Should your site's external links open in a new browser window? The question is better framed as "should my website force external links to open in a new window?"

While it may in fact be the best browsing experience to open a particular link in a new window, the crucial functionality is to let the user choose this method themselves. If they want a new window, they will open a new window.

The primary argument I hear against this idea is "I don't want visitors to leave my site. I want to make it easy for them to refocus their attention back to my site even if they click a link that takes them away from my site." You're afraid you'll lose visitors, and that by opening a new window, you'll keep them 'on' your site while providing a convenient way to see other web sites.

In fact, the opposite is true. Here's a couple of reasons why this is generally considered a bad idea:

1. It is unexpected behavior. The standard behavior for most every link on the internet is to open in the same window. Changing this behavior for your site visitors can be a surprise. While most visitors can deal with the unexpected window or tab popping up, it can damage their impression of you and your site. Some visitors may even get confused by it - if the new window opens unseen in a tab for instance (controlled by the browser setting), or if it obscures the original window underneath, and they think it is the same window (because they expected it to be).
2. You've taken some control away from the visitor by removing a navigation choice. If a user wants to open a new window, they have several options for how to do it themselves. Some prefer a new window, some prefer a new tab (in most modern browsers). Others will use the back button as it is intended.
3. A new window has no history: the back button is reset when a link opens in a new window. The back button is an important navigation feature, and once they are browsing in the new window, they cannot go back any farther than the pop-up. It is not difficult to forget which window was the original and which was the pop-up, especially if you've got a lot of pop-ups happening with every link. A user may accidentally close the original window, and then lose the history in the back button.
4. It can damage trust: By springing a new window on them, you've essentially communicated that you don't trust them to return to your site once they are done with the external link, and you've made the decision to keep two windows open for them. This is subtle but important. It is most respectful of the visitor to let them make this decision themselves. A useful site with good content and a pleasurable browsing experience is what brings people back. Is your site more important than the sites you link to?
5. It's not the same experience in every browser. Older browsers without tabbed browsing capabilities will open in a new window. Most newer browsers with tabbed browsing functionality will open a new tab. In such a case, the new tab may open unnoticed: it is easy to miss the new tab if you aren't looking right at the tab area. However, the new tab has no history- and the user may become confused as to why the back button doesn't work.

Exceptions

There's always an exception to the rule, and some situations will still benefit from this capability, despite the general recommendation against it.

1. When you are linking to a downloadable file such as a PDF or large image.
2. When the data displayed on your current page will be disturbed by reloading or returning via the back button. If there are dynamic elements embeded in the page, such as javascript, flash or others. If a user had to work to produce some of the content on the page, they may benefit from the convenience of leaving it undisturbed.

icon exampleIn all cases, however, notification should be given to the user so they know what to expect. This can be done with an icon, a text notice, or ideally both to indicate what will happen. In many cases, icons don't quite communicate the message alone- many visitors don't immediately recognize the "new window" icon.

It all comes down to providing the best user experience to your visitors. If the answer isn't clear, it may be a great opportunity for some user testing. Sit down and watch some people visit your site, and take note of how they respond. Set up several different variations on your outbound links and watch how users navigate. It is always an enlightening experience.

References

Webcredible | Beware of opening links in a new window

Smashing Magazine | Should Links Open In New Windows?

Jakob Nielson | The Top Ten Web Design Mistakes

Webnauts | Avoid forcing to open in a new window