Several recent interactions with friends, as well as the ongoing Olympics, have brought the topic of country identity to my mind. In this case I'm not talking about national symbolism, or flags, but about how people form a concept about another country.
We form patterns quickly. I think it's actually a biological imperative. If an unusual-looking bear is coming at you, and you hesitate because you can't figure out if it's really a bear or not... you get the idea. Typically we suspect a pattern after two data points conform to it, and we feel it's been confirmed when we see a third conforming example. From that, we expand our idea into a generalization that we try to apply in new contexts.
Lately, I've met more than one person who told me they didn't want to go to Australia. Needless to say, I found this surprising, since I'm married to an Australian and have been there lots of times (it's lovely). When I asked why, these people explained (not in precisely these words) that Australia was filled wall to wall with terribly dangerous animals from sting rays to jellyfish to spiders to crocodiles, and that going there would mean certain injury or death. My response? Um, wow. But I didn't ask where they got the idea, because I actually know where they got it. From the news. It doesn't take more than one Steve Irwin to make a big impression; add to that one story of lifeguards wearing panty hose to avoid jellyfish stings, and one story about the Sydney brown recluse spider, and voila! That's a pattern for generalized fear.
Just so you realize I'm not really much better than any of these people, I'll tell you how I felt when I first met my husband. I was incredulous, listening to all his stories about the big city of Melbourne (3 million people). I thought - with some embarrassment, mind you - "Wow, Australia has people?" All my data points came from stories or tv shows about Australian scenery and animals.
I heard another story about a pr video that was being made in France. The first people to judge it weren't impressed, because they wanted to get away from the typical "Baguette, beret, fromage" image. (Fromage is cheese.) France is quite strong in biotech, for example, and in this case, pointing out that fact was far more relevant to the video than the old-fashioned image.
So in general, I'll observe that people's mistaken first impressions of countries don't seem to come from erroneous information so much as bad luck in the first few pieces of data they encounter. When I watch the Olympics, I always feel like it functions as a force for good in world awareness, because at least it will give people a few more data points about a place they don't know well.
So in real life, we might advise people to be cautious about drawing larger generalizations from scant data - but the fact is, people do it. This is where it becomes relevant to writing alien and fantasy worlds. Chances are, residents of a fantasy or alien world will think in much this same way - and draw conclusions about humans, or about their neighbors in another country, accordingly. Think through how people think about each other. If you can, try to go as far as establishing the kinds of major events or rumors that might establish a country's reputation with its neighbors. It will make your whole world feel more real.