We all recognize the phenomenon inflicted by the Steam sale, which leaves us with heaps of unplayed games cluttering our libraries. That's just what Steam sales do to people, right? But why?

The Psychology of Video Games is run by Jamie Madigan, who holds a Ph.D in psychology and is a lifelong fan of gaming, and he recently outlined The Psychology Behind Steam's Summer Sale. New to the 2013 Summer Sale are Steam Trading Cards, which Madigan says nag at your inherent need to complete things you've started - a progress bar shows how much more you need to spend to get a random card, and this taps into a bias toward completion. Steam adds another layer after that with the Summer Getaway badge, which is crafted by collecting 10 of these cards.

Additionally, Steam has perfected the facade of a random reward schedule, something that keeps you coming back at regular intervals, hoping to see a good game in the Daily Deals, or in the eight-hour Flash Sales, or the Community Choice games. People respond to random rewards, such as good games, and even though Valve probably has its sale games strategically planned out ahead of time, they seem random to the audience, Madigan argues.

Steam also works the artificial scarcity angle, offering game sales for a limited time and heightening the chances you'll get it now, before the deal is lost, Madigan says. Just as people value diamonds and panda bears because they're relatively rare, you're more likely to spend more, more often, on things that have "limited availability."

Madigan offers five psychological tricks that Steam seems to be employing on The Psychology of Video Games.