Yes, there are a lot “defaults” under Admin → Settings → User Preferences
AFAIK most defaults are set for new accounts based on what the settings are at the time the account is created.
Note that the settings will affect only registered members and not anonymous visitors. (i.e. they are “User Preferences” as in “Member Preferences”)
Non-logged in visitors have the option to use their browser to either leave (a click or back button) or stay (a middle-click or right-click open new tab).
IMHO forcing new tabs for external links for them is a (albeit minor) form of browser hijacking because it alters expected behavior without their intent and is a poor idea more likely to irritate visitors than retain them.