Nine out of 10 mortgages bought by Fannie Mae in the first quarter were held by borrowers with credit scores higher than 700, according to regulatory filings. In 2003, the share was 68 percent. Credit scores, developed by Fair Isaac Corp., range from 300 to 850.

Higher loan requirements have displaced about one-third of people who might have gotten mortgages in the years before a collapse in credit quality led to the subprime crisis, Bernanke said at a June 22 news conference. That's an "important problem," he said.

Screening Out

"We screen out about 30 percent of the people who call looking for a mortgage, usually because of their credit scores," said Michael D'Alonzo, president of Creative Mortgage Group in Maple Glen, Pennsylvania, and head of the National Association of Mortgage Brokers in Plano, Texas. "A lot of people don't even try, because they've heard horror stories of how hard it is to get a loan."

Home sales tumbled in three of the past four months even with properties at their most affordable level in a generation, according to the National Association of Realtors. Real estate prices in 20 U.S. cities fell 4 percent in April from a year earlier, the biggest decline since 2009, the S&P/Case-Shiller index showed last week. Pending home sales, a measure of signed contracts, rose 8.2 percent in May, not enough to erase the prior month's 11 percent drop, the Realtors said June 29.

Seeking To Buy

Americans are still interested in buying residences, a sign that tighter loan standards are limiting sales. In May, 5.5 percent of people said they planned to buy a home in the next six months, a record, according to the Conference Board, a New York research firm.

Government efforts to bolster housing so far have had hefty price tags and mixed results. While the homebuyer tax credit of 2009 and 2010 initially increased transactions, sales dropped to a record low in July, three months after it ended.

The credit cost $16.2 billion in lost tax revenue, data from the Government Accountability Office in Washington show. It resulted in 1 million sales that wouldn't otherwise have occurred, according to an estimate by the Realtors association.

"Most of the sales affected by the tax credit were most certainly a change of the time of the purchase, not a change in the decision to buy," said the Cato Institute's Bandow.