In an attempt to make its Mac OS X software feel more like the iPhone's iOS operating system, Apple changed the default direction that users scroll on Mac trackpads in 2011.
Instead of scrolling the way we've learned to for years on a mouse (scroll down to drag the page down, scroll up to drag the page up), natural scrolling acts like the iPad or iPhone (scroll up to drag the page down, and scroll down to drag the page up).
Users can change that setting if they feel like their world has been turned upside-down.
