It took a few hours to sink in, but investors sold off stocks after some weak economic data was reported this morning. ADP payroll data for March showed a 158,000 increase in jobs, which fell far short of both February's 237,000 and the estimate of 200,000. So far we're overlooking the fact that ADP revised February numbers higher by 39,000 and projecting the possibility of a weak jobs report on Friday.

The other data point worrying investors is ISM's nonmanufacturing index, which fell to 54.4 in March, below the 55.8 estimate. This still signals expansion but shows more weakness in the economy than it did a month ago and adds to weak manufacturing readings over the past few weeks. With about half an hour left in the trading day, the Dow Jones Industrial Average (DJINDICES:^DJI) has fallen 0.68%, and the S&P 500 (SNPINDEX:^GSPC) is down 1%.

Bank of America (NYSE:BAC) and JPMorgan Chase (NYSE:JPM) have done the most damage to the Dow, falling 3% and 2.6%, respectively. One of the concerning numbers in ADP's report was a slowdown in construction hiring, which has driven a lot of the economic recovery. Both megabanks are tied closely to housing, and increased construction signals a healthy housing market, so a slowdown is troublesome. Keep in mind that this is just one data point, and many economists weren't expecting the economy to really pick up steam until the second half of the year, when the impact of tax increases and the sequester are no longer on the minds of consumers and businesses.

Merck (NYSE:MRK) is one of just four Dow components moving higher today, up 1.1%. There are likely two factors driving Merck higher today. First is the increase in Medicare Advantage reimbursements announced yesterday, which should keep Merck from feeling a bigger pinch from insurers. The second factor is that Merck is in a business that sees few swings in demand based on the economy. Investors are fleeing to stocks that won't be hurt if the economy slows over the next few quarters, and Merck is a beneficiary of that flow of money today.

Fool contributor Travis Hoium has no position in any stocks mentioned. The Motley Fool owns shares of Bank of America and JPMorgan Chase & Co. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.