Here’s my Q&A column from the WSJ this week — and if you have any questions for me, you can tweet them to @danariely with the hashtag #askariely, post a comment on my Ask Ariely Facebook page, or email them to AskAriely@wsj.com.

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Dear Dan,

For Halloween this year, we are going to leave a plastic jack-o-lantern full of wrapped treats on the doorstep along with a sign that says “Only one piece of candy per trick-or-treater, please.” Is there anything we can do to make sure children follow this rule?

—Julie 

You might be surprised to learn that this isn’t a new question. Back in the 1970s, a study tested if trick-or-treaters would take more candy from an unattended bowl than they were supposed to. Unsurprisingly, the answer was yes. But the researchers found that if they placed a mirror next to the bowl, children were less likely to take too much. Evidently, seeing ourselves increases our sense of self-awareness, which in turn leads to greater pressure to behave honestly. With that in mind, try adding a mirror to your Halloween display this year.

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Dear Dan,

My wife is from the Netherlands, and we’ve talked about moving there someday. I’d like to start learning Dutch to prepare for this possibility, and I’ve bought some textbooks and recordings to help me practice. How should I approach the task? Is it better to study a small amount every day or to have longer sessions on the weekends?

—Terrance 

Learning a new language is much harder for adults than for young children. It takes time and dedication, but we can easily get discouraged when we feel we’re not making progress. My recommendation is to set yourself the goal of practicing every day, but allow yourself to skip one day each week. Research shows that building some “slack” into our goals helps them to seem more attainable. Just as important, it helps us not to feel like complete failures when we inevitably slip, making it easier to get back on schedule.

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Dear Dan,

I recently attended my first art auction, but to my surprise it was nothing like in the movies—no raised paddles or emotional bidding wars. Instead, participants submitted private bids, and each lot went to the highest, with no chance for people to raise their bids. Why would an auction house use this method?

—Lisa 

What you witnessed was a “sealed-bid auction,” where each person submits a bid in advance and no one knows what other people are bidding. This is less exciting than an “English auction,” where participants bid against each other and the last person willing to raise their bid gets the item and pays for it. These are just two of the many possible auction formats, and new ones keep being invented. This year’s Nobel Prize in economics went to Paul Milgrom and Robert Wilson for their work on auctions.
Which auction format to use depends on the seller’s assumptions about how much buyers are willing to pay. If the seller expects the potential bidders to differ greatly in how much they will pay for an item, it’s advantageous to use a sealed-bid auction, since it keeps high bidders from discovering that they could have won the item for less. When potential bidders don’t know how much an item is potentially worth, an English auction will usually bring the highest price, because it allows bidders to establish the item’s value by learning from one another’s bids. As you discovered, the goal of the auction house isn’t to create the most exciting auction but to maximize sale prices.

See the original article in the Wall Street Journal.