Humans Are Diplomats

""Of course it is. For the simple reason that no one else would ever build a place like this. Humans share one unique quality: They build communities. If the Narns or Centauri or any other race built a station like this, it would be used only by their own people. But everywhere humans go, they create communities out of diverse and sometimes hostile populations. It is a great gift, and a terrible responsibility—one that cannot be abandoned.""

- Delenn, Babylon 5

The polar opposite of Humans Are Warriors, the idealistic reason why Humans Are Special and the reason why Humans Are Leaders: Humans are diplomats when compared to other races. They make friends easily, and have a talent for negotiations. Humans tend to favor the diplomatic approach, especially when compared to the Proud Warrior Race. Because humans have such a diplomatic talent, it's the reason they invariably are part of The Federation if it exists. This preference for diplomacy over violence can also be one of the reasons why Humans Are Good.

In video or Tabletop games this may overlap with Humans Are Average - rather than getting powerful bonuses and penalty, humans are given small perks to diplomacy-related skills.

Of course Humans Are Diplomats and Humans Are Warriors, though opposite, need not exclude one another. After all Humans Are Special and "Speak softly and carry a big stick" was coined by humans.

Live Action TV

 * Humans in Star Trek pretty much run The Federation, and the Federation has been shown to be by far the most diplomacy-inclined power in the Alpha Quadrant, especially when compared to the Proud Warrior Race Klingon or Mustache-Twirling Roman-wannabe Romulans or Fascist Cardassians. It's notable that the Federation is pretty much the only Alpha Quadrant power that is formed from a coalition.
 * Taken to an extreme in Star Trek: The Next Generation, where the Klingons call upon the human captain of the Federation flagship, Jean-Luc Picard, to decide the next leader of the Klingon council. There are valid reasons for them not to trust their own people, of course, but even so...
 * Star Trek: Enterprise shows that it was the humans who brought the first races forming the Federation together. Some of them had, in fact, been at war until then.
 * Stated outright by the Minbari ambassador Delenn as the reason that only humans could have built Babylon 5 - "Humans build communities."
 * She likes this so much, she becomes part human herself with that cocoon thing, and then marries and has the child of John Sheridan. Of course, it helps that she needed all that in order to be come one-third of The One.

Literature

 * In Keith Laumer's Retief books, humans operate the Corps Diplomatique Terrestrienne, which is dedicated to adjudicating contentions between non-terrestrials and Earthlings. Although the CDT seems to succeed less due to its diplomatic prowess than from Reteif's unconventional approaches...
 * In Alan Dean Foster's Design For Great-Day, humans have this role. It's suggested that this is due to humans having an exceptional flair with language (being able to "talk the legs off a crocodile and insult its parentage in the process").
 * Inverted and subverted repeatedly and brutally in Stationery Voyagers. Even the Voyagers' enemies on Markerterion treat them better than their human hosts. And it's not like Humans Are Warriors is the reason, more like Humans Are Violent, Axe Crazy Thugs.
 * In parts of the Star Wars Expanded Universe this trope seems to come up. Humanity is not the galaxy's most diplomatic species, and individuals vary wildly, but there's a joke that goes "How do you know a [species] is lying? Their mouth is open", in which [species] is filled in by Hutts, Bothans, or humans.
 * There are a lot of places where humans are the only/majority population though, so it'd be plain stupid to send a Bothan diplomat to Kashyyyk, or a Trandoshan one to Naboo.

Webcomics

 * This is a point of fascination for many of the Trolls in Homestuck. Sure, humans may be a bunch of wimps who haven't built a galaxy-conquering empire or developed time-travelling IM clients, and they may be completely screwing up at ... but human society seems so much nicer, and the Kids are really good at the whole "friendship" thing. The stand-out example of one of the Kids being diplomatic with a Troll is probably John with Vriska.

Video Games
""Always remember that expansion is achieved by diplomacy, not by the military.""
 * In the Master of Orion series, Humans have bonuses to charisma, making them better negotiators. Furthermore, they have no racial rivals, unlike every other species in the game (at least in the first two). They have, according to the third game's manual, twice dominated the galaxy, forming allied fleets to take down both the Orion Guardian and Antares. They did this not through military conquest, but by convincing everyone else to work for them.
 * The same happens in Galactic Civilizations 2, Humans are stated with bonuses to negotiation.
 * The fluff plays with this. Humans got to be so good at negotiation because as a race, they were atypically factional until reaching space; the other side of this is they are equally as unusually practiced at killing each other when negotiations fail. The one other race that has realized this is terrified of humanity for that reason.
 * In Space Rangers humans are this. They are also Space Jews.
 * The humans in World of Warcraft have a bonus to reputation with any faction, both reflecting and reproducing this.
 * In the game's lore. It's stated that the humans of the Alliance are the only thing keeping the races within it together.
 * The Elder Scrolls: While there are several human races, Imperials, the one fitting the classical fantasy-human mold best, are known to be skilled diplomats (in fact one of their abilities is to make people like them more and they often have bonuses to speech/diplomacy).
 * Deadlock, semi-obscure sci-fi empire-builder. Several races, and humans are diplomats and traders, with bonuses for making peace-treaties and establishing trade-routes.
 * ...and the ability to send infantry into a tank-munching berserker mode in battle. Wait, what?
 * Final Fantasy XII. So, so much. The entire game is about diplomacy, with the 12-year-old Lord Larsa Solidor (a diplomatic genius) running around the continent getting the various races to help him bring peace. The other races pretty much don't care until he shows up.
 * Mass Effect has humanity as the most diplomatic race, with all the backstabbing and double-dealing that proper diplomacy implies. Dig into the codex and you'll see that every other race, to some degree or another, has a leadership caste and societies that have made obedience to proper authority second nature to them. When humanity, with its every-man-for-himself ethos, hits the galactic stage, nobody really seems to know how to deal with it. The rules are right there, and everyone else has followed them, and then the humans show up and start taking shortcuts that nobody else even considered. This has lead to Humanity getting a representative on the ruling Council less than a century after they made first contact; other races have gone for millenia as clients to the previous three Council races.
 * To be fair, though, the asari are the ones with the interstellar reputation for diplomatic ability and cultural outreach; humanity, by comparison, is still rather supremacist.
 * It seems that the asari have the most experience dealing with other races, and are viewed neutrally to well by the other races, but they're a bit too complacent. Humanity is dynamic, forward thinking and creative, but still engender and harbour a fair amount of xenophobia.
 * Stonekeep, if the only human playable character is any indication. Let's put it this way: the game's dwarves are biased against every other existing race except humans, and representatives from a good half of the other races will join you.
 * In the Star Trek game Birth Of The Federation, The opening for the United Federation of Planets actually states this:

Tabletop Games

 * In Dungeons and Dragons, this overlaps with Humans Are Average - humans have average stats and they can learn any language at character creation, a trait shared with partially human races like Half-Elves.
 * In 3rd Edition at least, Half-Elves Are Diplomats even more than humans (or elves), being the only race to receive inherent bonuses to social skills. Which, admittedly, aren't very large, but dedicated "Diplomancer" builds are almost always half-elves.