I recently got massively addicted to an iPhone game called The Simpsons: Tapped Out. Its premise is fairly simple: After Homer accidentally destroys Springfield through a catastrophe at the nuclear power plant, you have to help him and Lisa rebuild and repopulate the town. You do this by collecting in-game money that's generated periodically by the buildings you own and by assigning your characters automated tasks like "Shop at Kwik E Mart" that pay different amounts based on how much time they take.

If the game is moving too slowly for you, you can spend another type of in-game currency called donuts to let your characters complete their tasks immediately, rather than waiting up to 36 hours for some of the more time-intensive ones, or to purchase buildings and decorations—Hank Scorpio's volcano lair, Mount Carlmore—that are unlocked at later stages, or that can only be bought with donuts. You earn donuts in the game, but at an excruciatingly slow pace. If you want enough of them to buy anything of substance, your only real choice is to buy them with actual money through an in-game transaction. A rack of 132 donuts costs $9.99, and will buy you Mount Carlmore with an even dozen to spare.

The Simpsons: Tapped Out is what's known in the gaming industry as a "free to play" game—it doesn't cost anything to play, but if you won't be able to complete every level or collect every item without shelling out real-world currency. Some people are capable of resisting the temptation to pay for exclusive items or boosts or whatever's being sold, but it's not uncommon for players who get seriously hooked to end up investing hundreds or thousands of dollars in an ostensibly free game.

Getting users to make the leap from free play to paid play doesn't happen by accident. Recently the community-curated blog Metafilter pointed to an article listing the most effective tactics in "coercive monetization," like "reward removal" and "progress gates." I clicked through expecting some kind of heads-up meant to expose this scammy aspect of the casual gaming industry, but actually it's something like a primer for game designers looking to improve their product's money-earning potential, which is even more educational.

Like all good cons, legal or not, free-to-play games exploit a number of common psychological traits, like the fact that people are far more likely to spend real-life money in games if it's first converted to a secondary in-game currency, like donuts. It's fascinatingly devious stuff, and after reading it you'll probably think twice before buying the next power gem or dungeon level or whatever your favorite time killer is trying to push on you.