Mortgage rates aren’t what they used to be — and not just because they’re higher.

You can normally predict the going rate for a 30-year fixed mortgage by looking at the yield on 10-year Treasury notes. If the yield’s 3.8 percent, as it was in the middle of this month, you’d expect mortgage rates would be a bit less than 5 ½ percent. Instead, they were hovering around 6 ½ percent.

As Treasury yields dropped earlier in the summer, in fact, mortgage rates stayed steady or even rose. Joseph Bell, a Wonk reader who’s thinking of buying a house, wonders: “Is there any reason for this?”

Yes.

Oh, you wanted to know what the reason is? (So much for an easy post. Can’t a wonk get a day off?)

Keith T. Gumbinger, a vice president at financial publisher HSH Associates, sums it up like this: “Risk means higher rates.”

Lots of people wanted to invest in mortgages back when home prices were rising faster than a helium balloon. Now, with prices falling and delinquencies multiplying, investors still willing to inject their money into the mortgage markets want a better return to make up for the risk.

Concerns about the health of Fannie Mae and Freddie Mac, the big buyers of mortgages, are also driving up rates. So are the additional fees Fannie and Freddie have been levying.

The difference between mortgage rates and the 10-year Treasuries isn’t the biggest it has ever been. But “it is unusually wide,” says Greg McBride, senior financial analyst at Bankrate.com.

Mortgage rates tend to move in concert with 10-year Treasury yields because “T-Notes” are a benchmark for the cost of borrowing, Gumbinger says. Treasuries, the government’s primary method of borrowing, are guaranteed to be paid back, so mortgage rates are set higher to account for the added element of risk. (Most homeowners refi or sell long before 30 years, hence the use of the 10-year note.)

What about the Federal Reserve, which sets the rate banks charge each other for overnight loans? Well … its effect on mortgages is complicated.

The Fed raises rates to combat inflation and lowers them to avoid recession, but it’s now faced with rising inflation and a slow economy. Since April it’s kept its rate steady — potentially adding to inflation. And when the cost of living rises, so do mortgage rates because investors don’t want to lose ground, Gumbinger says.

All this means borrowers are more likely to see rates go up than down for the rest of the year, both Gumbinger and McBride say. Gumbinger thinks rates could go as high as a half-percentage point above where they are now.

But McBride says buyers shouldn’t panic.

“With home prices still sliding, that removes some of the urgency for borrowers that would otherwise be laser-focused on the movements of mortgage rates,” he says. “Buy a house when you’re ready.”

Read the full How-to Monday: Mortgage rates story