The costs of gas and milk have risen about the same over 20 years, but filling up is like no other consumer experience.

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Outside a 7-Eleven in West Seattle the other day, gasoline cost $2.60 to $2.85 a gallon, depending on grade. Inside the store, milk cost $2.50 to $2.99 a gallon, also depending on grade.

Consumers probably grump over the price while pumping gas but don’t think twice as they buy milk. That’s because few people realize that over 20 years, prices of these two commonly purchased liquids have climbed by nearly identical amounts. And grocery prices overall have risen even more than gas prices, according to the federal keepers of price data.

So why don’t you hear legislators fulminating about extortionate milk prices? Why do newspapers such as this one run regular updates on the price of gas but not of bread, which has more than doubled over the past 20 years?

Why, in short, are we as consumers far more sensitive to the price of gasoline than practically anything else we buy?

The reasons have a lot to do with our perceptions as consumers, say experts on buying behavior. Because gasoline is sold separately from other goods and goes through big, unpredictable price swings, each increase stands out and reinforces our sense that current prices are unfair and unprecedented.

“Gasoline is a unique product,” said David Stewart, who teaches marketing at the University of Southern California’s Marshall School of Business. “You have to go to a specific place, a service station, to fill up, so it becomes an event that really stands out relative to other kinds of shopping.”

Buying gas is different

Gas prices have soared in recent years, provoking howls of protest from consumers and worried frowns from many economists. Gasoline in Seattle costs an average of 54 percent more than two years ago, according to the Energy Department. Over the same period, milk prices nationally are up only about 15 percent.

But look out over the long term and a different picture emerges. Since the early 1980s, milk prices have gone up about as much as gas prices — more than 80 percent in both cases, according to the latest available federal Bureau of Labor Statistics (BLS) figures, which run through June. What’s more, grocery prices overall have risen more than gas prices over the past two decades.

One reason for our touchiness, of course, is that we buy a lot of gas. In 2003, the U.S. economy consumed 137 billion gallons of gas — 471 gallons for every man, woman and child in the country. That same year, per-capita milk consumption was just 21.64 gallons.

But as much as we pay for gas, we pay a lot more for groceries — typically the other big weekly purchase. According to the Food Marketing Institute, households spend an average of $92.50 a week on groceries, and more if they have kids. By contrast, the average household spent $25.63 on gas and motor oil in 2003, the latest year for which federal Consumer Expenditure Survey data are available.

In fact, according to the BLS, keepers of the Consumer Price Index, long-term grocery inflation exceeds gas inflation — 89.4 percent versus 84.6 percent over the past two decades.

Bombarded with data

Experts in consumer behavior and behavioral economics say buying gasoline is different both physically and psychologically from the way we buy milk, bread and other staples. At a special place — the service station — we buy gas 10 to 20 gallons at a time, rather than picking up one gallon of milk along with the rest of the groceries. And when we swipe our card or haul out our wallet, all we’re paying for is gas (and the occasional Slushie).

At the grocery store, by contrast, higher prices for milk, meat or bread are subsumed within the overall bill, Stewart said: “If we had to go to the milk store or the bread store, that might become a more salient price point.”

In addition, the big, brightly colored signs at gas stations bombard us with price information every day, noted Suzanne Shu, a professor at Southern Methodist University’s Cox School of Business.

Shu, who uses the insights of behavioral science to study how consumers make choices, said that steady flow of data leads us to judge price changes in the short run — week to week or even day to day — rather than over the long term.

“If I see the price go up 2 cents it bothers me, even if it’s gone back down by the next time I fill up,” she said. “There are all sorts of examples in life where checking more frequently causes us more pain. It’s like weighing yourself every morning versus once a week — every little pound gets exaggerated in importance.”

Robin Segal, author of “The Car Buyer’s Bible,” agreed that short-term price jumps are more likely to cause people to consider ditching their SUVs for small hybrid cars.

“Right now it’s not really cost-effective to buy a hybrid, but what the repeated price spikes do is make people scared that the price will continue to spike up,” she said. “If the price of gasoline had gone up slowly, like milk, I don’t think it would concern people as much, because they can plan for a steady, constant rise.”

20 years of cheaper gas

Just looking short-term, though, can give a distorted picture.

Based on the Consumer Price Index data, after the second big oil shock in 1979-80 and the subsequent collapse in energy prices, gasoline prices generally stayed flat or even fell for most of the next two decades. As recently as February 2002, people paid about as much for gas as they did in February 1982.

That long period of near-stability likely created a “reference price” for gas in people’s minds. An important concept in behavioral economics, the reference price is the idea we carry around in our heads of what something ought to cost. A Baby Boomer might think a Milky Way bar should cost no more than a nickel, while a Gen-Y’er won’t blink at paying 50 cents or more.

When prices suddenly rise above our reference price, SMU’s Shu said, we tend to assume they’re unfair. That plays into another pattern identified by behavioral economists: People feel losses, such as gas-price increases, much more keenly than they do gains.

“When we see gas prices go down we think ‘Oh, good — they’re back where they should be,’ ” Shu said. “But when they go up, if there’s not a cause we can immediately pinpoint in our mind, we think it’s unfair.”

And it’s not just your imagination: Gas prices really do move around more than the prices of other products. A study last year found that, of 350 product categories tracked by the BLS, the three with the most frequent price changes were — ready? — premium-grade gas, midgrade gas and regular gas.

Experiments have shown that most people are willing to accept as fair a price increase caused by supply shortages — for example, from a drought or a transportation mixup — or if a vendor is losing money. But when stores raise prices on a sudden surge in demand — say, for snow shovels the day after a blizzard — people condemn that as unfair price gouging.

Various studies of gas-price spikes over the years have pinned the blame on a combination of factors: OPEC limits on crude-oil production; increased competition for crude from other countries, particularly China and India; U.S. refinery capacity that hasn’t kept pace with demand; local events such as pipeline ruptures and refinery shutdowns; and on the West Coast, the region’s near-isolation from the rest of the nation’s pipeline network.

But none of those reasons, valid though they may be, readily explains why a gallon of regular unleaded at the local Kwik-E-Mart was $2.55 one night and $2.60 the next morning — just the kind of sharp, short-term rise that sticks in consumers’ craws, as well as their minds. “We all have this feeling that there’s this group of guys in a back room somewhere setting the price to maximize their profit,” Shu said.

Spikes vs. steady rises

By contrast, prices for milk and other grocery items have been rising slowly but steadily for decades, with only a few noticeable spikes. As a result, consumers have grown accustomed to prices going up a bit each month.

And, since we generally buy milk a gallon at a time, even if the price spikes it adds only a few cents to our total grocery bill — which is what we really pay attention to. If that number strikes us as too high, we can switch to store-brand orange juice or tell the kids no Snickers snack this week.

But there are few such alternatives to gas, and prices have to rise a lot before we start cutting back on driving.

As a recent Federal Trade Commission report on gas-price changes put it, “[G]asoline consumers typically do not reduce their purchases substantially in response to price increases [which] makes them vulnerable to substantial price increases.”

Drew DeSilver: 206-464-3145