A Google Webmaster Help thread has one webmaster being a bit obsessed about making sure Google does not know he has affiliate links on his web site.

He is under the belief that once Google figures out his site links out as an affiliate, Google will automatically downgrade the site.

Over the years, webmasters have come to learn that affiliate sites are hard to rank in Google. But why? Is it because they are linking to other sites that sell the same product? Or is it because the content is often not unique?

Google’s John Mueller said in a Google Webmaster Help it is because of the unique content not being there. John wrote:

This is pretty obvious stuff but when SEOs get so deep into their daily grind, they don’t always step back and look at the whole picture.

As a side note, I’d nofollow affiliate links just to be careful.

Forum discussion at Google Webmaster Help.