Not too many years ago, an early frost wiped out half the Florida orange crop. There was no need to weep for the orange growers. Prices more than doubled, and revenues hit an all-time high.
Network news commentators cried, "A-ha! Evidence of monopoly power in the orange groves!" Economists cried, "A-ha! Evidence of economic illiteracy among network news commentators!" Not only did the commentators have it wrong; they had it exactly backward: The dramatic price increase actually proved the absence of monopoly power. After all, it had taught us two lessons: First, interruptions in supply are good for orange growers; and second, it takes a frost to interrupt supply. A monopolist wouldn't have waited for a frost; he'd have cut down trees or thrown away oranges long before the weather went bad.
The commentators went astray because they asked themselves the wrong question. They asked, "Why are orange prices high?" when they should have been asking, "Why did orange prices rise?" A uniformly high price is evidence of a monopoly power, but a suddenly rising price is something very different.
To put this another way, you can ask yourself, "Why were prices so much higher after the frost?" or you can ask yourself, "Why were prices so much lowerbefore the frost?" Except for the wording, those questions are identical. If your theory fails to address the second question, then it fails to address the first.
Which brings me back to the great shopping-cart controversy. Last month in this space, I asked why shopping carts have gotten so much bigger over the past 30 years and solicited the speculations and theories of Slate readers. The e-mail response has been overwhelming. I'm trying to read all of the e-mails, but as they continue to trickle in, I keep falling farther behind. For the moment, I'm concentrating on the short e-mails. Anything longer than about 80 lines—and anything, regardless of length, that is littered with html code—is sitting on the bottom of the virtual pile.
With shopping carts, as with orange prices, the problem is to explain not the size but the change in size. The question is not why carts are big but why they've gotten bigger. If your theory doesn't explain why carts were so much smaller in the past, then it can't explain why carts are so much bigger in the present.
So all theories about the advantages of large carts—room for your purse, room for your kids, room for your shopping list—founder unless they are accompanied by explanations of why all that extra room is more valuable this year than in 1970. That's why I liked the e-mail from Susan Provan, who argues that room for the kids is more important in an era with more single mothers.
Quite a few of my correspondents pointed to the fact that supermarkets now carry televisions, VCRs, garbage cans, and other non-food items. I like this story, but it's incomplete without an auxiliary story about why the mix of supermarket items has expanded. The answer, I suspect, is that working women don't have the time to shop at six different stores, and there are a lot more working women these days—so it makes sense for one store to offer everything from pet food to lawn furniture.
Several readers pointed out that shoppers today are more likely to be driving, and those who are driving are more likely to be driving SUVs and minivans, which have a lot more room for shopping bags. I received particularly thoughtful e-mails on this point from both Pamela Nadash and D. Gregg Doyle, who offers this observation: More cars require bigger parking lots. Bigger parking lots require grocery stores to locate on the outskirts of town where land is cheaper. Faraway grocery stores mean longer trips. Longer trips inspire shoppers to stock up and shop less often.
Quite a few readers asked why I don't just call cart manufacturers and ask why they're making carts bigger these days. But I doubt there's much to be learned from that exercise. I'm sure the manufacturers are aware that their customers want bigger carts, but I'm not sure they'd know why their customers want bigger carts. On the other hand—if there's a cart manufacturer out there with some special insight, I'll be thrilled to hear from him. Send e-mail (but no html e-mail) to email@example.com.