Hello, genetic diversity.
Icelanders have a peculiar problem: they are descended from five Viking men and four Irish women (how does that math figure?)
Because of this, the most genetically homogeneous nation's government have created a genetic database to help fight inbreeding.
When you tap phones with someone who has the app, it brings up an alert if the owners of the two phones share a grandparent. (Of course, if you don't already know who you share a grandparent with, incest may be the least of your problems, but the team says it is looking into functionality for spotting common great grandparents, too.)
The app, called Sifjaspellsspillir translates to "Incest Destroyer."
An iPhone counterpart is currently in production.