The go-go days of marketplace lending have screeched to a halt in recent weeks, as anxiety spreads among the big-money investors who fueled the sector's rapid growth.

The worries are being fanned by an uptick in delinquent loans at some of the leading online platforms, which has contributed to a decline in the returns enjoyed by loan purchasers, as well as by mounting doubts about the industry's short track record and diminishing confidence in the U.S. economic outlook.

"Investor sentiments are changing now that the asset class doesn't look invincible," said James Wu, the chief executive of MonJa, an analytics firm that caters to hedge funds and other big investors that purchase many of the sector's loans.

In December the monthly yield on loans originated by two leading consumer loan platforms, Lending Club and Prosper Marketplace, fell to its lowest level in more than four years, according to an index developed by the technology provider Orchard Platform.

Investors in those loans earned 0.32% in December, which was down from 0.56% in the same period a year earlier, according to the index.

Part of that decline is attributable to increased competition in the online lending industry. As a flood of venture capital money seeded dozens of startups in 2014 and 2015, incumbent lenders were forced to respond by lowering the interest rates they charged to borrowers, as well as by spending more on customer acquisition.

The recent dip in returns also appears to be connected to the fact that more consumer borrowers are missing loan payments, though 30-day delinquency rates remain below the levels online lenders saw from 2010 to 2012.

"Given the steady increase in delinquency rates, we might expect to see further increases in chargeoff rates in the coming months," Orchard wrote in a February report about the consumer lending side of the industry.

The recent uptick in delinquent loans is not due to changes in underwriting standards, which have remained the same, said Ram Ahluwalia, the CEO of PeerIq, which provides data about the industry to institutional investors.

Ahluwalia said that more recent loans with the same borrower characteristics are performing worse than their earlier counterparts, suggesting that changes in the economic environment are to blame.

"There's a lot of investor apprehension," he said. "Everyone's waiting for the shoe to drop."

As investors grow more skeptical of marketplace lending, loan platforms are reversing course and raising the interest rates they charge to borrowers, in an effort to remain competitive with other investment opportunities.

Within the past month Lending Club and Prosper, as well as business lender Funding Circle, have all raised their rates.

At San Francisco-based Lending Club, interest rates rose more substantially for higher-risk borrowers, while applicants with more pristine credit histories actually saw a slight decrease between late December and late January, according to an analysis by Compass Point Research & Trading.

Lending Club's decision to raise rates for less creditworthy borrowers tracks with the industrywide data, which shows that loans to borrowers with lower credit scores are performing worse than they were a year ago, while loans to stronger borrowers are not showing the same signs of deterioration.

In an interview Lending Club Chief Executive Renaud Laplanche said that if the U.S. economy becomes more distressed, consumers with poorer credit histories will be hurt first.

He explained Lending Club's decision to raise its rates in the context of the gloomy global macroeconomic picture. "So with the likelihood of a U.S. slowdown becoming more prominent, we decided to raise interest rates," Laplanche said.

Funding Circle, a U.K.-based small-business loan platform that also operates in the U.S., hiked its rates earlier this month by an average of 1.8%. In an email, the company attributed the decision to changes in the macroeconomic environment, along with dynamics in the marketplace.

San Francisco-based Prosper raised its interest rates last week by an average of 1.4%, from 13.5% to 14.9%. Again, the rate hikes fell most heavily on less creditworthy borrowers.

Last week's move was only the latest in a series of rate hikes by Prosper since August 2015, Chief Executive Aaron Vermut said in an interview.

"It's very important to us that our investors are being compensated for the actual risk they're taking," he said.

Earlier this month Moody's Investors Service placed on review for possible downgrade certain notes in three securitizations of Prosper loans. The loans were securitized by Citigroup in the second half of 2015, and Moody's stated that delinquencies and chargeoffs were building up faster than it expected.

Moody's has faced criticism for its handling of the matter, with some analysts arguing that the rating agency initially had overly rosy expectations for the performance of the bonds.

Still, the negative action by a major rating agency, on bonds issued by one of the nation's largest banks, has fed into investors' jitters about the sector.

Some large purchasers of the industry's loans went in with plans to securitize them, and those firms are now encountering less favorable pricing.

But at a time of widespread unease in the financial markets, it is difficult to sort out the headwinds facing marketplace lending from broader investor worries about asset-backed securities.

"There is a lack of liquidity across all ABS markets," said Vermut, "which is definitely impacting the ability of platforms to originate new products."

The flagging enthusiasm for marketplace lending among investors is also reflected in the share prices of the sector's only two publicly traded companies, Lending Club and OnDeck Capital.

Lending Club's stock has fallen by 23.6% since Jan. 1, while shares in OnDeck have dropped by 35.3%. The KBW Nasdaq Bank Index, which measures the performance of U.S. bank stocks, has fallen by 15.1% during the same eight-week period.

In addition, investment by venture capital firms in the online lending sector appears to have slowed in 2016, after reaching fever levels last year.

As during any downturn in investor sentiment, it is hard to say whether this one represents ill-considered herd thinking or a level-headed retreat following a period of irrational exuberance.

And it would be wrong to exaggerate the impact of investors' newfound wariness. During the fourth quarter of 2015, OnDeck reported loan originations that were 51% higher than during the same period a year earlier. Lending Club recorded an 82% jump in originations.

The more established platforms argue that they are better positioned to weather a worsening credit environment than smaller, newer competitors that rely on less diverse sources of funding.

But nearly everyone seems to agree that the marketplace lending business, which blossomed during an era of historically low loan delinquency rates, is facing a new test.

"We think we're approaching the end of this credit cycle," said Harlan Peltz, a founding partner in iBorrow, which specializes in commercial real estate transactions. "Most people who are in this environment would agree that capital is harder to find."

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