I want to thank Sven Dahlstrand for taking the time to explain to me why opening external links in a new tab is probably not a good practice. Sven helpfully referred me to a page written by usability experts   Jakob Nielsen and Anna Kaley explaining:

Since 1999, it’s been a firm web-usability guideline to refrain from opening new browser windows for several reasons. All of these also apply to opening new browser tabs and are still valid today:

  • More windows or tabs increase the clutter of the user’s information space and require more effort to manage.
  • New windows or tabs can cause disorientation, with users often not realizing that a new window or tab has opened. This problem is exacerbated on mobile, where the old window is never visible.
  • Less-technical users struggle to manage multiple windows and tabs, especially on mobile. (On tablets, where users can have both multiple windows and tabs for the browser, it’s even more confusing.)
  • New windows or tabs prevent the use of the Back button for returning to the previous page and force the user to spend effort to find their way back to the previous content.
  • New windows or tabs are not inclusive for blind or low-vision users — especially when they open outside of the area that’s magnified.

I had been opening external links in a new tab in the hope of keeping visitors on my site but I had not thought about the confusion this can cause, especially on mobile:

Designers open new browser windows on the theory that it keeps users on their site. But even disregarding the user-hostile message implied in taking over the user’s machine, the strategy is self-defeating since it disables the _Back _button which is the normal way users return to previous sites. Users often don’t notice that a new window has opened, especially if they are using a small monitor where the windows are maximized to fill the screen. So a user who tries to return to the origin will be confused by a grayed-out _Back _button.

Jakob Nielsen