Here’s a bit of a tricky conundrum. As a rule, it’s assumed (almost certainly correctly) that every user will expect internal links (i.e. links that are taking you to a different page on the same site) to open the page in the same window rather than loading pages in endless new browser windows or tabs.
When it comes to external links, however, the waters are muddied slightly by the fact the website owner usually wants to keep people on their site, so the default behaviour of links is thrown out in favour of forcing the link to open in a new window or tab. In my experience, you can guarantee that any client will ask this to be done for external links. The thought of taking the user away from site is just not an appealing one, which is of course understandable but the problem is that decision is almost always made without thought for what the user will be expecting to happen when clicking the link. For example, if you have an external link that, by default, opens the site in the same window then the user has the choice based on their knowledge of the link.
If it’s clear that the link will be taking them to another site, then they have the option to force the link to open in a new window or tab using the right click menu or the middle button on their mouse. If a developer has instructed the link to open in a fresh window, however, the choice is completely removed and the user could end up having the website behave in a way they are not expecting.
Counter arguments are all based on the assumption that the website knows better than the user. “What if they don’t know how to open a link in a new window?” is the main point, but personally I would rather trust the user to know this basic functionality and leave the choice with them.
At the end of the day, if your site is interesting and appealing enough, having a link take the user away from it shouldn’t matter. Tricking the user into non-standard behaviour just to keep your site open is, at the end of the day, just not worth sacrificing usability and consistent behaviour.
