I assumed this must be happening — people who default on their mortgages even though they can afford to make the payments — but I’ve never seen numbers until now.

“Who Walks Out? New Studies Shed Light on Strategic Defaults” (Emphasis is mine.)

According to Experian and Wyman, numbers of strategic defaults are far greater than you might expect. Nearly 600,000 borrowers nationwide fell into this category in 2008, more than double the number in the previous year. That number also represents 18 percent of all serious delinquencies from last year.

So what kind of people turn in the keys and walk out on their homes, even when they can pay the mortgage? It’s not who you think ““ not entirely, anyway.

The Experian report looked at 24 million U.S. credit records and found that borrowers with the highest credit ratings are 50 percent likelier to strategically default than lower-rated homeowners. The defaultees often have no adverse credit history, going from a record of perfect payments to no mortgage payments at all.

It’s not longtime homeowners; the Northwestern report said borrowers who bought more than five years ago were less likely to default. Surprisingly, though, ” young people” don’t account for that many walkouts, either. ” The young are more dependent on the loans market and thus face higher reputation costs from defaulting,” the report says.

Above all, though, the studies agree that negative equity ““ being severely ” underwater” in a mortgage ““ is the biggest factor in strategic defaults. ” The homeowners who walk away know full well they are damaging their credit records, but are making a calculated decision that sticking it out over the long-term would be worse,” writes Boston Globe real estate reporter Scott Van Voorhis.

Not all underwater borrowers are equal, however. The Northwestern study says homeowners never walk out if their negative equity totals less than 10 percent of the home’s value. Once that shortfall reaches 50 percent, though, a significant number of borrowers will default strategically.

The outlook can’t be good given those demographics and the fact that the prices of more expensive homes continue to fall.