Earlier this month, Google announced that its Personalized Search results would come to even computers that weren’t signed in to Google accounts. With the opt-out system in place, many users and public computers can’t help but use personalized search by default—and it may be AdSense publishers paying for it.

The logic goes something like this: if Google is now storing information about what you’re searching for even if you’re not signed in, it may also serve ads based on your search and browsing history, which you’re less likely to click on than ads only relevant to the page’s content. I’m not 100% sure that theory holds true (need more data!), but at least some AdSense publishers are seeing definite drop-off since Google threw the switch.

A thread on WMW documents some of publishers seeing this problem—and others who haven’t. Among those who’ve spoken up, seven of eleven have seen some sort of decrease in December (the original poster cited a significant slide in CTR and clicks from December 5 versus prior years, with CTR down 12.3% to 22.6% of normal average and clicks down 22.8% to 35.2% of normal average).

Of course, the personalized search change isn’t the only explanation in the first 30 messages of the WMW thread, alternate explanations offered include:

• the “Caffeine” update in Google’s index
• annual holiday decline (though the OP and some others note that this is more significant than previous years)
• the wider rollout of interest-based ads
• short sample skewing the results
• sector-specific slowdowns

Interestingly, the original poster returned the day after posting and noted that his CTR had jumped that day. He hadn’t seen a dropoff in his earnings per click (though the new high day carried much higher earnings).

What do you think? What could be behind a decrease—and have you seen one? Is personalized search affecting your AdSense results?