Back in the 1960s, Ruth Van Reken studied the impact of international life on the children of missionaries in India, realising that their development was being shaped by their unique environment. Thus, after research with others, she developed the concept of what she would call Third Culture Kids.
It's not just a matter of counting though, “Third Culture” represents the culture created by people who are somewhere between their home (first) culture and their host (second) culture. It is nebulous and difficult to define, but an observable phenomenon. Children who grow up following their parents' international careers in business, diplomatic, military, missionary or other work develop differently. They are influenced by the culture of their parents, of their surroundings, of the (often, but not exclusively) international schools they attend, and most significantly their friends from their own mixed and various backgrounds.
That shared community of children brought together through a whole mix of influences creates the typical Third Culture Kid. The “Kid” aspect is important, because of how this experience affects children during the important developmental stages of their lives. Adults who travel extensively also undergo a lot of change, but this is building on the foundation of their childhood, and if that's more geographically stable and less culturally mobile, will be a strong marker when it comes to a sense of home or cultural identity.
People who identify as TCKs tend to have both advantages and disadvantages, though of course since the breadth of experiences are varied, so too is the spectrum of those. Languages are often the first benefit, where being in a position to learn languages during the developmental stages of their lives not only allows them to learn them, but also make it easier to learn others. Since their cultural identity usually doesn't develop according to standard interpretations, they also tend to be more flexible with cultures and social groups. That comes at a cost as well, as they often have trouble settling down, and have trouble with their cultural identity when not in an international environment.
Speaking personally, I had a lot of trouble returning to the US when I was 18. Between my parents being Vietnamese, being born in Texas - but never living there, growing up in Indonesia and attending a British school there, this was something perfectly normal among my friends growing up but was alien to the American friends I made, who all struggled to find a box to fit me into that they could understand. While every young person has to learn who they are as an adult, adding a cultural dimension to that does make things complicated.
The term “TCK” has expanded in the last several years, now including “Cross Cultural Kid” as a broader umbrella term, in recognition of how any kind of significant cultural move and change can dramatically influence a child's life. The “TCK” term has also become better known across international schools around the world, and teachers are better prepared now than ever to help children adjust into a world which doesn't always know where to fit them.
While children at EF tend to be from their local countries, we as international teachers do provide some semblance of an external cultural influence on them. Sometimes, even the smallest influences can be flapped at by the butterfly of chaos to dramatic effect, and create both new opportunities and new challenges for them. As international teachers, it's also good to be aware of what's happening to us as we travel and acclimatise.
A bit of food for thought for any international teacher, really, both for themselves and their students.