(6) We let our emotions get the best of us. In a brilliant experiment from Poundstone's book, volunteers are offered a certain number of dollars out of $10. Offers seen as "unfair" ($1, let's say) activated the insula cortex, "which is otherwise triggered by pain and foul odors." When we feel like we're being ripped off, we literally feel disgusted -- even when it's a good deal. Poundstone equates this to the minibar experience. It's late, you're hungry, there's a Snickers right there, but you're so turned off by the price, that you starve yourself to avoid the feeling of being ripped off. The flip-side is that bargains literally make us feel good about ourselves. Even the most useless junk in the world is appealing if the price feels like a steal.

(7) We're easily made dumber by alcohol, time, decisions. When you're young and drunk at a bar, you're more likely to do stupid things with strangers. "Am I fully assessing this complex romantic situation?" is a difficult question to answer on seven glasses of wine, so we're more likely to ask ourselves a simpler question: "Is s/he hot?" When we're drunk, stressed, tired, and otherwise inattentive, we're more likely to ask and answer simple questions about buying things. Cheap candy bars and gum are situated near the check-out at grocery stores because that's where exhausted shoppers are most likely to indulge cravings without paying attention to price. Boozy lunches are good for deal-making because alcohol narrows the range of complicating factors we can hold in our heads at once. If you want somebody to take an under-examined risk, get him boozed, tired, or ego-depleted.

(8) We're pained by transaction costs... In a personal finance column here, Megan McArdle implored her readers to give up recurring payments like gym memberships and subscriptions to papers and services they don't use. "Don't buy stuff you don't consume" seems like obvious enough advice, but Megan had a great point. We're drawn to subscriptions and memberships and bundles partially because we seek to avoid transaction costs. We'd rather overpay a little than suffer the psychological pain of pulling out a wallet and watching our money go to each gym season/movie/etc.

(9) ... but we're weird about rebates and warranties. Now that I've just told you that consumers try to avoid additional payments, I should add that there are two additional payments we love: rebates and warranties. The first buys the illusion of wealth ("I'm being paid money to spend money!"). The second buys peace of mind ("Now I can own this thing forever without worrying about it!"). Both are basically tricks. "Instead of buying something and getting a rebate," Poundstone writes, "why not just pay a lower price in the first place?'

"[Warranties] make no rational sense," Harvard economist David Cutler told the Washington Post. "The implied probability that [a product] will break has to be substantially greater than the risk that you can't afford to fix it or replace it. If you're buying a $400 item, for the overwhelming number of consumers that level of spending is not a risk you need to insure under any circumstances."