A gallon of organic skim milk in the cooler at Lunds contains organic skim milk. No one gives a thought to actually checking before placing it in the cart.

Freeway Ford’s technician puts new motor oil into an Explorer in the service bay. And when Xcel Energy sends an electronic bill, it can be paid with one click of a computer mouse at wellsfargo.com.

There’s trust involved in all these transactions. And while more complex buys take more checking, the idea is the same. Trust lubricates the engine of a market economy. But we live in a high-trust culture, so it’s one of those luxuries we enjoy without always fully appreciating.

Five years ago we were forced to think about trust, and one of the most bitter lessons learned in the financial crisis is that trust is very fragile. Even five years later, we can still see how very difficult it is to put back together what was once shattered.

Five years ago this weekend was when New York bankers and regulators gave up on the idea that the investment bank Lehman Brothers could be saved. In the early-morning hours of Monday, Sept. 15, 2008, Lehman Brothers said its U.S. holding company would file for bankruptcy.

A bankruptcy usually settles things down. Not this time.

That Monday, the managers of billion-dollar hedge funds decided they couldn’t trust leaving any money with Morgan Stanley or Goldman Sachs, marquee names in U.S. finance and competitors of Lehman’s.

Maybe decided is not quite the right word. Scrambling to pull assets doesn’t reflect a coolly analytical mind, but one gripped by fear.

A day later, the managers of the Reserve Primary Fund, a $64 billion money market fund, told investors that they had “broken the buck,” meaning the net asset value of a share would be less than $1.

Money fund shares were thought to be like cash, and investors put billions of dollars into funds with little thought. And they were going to be worth less than a buck? You can’t trust a money market fund?

Money market funds buy commercial paper, the short-term IOUs many top companies issue every week to fund operations. With billions now being withdrawn from money market funds by institutional investors, the commercial paper market seized.

So not only were investment banks teetering and banks not lending to one another, some big American companies suddenly were confronted with the thought that they just might run out of money.

By Thursday evening, Federal Reserve Chairman Ben Bernanke was telling congressional leaders that a massive intervention was required or “we may not have an economy on Monday.”

“Markets failed,” said Bruce Yandle, an economist now with the Mercatus Center at George Mason University. “Markets fail to operate when trust has been vacated.”

Yandle said he did not really grasp what he was seeing in the fall of 2008 until he started thinking about how professional investors in 2008 woke up and found that securities rated AAA by Moody’s or Standard & Poor’s, the highest grade, were now in default.

A credit rating is one of the things Yandle called “trust-forming devices.”

A buyer of mortgage-backed securities couldn’t invest the time to analyze every mortgage in the pool that backed the security, but he can trust the work of Moody’s.

And Moody’s couldn’t be trusted.

Bond ratings were far from the only trust-forming device that blew up.

The point of Yandle’s best-known paper on the topic is that this is the only financial crisis in our history that resulted from a collapse of trust. It wasn’t a failure of central bank policy, as in 1933, or a lack of liquidity in the market, as in the panics of the 19th century.

And once gone, trust takes a very long time to come back.

Transaction costs up

Yandle answered a question about the return of trust by telling a story about neighbors, when one leaves town and asks the other to please keep an eye on his house.

That works fine until the new big-screen TV goes missing and investigators later find it next door at the neighbors’. That’s a case where those neighbors could live side-by-side 20 more years, and trust never recovers.

The reason trust matters in a market economy is that lack of trust is a major factor in what are known as transaction costs. The money that goes to lawyers and regulators is one example, and so is the time people have to spend digging into the background of a person or firm on the other side of a proposed transaction.

Low transaction costs make markets far more efficient. Capital will flow more easily to the most promising opportunities.

Consumers will readily choose the goods they think best meet their needs and put their savings into investments rather than in a coffee can buried in the back yard. All of this spurs economic growth.

Yandle said there are signs of a recovery in trust, as the volatility in financial markets has settled down, retail sales have recovered and so have some lending markets.

On the other hand, he said, slow bank lending and the increased regulatory burdens on bankers are some signs that trust has a long way to go to fully recover.

“I knew it was serious,” he said, looking back to the fall of 2008. “I knew we were in trouble. But I had no idea we would still be mired in the effects of that collapse five years later.”

There aren’t many benefits that came out of the experience of 2008. About the only one that comes to mind is perhaps a bit of clarity.

If anyone wondered whether the people who run America’s biggest financial institutions deserved our complete trust, the events of 2008 should have settled that.

Of course they don’t.