This is the advice given to most writers when it comes to, well, everything. Telling your audience that the protagonist likes to eat pizza on the weekend isn't as effective as having the person simply order the pizza and enjoy eating it while watching TV.
I just read a post by Paperback Writer, where she relates a story that happened to her in a restaurant. She'd recently had surgery and her eye was affected by it. Her waitress believed - due to the 'shown' information, that her husband had hit her and treated the couple accordingly. Paperback Writer concludes with the moniker that we need to be aware that our readers may not see our hints the way we intend, and that sometimes we need to tell to get the right message across.
This actually brings up a different question for me. How much of stereotypes, how much misinformation about people, races, cultures, religions is due to literature showing rather than telling? We've been trained to take a few pieces of often disjointed information and pull together a so called composite picture from that.
If you saw a woman, sitting with her back to the room at a restaurant, wearing sun glasses that - once removed - prove she's sporting a black eye, what conclusion would you jump to?
What about the black man standing in line behind you, one hand in his coat pocket, obviously covering something? Would your first thought be that he's holding his wallet because it's a bad neighbourhood? Or would you think it was a gun - because that's the stereotype we've been trained to assume is the truth?
How about the Muslim woman wearing the hajib. Do believe she's wearing it because her husband is forcing her to or because she's a pious woman who honestly prefers it?
And why are we too afraid and ashamed to ask and discover the truth?
I'll never forget reading Black Like Me by John Griffin. I shook my head at a lot of the racist slurs being perpetrated against the African American population as described in his book. Growing up in Canada, born after the civil rights movement ended, I had never heard these things before. Nor had I realized how many people believed such slurs to be the truth. Or how many people continue to perpetuate such insidious lies rather than discover - or accept - the truth. The truth that people are inherently all the same. We have the same needs: food, shelter, heat and love, the same wants: to provide for our families, be successful and find happiness.
Maybe we need to discover a better way of writing. One that doesn't require jumping to conclusions. Or perhaps we need to realize that jumping to conclusions about characters in a novel is harmless - while doing the same with real people can be devastating.