We are at the end of the credit boom — certainly the six-year boom and maybe the 60-year boom. Has any society ever created so many ways for people to go into hock? In 2003, Americans had 1.46 billion credit cards, or five per person. Home mortgages total $9 trillion, and some initially don’t require borrowers to repay all their annual interest. In 1946, households had 22 cents of debt for each dollar of disposable income. Now they have $1.26. Behind these numbers lies a profound social upheaval: the “democratization” of debt. Everyone gets to borrow. But this process may now have reached its limits.

Although Americans are routinely stigmatized as credit junkies, that’s unfair. Of course, some people over-borrow, and some financial institutions lend abusively. Still, the democratization of debt has generally been a good thing. Millions of families can now borrow for college, cars, and clothes. The biggest boon has been the expansion of homeownership, up from 44% of households in 1940 to 69% today. (Three-quarters of household debt consists of mortgages.) At heart, Americans’ appetite for credit reflects national optimism. We presume that today’s debts can be repaid because tomorrow’s incomes will be higher.

The origins of today’s credit culture date to the 1920s with the advent of installment lending for cars and appliances (stoves, refrigerators, radios), says economist Martha Olney, author of “Buy Now, Pay Later.” Attitudes changed. In the 19th century, “it was thought that only irresponsible families bought on credit,” she says. “By the 1920s, it was only foolish families that didn’t buy on credit and use it while they were paying for it.” In the mid-1920s, 60% to 70% of cars were sold on one- to two-year loans.

After World War II, credit became part of the mass market. In 1958, Bank of America introduced a credit card that (in 1976) was renamed Visa. The combination of aggressive merchandising and government laws prohibiting racial and ethnic discrimination in lending led to a huge expansion of borrowers. One reaction to the anti-discrimination laws was the use of impersonalized computer-driven credit scores to determine loan eligibility. Now, American businesses buy 10 billion FICO scores annually.

Credit is about more than selfishness and impatience. “Once consumers step onto the treadmill of regular monthly payments, it becomes clear that consumer credit is about much more than instant gratification,” writes historian Lendol Calder in his book “Financing the American Dream.” “It is also about discipline, hard work” — the attributes necessary to repay the debt and borrow more. Ironically, our optimism feeds our stress.

The trouble is that no society can forever raise its borrowing faster than its income — which is what we’ve been doing. Sooner or later, debt burdens become oppressive. One reason for thinking we’ve passed that point is that the last spasm of credit expansion was partially artificial. To soften the 2001 recession the Federal Reserve embarked on an audacious policy of easy credit. From December 2001 to November 2004, it held its key short-term interest rate under 2%.

A real-estate bonanza ensued. From 2000 to 2005, sales of new and existing homes increased by almost 40%. In hot metropolitan markets, prices more than doubled over five years. Nationally, the increase was 57%. The frenzy depended heavily on low-interest-rate mortgages. In 2005, about half of new home loans had variable interest rates (often with low, introductory teaser rates) or required only interest payments.

What the Fed giveth, the Fed taketh away. Since June 2004, it’s raised short-term interest rates from 1% to 5.25%. Whether the Fed achieves the vaunted “soft landing” — an economic slowdown that reduces inflation without causing a recession — hinges heavily on how the credit boom of the last few years unwinds. If it ends violently, with a crash in home prices and housing construction, a recession could follow. What the Fed wants is a gradual and graceful unwinding.

This turn of the credit cycle could signal the end of the decades-long rise of personal debt to income. It is not just that debt service — interest and principle — is at an historic high, almost 19% of disposable income. Credit standards may have been stretched too far. Since 1989, the share of households with debt has risen from two-thirds to three-quarters. There are other reasons, too: much recent debt was contracted at artificially low interest rates, which have risen (or will rise). Aging baby boomers will reduce debt by repaying mortgages and many Americans will replenish their savings.

For years, the democratization of debt stimulated the economy. What happens without that prop? For better or worse, we may soon learn.