One in five searches performed in Google’s search engine are tailored to the user’s particular location, web history, or online contacts , according to software engineer Bryan Horling, who works on the Google personalized search team.

Search results are different from country to country – that much is known. Now, Google goes into the more specific details and data known about the user, and is tweaking results based on the individual user’s behavior. Horling states, though, that the differences are minor – results may move a few spots up or down, but Google is not changing the entire character of the page.

You don’t have to be signed in to a Google account to be subject to this personalization. Google will attempt to deliver personalized results to any user using its search engine, depending on their web history and cookies. If you are signed in, the results will even vary according to your Gmail contacts or people you follow on Buzz. Google calls this “Social Search”.

SEO companies, of course, aren’t happy. This personalization process by Google also means that soon every user will get different results, and it will be very hard to determine whether SEO efforts are successful and efficient. The fact that even logged-out users are getting personalized results is a real problem for SEO people.

Google urges site owners to start making sites for users, not for Google – a lesson that a lot of sites will learn the hard way as time passes.