Here's my favorite headline of the day: "Retail Investors Are Back! But Don't Hit 'Sell' Yet," courtesy of our friends over at CNBC. The point of the article is simple. It's long been assumed by professional traders that the time to start selling out of a market is when mom-and-pop investors start getting in. If you haven't already done so, I suggest you consider the implications of that.

With this in mind, it should be no surprise that the Dow Jones Industrial Average (DJINDICES:^DJI) is tanking today. According to data cited in the article, mutual funds and exchange-traded funds recorded net inflows of $4.5 billion over the last week, adding to an already considerable influx since the beginning of the year. "It's driven today by people who feel that they've been missing out, but we're still in the early innings of retail investors coming back," an analyst told CNBC.

Of course, the other explanation is that today's triple-digit sell-off was triggered instead by a handful of disappointing economic releases. In the first case, payroll-processing company ADP reported that private-sector employment increased by only 158,000 jobs last month. Economists surveyed by Bloomberg called for a gain of 200,000. And in the second case, the Mortgage Bankers Association said this morning that mortgage applications dropped last week by 4% despite the fact that mortgages rates declined as well.

While there's probably some truth to both, the performance of the Dow's banking stocks today certainly adds credibility to the latter. With about an hour left in trading, JPMorgan Chase (NYSE:JPM) and Bank of America (NYSE:BAC) are the index's worst-performing components, down 2.5% and 3%, respectively. Given the importance of mortgages and the associated fees to these businesses, this should be no surprise. In the fourth quarter of last year, JPMorgan originated $51.2 billion in mortgages, while B of A notched $22.5 billion -- though both were dramatically outdone by Wells Fargo.

Alternatively, the best-performing stock on the Dow this afternoon is Merck (NYSE:MRK), up 1.6%. As my colleague Dan Dzombak observed earlier, the pharmaceutical company's performance today comes on the heels of the government's surprise decision to expand, rather than contract, the amount of support it's offering to Medicare Advantage customers. According to Dan, "While [the changes] don't benefit Merck directly as they would health care plan providers, higher reimbursements should mean that drug revenue will not fall as previously expected."