over a social network in two ways: homophily affects the way a social network develops, and individuals are more likely to successfully influence others when they are similar to them.
Homophily often leads to homogamy—marriage between people with similar characteristics. Homophily is a fertility factor; An increased fertility is seen in people with a tendency to seek acquaintance among those with common characteristics. Governmental family policies have a decreased influence on fertility rates in such populations.
- Ferguson, Niall (August 15, 2017). "The False Prophecy of Hyperconnection". Foreign Affairs. Retrieved October 1, 2017.
At the same time, birds of a feather flock together. Because of the phenomenon known as “homophily,” or attraction to similarity, social networks tend to form clusters of nodes with similar properties or attitudes.
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