Matthew Grove of MailChimp has run some analysis trying to figure out the impact of the new Gmail tabbed interface on email marketing effectiveness. (For those who haven’t heard of or seen the new interface, it’s been written up and Google has explained how it works.)

The new interface creates this tabbed view, with what I call the “outer tabs” — social, promotions, and updates — pulling less critical mail out of the primary stream and leaving behind mail from actual people you are generally corresponding with.

The short version of the research is that the Gmail tabbed interface seems to be decreasing clickthroughs. Here’s Grove’s chart:

Over a three-week period, Grove’s numbers are down around 1 percent, which is a lot when you are only getting clickthroughs of 10 percent to 13 percent in the first place.

Email marketing effectiveness is variable and seasonal, but the numbers and my personal experience has led me to this conclusion: When emails from brands, media, and groups are automatically aggregated into the “outer tabs” in the new Gmail tabbed interface, they are more likely to be ignored at the time of their receipt and not handled in the way that primary mail is. As a result, when users gets around to looking at these non-primary messages they process the mails in a batch triage mode, throwing a lot away unread. I know that’s what I am doing.

Marketers, brands, media companies, and group organizers are going to have to convince Gmail users that their mails are primary if they want to get back to or exceed their old click rates. Google makes it simple to overrule the algorithmic processing of emails — you can simply drag an email from one of the outer tabs to the primary tab, and Google will ask if you’d like all emails from that sender placed into the primary tab in the future or just the selected email — but getting people to do that is going to put new pressure on the email senders to make users want to do so.