Micro investing app, Moneybox, allows users to roundup their transactions and save the balance. The app is gaining ground with fresh figures showing that it has more than 100,000 savers having launched in August 2015. And this success has just promoted a new funding round of £14 million. The key question is why. Why is the app proving successful when you could just as easily go into your banking app and set up a direct debit into an investment account and achieve the same result? In truth I'm not really sure but I suspect part of the answer lies in the design of the user interface. There's something appealing, almost addictive about swiping a transaction left or right depending on whether you want to round it up and watching your investments steadily grow week-to-week. This, in my view, is down to the interface tapping into a powerful human behavioral trait that psychologists term ‘cue, routine, reward’. Prevalent in consumer goods marketing and UX design, it’s fascinating to see psychology being applied in this way in financial services.