MacOS – Inverted scrolling for external mouse

AppleScript is a nice way to go !

tell application "System Preferences"
    reveal pane "com.apple.preference.mouse"
end tell
tell application "System Events" to tell process "System Preferences"
    tell checkbox 1 of window 1
        click
    end tell
end tell

It works on Mavericks. It might need some modifications on older OS X because the UI of the mouse preferences changed.

Update 2020/12/03: As other's have pointed out, this is not a feature of Karabiner in newer versions.

It appears that this is integrated in the Karabiner app so it works out of the box without any custom XML (which are described above).

You'll need the following settings:

• Karabiner Preferences > Change Key > Karabiner core settings > Exclude devices > Don't remap Apple's pointing devices
• Karabiner Preferences > Change Key > Pointing Device > Reverse scrolling direction > Reverse Vertical Scrolling

Set the Enable Natural Scrolling setting in the Mouse/TrackPad PrefPane to 'enabled'. This allows for the internal trackpad to work as it's supposed to. Any connected non-Apple external mouse will scroll in their own natural way.