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There are many reasons for the volatility in the S&P 500 and other markets at the end of 2018, but President Trump’s erratic tweets probably didn’t help. After tweeting that he’d reached a truce in the trade war with China, he seemed to reverse himself, posting, “I am a Tariff Man.” He then issued several tweets critical of the Federal Reserve, prompting two senior advisers to try to reassure the public that the Fed chairman wasn’t about to be fired.

How much do tweets really influence financial markets? From anecdotal evidence, the picture is cloudy: earlier in his presidency, Trump tweets seemed to cause declines in the stock of companies like Boeing (BA) and Amazon.com (AMZN), but those stocks quickly rebounded. The Turkish lira was less fortunate: on August 10, President Trump tweeted that he’d double tariffs on Turkish steel and aluminum. The Turkish Lira promptly dropped 16%, not returning to its previous value until the end of October.

We wanted to see if we could quantify the impact of tweets on the financial markets. In new research, we compare news from Twitter (TWTR) with news appearing in traditional news outlets with respect to the euro zone’s sovereign bond market.

More specifically, we examine the following two questions: First, is there a two-way flow of information between Twitter and traditional news outlets? Second, which source of information influences the bond market more?

We focused on tweets and traditional news announcements relating to the Greek debt crisis between 2012 and 2016 and looked at their impact on the sovereign bond market. We chose to focus on the Greek crisis because it was of global interest and could be easily tracked because the word “Grexit” tended to appear in each announcement regardless of the language in which it was written. We examined the sovereign bond market since sovereign risk affects not only the ability of governments to borrow, repay, and roll over their debt obligations in international markets, but also because it affects borrowing costs of that country’s banks and private sector firms.

First, our data showed a two-way information flow between Twitter’s “Grexit” tweets and the respective mentions in traditional news outlets. In other words, the tweets influenced the news, and the news influenced the tweets. But the influence of Twitter on the traditional news was almost twice as strong as the reverse.

Second, we also found that information on both sources—Twitter and the traditional media—affected the sovereign bond market over and above the impact of financial fundamentals (namely country-specific Credit Default Swap premia and international risk; the latter measured by the Global Financial Stress Index constructed by the Bank of America Merrill Lynch). Again, Twitter had a stronger effect. Specifically, we find that the information on Twitter and traditional news significantly affects the Greek bond spread for up to twenty days; the cumulative impact is 329 basis points (0.33%) for the former and 215 basis points (0.22%) for the latter.

This should not necessarily come as a surprise. With the help of social media, news travels much faster and wider compared to the recent past. And social media of all types—Twitter included—has only grown since the period in our dataset.

Particularly in recent years, it’s become clear that social media cannot only contribute to the efficient sharing of information, but the rapid sharing of misinformation: for instance, it has been found that automated Twitter accounts (bots) can pump out messages that have the ability to affect public opinion and the stock market. And academic researchers have found that rumors can influence the actions of bank users more than reality. And it’s not just high-profile individuals or large groups who might move markets: a 62-year-old Scottish man was charged with spreading false information and costing shareholders $1.6 million in 2015.

The question for investors, then, is perhaps not whether Twitter does move markets, but how much it should.

Costas Milas is a Professor of Finance at the University of Liverpool. Theodore Panagiotidis and Theologos Dergiades are at the University of Macedonia, at the Department of Economics and the Department of International and European Studies, respectively.