Thank you to Adam for writing these instructions and emailing to us:


Starting with 10.7 a new feature has been added to Mac OS X called Gatekeeper. Let me quote Wikipedia here:

“Gatekeeper is an anti-malware feature of the OS X operating system. It allows users to restrict which sources they can install applications from, in order to reduce the likelihood of inadvertently executing a Trojan horse.”

So it’s a feature that prevents you from accidentally launching a program that may be harmful.

Programs (apps) fall into three main categories:

- Apps installed from the App Store (safe)
- Apps installed from registered Apple Developers but not distributed via the App Store (usually safe)
- Everything else, i.e. apps from unidentified, unregistered developers (possibly unsafe)

By default Gatekeeper will not let you open apps from the last category: unidentified developers. 

The fact that a program comes from an unidentified developer doesn’t inherently mean it’s suspicious or unsafe. It just means that whoever made it is not registered with Apple so Gatekeeper doesn’t recognize this developer. This usually means that the developer doesn’t want to pay the annual Apple developer registration fee which is common for those who develop free apps. So there are legit apps out there, such as ImageJ, that Gatekeeper will prevent from being launched on your Mac.

When you double-click on such program you will get this message and you will not be able to proceed:

But there is an override! In order to open a program from an unidentified developer you have to right-click (or Control-click, two-finger-tap) on the app icon and select Open:

This is to prevent you from accidentally double-clicking on something you didn’t mean to open and requires you to take an extra, conscious step to launch the software to confirm your intentions;) Now you will get a different message that just warns you but allows you to proceed:

Gatekeeper will remember your decision and will not bug you about this program again unless the program is updated, replaced or changed in some way.

There is a way to change the default Gatekeeper behavior to allow opening of applications from any source but it’s not recommended for apparent safety reasons.


last updated 20150415