Crop circles, twinkling stars, government conspiracies, all of the hallmarks of good alien folklore. Just because the chances of Earth being already visited are considered almost zero by any scientific look at the matter, that does not mean that life could not exist out in the cosmos somewhere.
Even so, there is a large amount of the population, on every country on the planet, that believes that aliens have either visited us, or walk among us already (think Men in Black).
Here are the numbers:
Personally, I’m surprised that the numbers for the USA aren’t higher. We have entire industries, such as the movie, video game, and book industry, that make stories of alien invaders their bread and butter. And this is what I consider to be the genesis of this belief in visitors: humans see weird things in the sky, and in an effort to explain them, sensationalize the story and pass it through a cultural filter of “little green men”.
Do I think life exists out there? Probably. Have aliens visited us? Nope.