Keynes famously wished that economists would one day become as useful as dentists. But every time I go to the dentist, it’s clear that knowledge of economics would be useful to to dentists.

The whole idea of cost-benefit analysis seems foreign to them. Every patient gets the same lecture: “If you don’t floss, you’ll loose your teeth. I told you this last time, and you’re still not flossing!” Has it ever occured to them that the marginal benefit of flossing may be less than its marginal cost?

When I ask such questions, I always get the same reaction: “Here’s how gum disease works. Blah, blah, blah.” I want to talk about marginal benefit, and the dentist gives me an anatomy lecture. When I press the issue, and demand, “How many teeth does daily flossing save, on average? What is the probability that I get gum disease conditional on flossing versus not flossing?” they dodge the questions or admit they don’t know.

When experts can’t answer my questions, I google. And while google failed to answer these pointed questions, I learned a lot more in five minutes than I ever learned from listening to dentists. The single most informative piece: “Total Tooth Loss in the United Kingdom in 1998 and Implications for the Future” (Steele et al, Nature, December 2000). What I found out:

• Dentists who warn you that you will lose teeth if you don’t get regular check-ups are half-right. What they don’t tell you is that you are going to loose teeth even if you do get regular check-ups! Here a nice figure showing average number of teeth as a function of age and dentistry.

The biggest gap – 5 teeth – appears in the 55-64 age bracket, when people who get regular check-ups average 21 teeth, and the rest have 16. By the time you’re 75+, average number of teeth is about 17 if you had regular check-ups, and 14 otherwise.

Of course, this assumes the difference is 100% attributable to dentistry. In all likelihood, people who get regular check-ups have better hygiene. Think of a marginal benefit of 3-5 teeth for a lifetime of dentist appointments as an upper bound on the benefit.

• The main reason people loose all their teeth (i.e., become “edentate”) is tooth decay, not gum disease, as this figure shows, and as the article explains:

Decayed teeth was the most frequently cited cause for final clearance (in 64% of cases) and 28% said it was caused by bad gums. However, when asked what problems they had experienced just prior to having all their teeth taken out, over a half mentioned having had some gum problems or loose teeth.

The last sentence seems intended to maintain readers’ fear of gum disease, but it’s not surprising that people with badly decayed teeth also have some gum problems or loose teeth. The basic fact still stands: tooth decay is more than twice as common a reason for loosing your teeth. This in turn suggests that the lazy man’s strategy of brushing but not flossing is a reasonable trade-off.

• Forecasts indicate that people in the future will have more teeth. And since, as every nagging dentist knows in his heart, hygiene is highly unlikely to improve, we’re going to get these extra teeth the lazy way. Impossible? Think about fluoride.

In the end, I still don’t have the answers to the really interesting questions: How many teeth does brushing once per day save? Twice? Thrice? How about flossing daily? Weekly? If anyone knows a dentist who knows the answers – and spares me the anatomy lecture – I’d be most grateful.

P.S. If you’ve got Jstor access, check out Alan Blinder’s classic piece of ecomedy, “The Economics of Brushing Teeth.”