"Even as we suffer more frequent attacks, the stock market has continued to recognize the limited economic impact," Kleintop says. And so far, the response to bombings in Brussels seems fairly true to type; while travel stocks have taken a hit, the U.S. markets have been calm. The Dow Jones industrial average and the Standard & Poor's 500-stock index closed with slight losses Tuesday, while the Nasdaq composite index posted a small gain.
Why do investors seem so cavalier about the impact of terrorism? As the Economist explained following the last major attack to hit Europe — last November in Paris — consumers tend to simply defer any purchases or trips during periods of high anxiety rather than forgo them entirely. They will typically start shopping again as they soon as they feel a sense of normalcy return. Or maybe even earlier: Some analysts say that consumers shop in defiance of such attacks, as a way of proving the terrorists haven't won.
However, that relatively superficial analysis may hide some smaller, more specific effects. A 2013 study published in the Journal of Applied Business and Economics found that smaller countries trading a lot with larger countries hit by terrorist attacks are more vulnerable to spillover effects on their economies. Another 2013 study by the same authors, Valparaiso University associate professor Sanjay Kumar and Jiangxia Liu, found that the negative impact of attacks on businesses from 1991 through 2009 rose after September 11, 2001, perhaps because of how that cataclysmic event focused public attention on the issue of terrorism.
The increasing frequency of attacks could impact markets in a couple of different ways.
On the one hand, investors may have begun to price such disruptions into their stock valuations, decreasing volatility in response to attacks. "They may have already accounted for such events, and in the short run stock markets only react to 'unexpected' events," Kumar said by email. "Terrorist attacks may not be one of those anymore."
On the other, investors might also start to look at the cumulative impact that terrorism could have on policy decisions. For example, the rising drumbeat of attacks has fanned calls in the United Kingdom for an exit from the European Union in order to avoid requirements to admit Syrian refugees, which could significantly disrupt cross-border commerce. "You have to begin to wonder if, as the frequency increases, we start to see a collective effect," Kleintop says.
For now, investors seem mostly concerned about the future for travel. At mid-day, according to the Associated Press, shares of Royal Caribbean Cruises had fallen $2.45, or 3.1 percent, to $75.78, while Carnival lost $1.24, or 2.5 percent, to $48.54. American Airlines Group fell 49 cents, or 1.1 percent, to $42.98. Delta Air Lines fell 78 cents, or 1.6 percent, to $49.34. Priceline Group slid $34.43, or 2.5 percent, to $1,316.08, while Expedia fell $1.53, or 1.4 percent, to $109.35.