Reciprocity is a simple dating site, created by some friends of mine. You sign up and see a list of all your Facebook friends who also signed up. You can put a checkmark next to their name to indicate you want to date them (they can't see this). If you both checkmark each other, then the site reveals you've matched.

This seemed like an obvious great idea. But I started to hear a lot of stories like the following: "I checkmarked Alice's name on Reciprocity, and the system didn't notify me that there was a match, so I assumed Alice didn't like me. Later I asked her out in person, and she said yes and we had a great time."

I always figured Alice was just a jerk who was ruining the system for everyone else. After all, the whole premise was to incentivize honesty. Checkmark the names of people you honestly want to date. If they don't want to date you, they never hear about it, and you would be no worse off. If they do want to date you, the system will let you know, and you can arrange a date. If your pattern of checkmarks doesn't really match who you want to date, you're just screwing yourself and everyone else over for no reason.

A few months ago, someone asked me out on a date and I said yes. And I realized I hadn't checkmarked them on Reciprocity. This caused a crisis of self-loathing. What's wrong with me? Why would I go against my own incentives and ruin things for everyone else?

I asked a friend, who admitted she had done the same thing. Her theory was that asking someone on a date (with all of its accompanying awkwardness and difficulty) was a stronger signal of interest than ticking a checkbox. And potentially there's a grey zone of people who you would only date if you thought they liked you more than a certain amount. And asking them in person is hard enough to be a costly signal that you like them at least that amount, but ticking a checkbox isn't.