It was recently brought to my attention by @jselss that Google saves your web history and has been doing so for some time.  Now, they are also using this information to fine-tune any searches you do.  See Google’s explanation of Personalized Search below:

I understand that Google is trying to make their search engine better, but I don’t like this idea AT ALL!!! I have been known to search some crazy things in my time.  Do I really need Google to remember these silly searches? Luckily, for all of us, according to Google, there is a way to opt-out:

Signed in searches
To disable history-based search customizations while signed in, you’ll need to remove Web History from your Google Account. You can also choose to remove individual items. Note that removing this service deletes all your old searches from Web History.
Signed out searches
If you aren’t signed in to a Google Account, your search experience will be customized based on past search information linked to a cookie on your browser. To disable history-based customizations, follow these steps:
1.    In the top right corner of the search results page, click Web History.
2.    On the resulting page, click Disable customizations.(Because this preference is stored in a cookie, it’ll affect anyone else who uses the same browser and computer as you).
Or, if you’d rather just delete the current cookie storing searches from your browser and start fresh, clear your browser’s cookies.
Note: If you’ve disabled search customizations, you’ll need to disable it again after clearing your browser cookies; clearing your Google cookie turns on history-based customizations.

*A special thank you to my friends at Remarkable Search for letting me know about this!