The American exchange student is born with one purpose, to go on an exchange to another country, where they will live somewhere between a couple weeks and a year. There are two subspecies of the exchange student: one being shy and generally reluctant to talk about their experience and the other whose life was changed by the experience and it's all they can talk about. It is very possible you know some of both as they are endemic to all American colleges and universities.

The first is generally harder to find, as they are far less vocal. It is suspected that they too had deeply life changing experiences while abroad, but out of respect for those experiences or their listeners, are loathe to bring it up. These are a rare variety and often go on to live unremarkable lives, traveling the world. Not that you would know, if you follow them on social media. 

The second type are generally extroverts and they want you to know how much their life was changed by the time spent in another country. Their life was "just so different, you know? They do it all so differently and better," they are wont to say. You can recognize them by their carefully cultivated instagram pages gushing about their experiences and the people they met abroad. Most will continue to visit this country until they die, making similar observations on their social media for the duration. Some never return, but will incessentally talk about how their life was changed by the exchange. 

Both of these are categorized as a juvenile before they embark on their exchange. Once they embark on their exchange, the begin a process of metamorphosis, similar to a butterfly in a cocoon. There can be signs that an adolescent exchange student tends toward a certain type during this metamorphosis. However, they can only be officially categorized after they return home, reaching full maturity. 

These exchange students are of human descent, popping up randomly in the population. The mutation that causes an exchange student is not known to breed true, although it certainly can appear in their offspring.