What's the cost?

I believe that it's really important for this app to be free.
I could've ordered in a button and wired it up to the phone, but using the headphone buttons means that the app is freely available to anyone.
This means that the data will reflect opinions and input from a variety of people, not just a select group who could be bothered to order a button.

This app is designed to change people's everyday lives for the better. Not just for stress reduction, but for safety too. I think that if the app is free and easy to use then more people use it, and the more people use it, the better the data will be and more useful it will be to users in turn.