Notifications are nothing new. A doorbell is a notification system to let me know that someone is outside my house. The ring of a telephone is a signal that someone is waiting to talk. And an SMS ringtone notifies me that a message has arrived.

However, since the arrival of smartphones, notifications have been subtly changing. First, and above all, the apps and websites we use every day love to notify us of everything they can. Our phones no longer just alert us to calls and texts; they now remind us about game activity, tell us when people are tweeting, and nag us to take our 10,000 daily steps. Unlike doorbells, the things that today’s apps and websites deem worthy of a notification often don’t really require our immediate attention at all.

Second, notifications increasingly reach us by whatever means, and in whatever context, they can. Whether it’s an icon with an unread count, text scrolling in from the top of a smartphone screen, a special ringtone, or a disembodied voice assistant, notifications are designed to penetrate our senses and interrupt whatever we’re doing. Distraction isn’t an unfortunate side-effect of notifications – it is one of their core functions. Notifications are intended to draw us away from our current activity, and refocus our attention on wherever that notification is coming from.

Notifications are a powerful tool for UX designers and developers, because they work by pushing people’s psychological buttons. Appealing to a deep human instinct to be socially integrated and accepted, that number in a little red circle subtly tells us both that there’s a social transaction awaiting our attention, and that by ignoring it, we’re missing out on something. By now we’ve probably all experienced that seemingly irrepressible urge to tap an icon just because it has an unread count – even if we already know we’re not interested in looking at whatever’s there.