An anecdote about buying the wrong ice cream reveals a jarring truth about relationships

Steven Depolo/Flickr

On a recent trip, Nicholas Epley and his family went out to get some ice cream.

Since they'd brought their dogs with them, too, Epley volunteered to wait outside with them while his wife ran into the store.

"You go ahead," Epley told her, "and you just get me whatever you think I'll like."

A few minutes later, she returned, coffee-flavored ice cream in hand.

"I was surprised by that," Epley told me later, "because I thought she knew that I hate coffee-flavored ice cream."

In fact, he added, "it's probably the last flavor I would have picked out."

The mistake surprised his wife, too, because they've been married 20 years. "Of course she knows what kind of ice cream I like," Epley told me, "but in that case she didn't."

The incident might have gone unremembered, but Epley is a psychologist and a professor at The University of Chicago Booth School of Business. And he's all too familiar with the idea that we think we know our partners a lot better than we really do.

In his 2014 book, "Mindwise," Epley cites scientific evidence of that very phenomenon.

In 1997, researchers at the University of Texas at Austin recruited about 60 heterosexual couples at the school, who had been dating for between three weeks and six years. Members of each couple were randomly assigned to either respond to a bunch of survey prompts or guess how their partner would respond to those same survey prompts.

For one survey, participants had to indicate how much they agreed with statements like, "I perform well at a number of things." Another survey measured participants' self-perceived intelligence, athleticism, and attractiveness, among other things. The final survey asked participants to indicate how much they enjoyed different activities, like going to a bar and playing board games.

Steven Depolo/FlickrSure enough, couples who had been together longer were more likely to express confidence in how well they knew each other. Yet results showed that relationship length was not related to accuracy. Moreover, couples were in general much more confident about how well they knew their partners than they should have been.

For example, on the survey that asked participants to rate their own intelligence, athleticism, and attractiveness, people thought they could correctly guess their partners' responses about 80% of the time. Unfortunately, they only guessed right 30% of the time — which isn't that much higher than if they'd guessed randomly (10%).

The illusion of familiarity can have real consequences — more serious, of course, than buying your partner the wrong ice cream. Maybe you have no idea that your partner feels badly about his looks, or that she hates going to bars even though you go all the time.

Perhaps the best solution is one that Epley puts forth in "Mindwise": Just ask. It's not easy, because we presume we should know what our partner is thinking and feeling, but it's probably better than guessing wrong.

NOW WATCH: What you should talk about on a first date, according to research