In a medical study, it turned out that obstetricians in areas with declining birth rates are much more likely to perform cesarean-section deliveries than obstetricians in growing areas-suggesting that, when business is tough, doctors try to ring up more expensive procedures.

It is one thing to muse about experts abusing their position and another to prove it. The best way to do so would be to measure how an expert treats you versus how he performs the same service for him- self.

Unfortunately a surgeon doesnt operate on himself. Nor is his medical file a matter of public record; neither is an auto mechanics re- pair log for his own car.

Real-estate sales, however, are a matter of public record. And real- estate agents often do sell their own homes. A recent set of data cover- ing the sale of nearly 100,000 houses in suburban Chicago shows that more than 3,000 of those houses were owned by the agents them- selves.

Before plunging into the data, it helps to ask a question: what is the real-estate agents incentive when she is selling her own home? Simple: to make the best deal possible. Presumably this is also your in- centive when you are selling your home. And so your incentive and the real-estate agents incentive would seem to be nicely aligned. Her commission, after all, is based on the sale price.

But as incentives go, commissions are tricky. First of all, a 6 per- cent real-estate commission is typically split between the sellers agent and the buyers. Each agent then kicks back half of her take to the agency. Which means that only 1.5 percent of the purchase price goes directly into your agents pocket.

So on the sale of your $300,000 house, her personal take of the $18,000 commission is $4,500.

Still not bad, you say. But what if the house was actually worth more than $300,000? What if, with a little