No, homophily has nothing to do with sexual orientation. In the 1950s a pair of sociologists, Paul Lazarsfeld and Robert Merton, coined the term homophily to refer to the pervasive tendency of humans to associate with others who are similar to themselves.    

Even if you do not know homophily by name, it is something you have experienced throughout your life. In whatever elementary school you went to, in any part of the world, girls tended to be friends with girls, and boys with boys. If you went to a high school that had people of more than one ethnicity, then you saw it there. Yes, you may have been friends with someone of another ethnicity, but such friendships are the exception rather than the rule. We see strong homophily by age, ethnicity, language, religion, profession, caste, and income level. 

Homophily is not only instinctual—just watch people mingle at any large social event in which they are all strangers—it also makes sense for many reasons. New parents learn from talking with other new parents, and help take care of each other’s children. People of the same religion share beliefs, customs, holidays, and norms of behavior.  By the very nature of any workplace, you will spend most of your day interacting with people in the same profession and often in the same sub-field.  Homophily also helps us navigate our networks of connections. If you need to ask about a doctor’s reputation, which one of your friends would you ask? Someone in the healthcare industry, of course, as they would be the most likely of your friends to know the doctor, or know someone who knows the doctor. Without homophily you would have no idea of whom to ask. 

As simple and familiar as it is, homophily is very much a scientific concept: It is measurable and has predictable consequences. In fact, it is so ubiquitous, that it should be thought of as a fundamental scientific concept. But, it is the darker side of homophily that makes it such an important scientific concept.   

As the world struggles with inequality and immobility, we can debate how much a role accumulation of capital plays, or political regimes, but we miss a primary constraint on mobility if we ignore homophily. To understand why many American youths join gangs, and so many end up shot or in jail before their twenty-fifth birthday, one only has to look at what they observe and experience from a young age. If we want to understand why universities like Stanford, Harvard, and MIT have more than twenty times more students from the top quarter of the income distribution than the bottom quarter of the distribution, homophily is a big part of the answer. High school students in poor neighborhoods often have little idea of the financial aid available to them, or what the benefits to higher education really are, or even what higher education really is. By the time they talk to a high school counselor who might have a few answers, it is much too late. Homophily affects the way that their parents have raised them, the culture that they experience, the role models they see, the beliefs that they have, the opportunities that come their way, and ultimately the expectations they have for their lives.    

Although we are all familiar with homophily, thinking of it as a scientific, measurable phenomenon, may help it become a bigger part of the discourse on how we can increase mobility and decrease inequality around the world. Solving such problems requires understanding how persistent segregation by income and ethnicity prevents information and opportunities from reaching those who need them most.   Homophily lies at the root of many social and economic problems, and understanding it can help us better address the many issues that societies around the globe face, from inequality and immobility, to political polarization.