I am often asked by clients whether the external links in their websites should open in a new tab (or window) by default, so I'll set the record straight here. All links, regardless of their destinations, should open in the current tab, not in a new one. There are a few reasons why this is good practice:
Links were never meant to open in new tabs by default. In its most basic form, The code to create a link includes no indication of what tab to open it in, so it opens in the current one, replacing the page where you found the link. To make it open in a new tab, more code - a target attribute - needs to be added: target="_blank"
Every browser has a Back button that lets you return to the previous page. Forcing links to open a new tab is known as "breaking the Back button." The problem is that some people may not realize that a new tab has opened, and when they try to click the back-button to get to the previous page, it won't work.
The most important reason to have links open in the current tab is that it leaves the decision of same tab / new tab in the hands of the visitor. When links open in the current tab, the user still has the option of opening them in a new tab if she so chooses. Any of these methods will open a link in a new tab on most browsers:
If, on the other hand, links open in a new tab by default, then that would be the only option. It's always good practice to give the visitor as much control as possible with the user experience. Let links open naturally. If the user wants to open them in a new tab, she can choose to do so. Don't make that decision for her.
One exception might be pages that contain forms: If I'm filling a form on the page, and then I click a link, I don't want the new page to replace the form page and erase my content. But normally, clicking the Back button will return you to the form with your content still in it, so even this is not so much a concern.
I sometimes hear the argument that opening links in the current tab drives traffic away from the website. If you believe that, then why include external links in the first place? Furthermore, a visitor could just as easily get distracted by something in a new tab and still decide to abandon your site. Besides, if you're concerned that visitors will lose interest in your content just because they clicked a link, perhaps you need to work on your content!
Have I made you consider a viewpoint you hadn't before? Or do you and I think alike? Either way, maybe we should work together?