A voter wears an “I voted” sticker after participating in the Indiana primary on May 3, 2016, in Noblesville, Ind. (Darron Cummings/AP)

In July, WikiLeaks published 20,000 emails from DNC computers, a move designed to embarrass the Democratic Party on the eve of its national convention. Shortly afterward, Russian hackers were caught infiltrating voter registration systems in Arizona and Illinois.

Cyber experts and the U.S. intelligence community seem confident that the Russian government is behind these covert operations. Some U.S. officials are becoming increasingly worried that there will be more such acts before Election Day.

My research on this topic looks at the long history of foreign governments attempting to help or hinder candidates or parties in elections in other countries. When it comes to covert operations, it usually takes a while until either a smoking gun is found or the smoke clears. With that in mind, here are five things you need to know about such partisan electoral interventions — and what Russia might be up to in the 2016 U.S. election.

1. Major powers have frequently targeted U.S. elections

In October 1796, the French ambassador to the United States published a decree ordering the French navy to restrict America’s trade with Europe. The ambassador threatened that France would “become its enemy” if the United States continued its foreign policies, a lightly veiled reference to the election of John Adams.

John Adams was no more pleased than Hillary Clinton is now with this foreign electoral intervention. In his inaugural address, Adams warned Americans against “the danger to our liberties” that a “solitary suffrage” of this kind poses.

Foreign intervention in U.S. elections became more frequent as the country emerged as a major power. In October 1940, the Nazis leaked a captured Polish government document, hoping to expose Franklin Roosevelt as a “criminal hypocrite” and “warmonger.” The German embassy in Washington gave a U.S. newspaper a bribe to publish the document.

Eight years later, the Soviets intervened against Harry Truman, and in 1984, they intervened again, this time against Ronald Reagan’s reelection bid. Accordingly, if Putin is intervening in the 2016 presidential election, this would be at least the third such intervention by the USSR/Russia.

2. These incursions happen worldwide

Partisan electoral interventions are a common phenomenon in world affairs. I tabulated U.S. and USSR/Russian interventions in a new data set called PEIG (Partisan Electoral Interventions by the Great Powers). I discovered that these two powers intervened in 117 elections around the world from 1946 to 2000 — an average of once in every nine competitive elections. This kind of election interference happened far more frequently than other regime-change efforts, such as the U.S. covert efforts in Iran in 1953.

Both sides used a variety of methods, including public threats or promises, the secret provision of money (frequently in bags of cash) to the preferred party or candidate’s campaign, “dirty tricks,” an increase in foreign aid or other assistance before election day or withdrawing this type of aid, or creating campaign materials and election strategies for the preferred side.

Although common during the Cold War, nearly a fifth (18 percent) of such operations took place in the early post-Cold War era. In the past 12 years, there have been electoral interventions in Moldova, Ukraine, Lebanon, Kenya and Afghanistan — and these are just some of the ones that are public knowledge.

3. Hacking may be new, but the general methodology isn’t

Activities such hacking and using WikiLeaks to influence a foreign election are just the latest “dirty tricks” — acts designed to directly harm one side’s election campaign or candidates. Indeed, my PEIG database shows that one-quarter of USSR/Russian interventions used these techniques.

For example, in the run-up to the 1980 West German elections, the Soviet Union’s KGB spread fabricated stories that connected CDU leader Franz-Josef Strauss with far-right elements in the German intelligence services. A decade earlier, the KGB leaked false documents tying senior figures in various Pakistani parties to political murders and to pre-1948 opposition to the creation of Pakistan.

The United States has engaged in a few dirty tricks as well. In the 1958 Japanese election, the United States gave the Liberal-Democratic Party damaging political intelligence on its main rival, the Socialists. The CIA acquired it from paid informants within the Socialist Party. In the 1990 Nicaraguan elections, the United States leaked damaging information on alleged Sandinista corruption and Swiss bank accounts, funneling the information to German newspapers. The Nicaraguan opposition then used these German media reports to great effect.

4. Putin is probably intervening because he is apprehensive about a Hillary Clinton presidency

Some argue that Putin is looking for payback for Clinton’s condemnation of the quality of the 2011 Russian parliamentary election. Others have argued that it’s simply Putin’s modus operandi as a former KGB agent — or else a component of a Russian strategy to sow chaos within the United States, weaken whoever succeeds Obama and tarnish America’s image.

My insights into why major powers intervene suggest a different motive: Putin likely believes that Hillary Clinton would have inflexible and divergent interests on issues that are very important to Russia. Putin seems to see the favorable resolution of the situation in Ukraine as Russia’s top foreign policy goal — and ending Western sanctions imposed on Russia after the annexation of Crimea is also quite high on his list.

However, Hillary Clinton has a long record of hawkishness on Putin. During a January 2008 town hall meeting, in a riff on George W. Bush’s famous remark in this regard, she half-jokingly doubted that Putin has a soul. As secretary of state, she seems to have been the most hard-line member of Obama’s foreign policy team regarding Russia — a fact that was probably no secret to the Russian government. And in March 2014, she compared Putin’s move into Crimea to Hitler’s strategy in Eastern Europe. Her remarks prompted a harsh public response from Putin.

Given this record, Putin probably believes that a Clinton administration would have an intransigent stance on the Ukraine situation — and would continue to press E.U. governments to maintain their economic sanctions. As a result, he seems to have adopted an “Anyone but Hillary” policy as to the upcoming elections.

5. Electoral interventions are usually effective — but not likely to be in this case

My research found that partisan electoral interventions usually do influence elections, increasing the vote of the preferred side 3 percent on average. This margin may be enough in many elections to determine the result. But in this case, there are three reasons that these recent interventions are not likely to have a significant impact on the U.S. presidential election.

First, my research found that covert interventions are much less effective than overt ones. Covert interventions are more limited in the amount of assistance they can provide because of the need for operational secrecy. So in this case, the choice of a covert intervention by Russia weakens its effects.

Second, electoral interventions by major powers in U.S. presidential elections have historically been ineffective or counterproductive. John Adams still won the presidential election of 1796, and I found that the backlash against the intervention led him to gain more Electoral College votes than he lost.

Likewise, historians who studied later interventions in U.S. elections in 1940, 1948 and 1984 generally concluded that none had any significant effects on these elections. Historian Christopher Andrew saw the Soviet intervention against Reagan as “striking evidence” of the limits of Soviet covert efforts within the United States during this period. The United States has so far been an unusually “hard target” for such interventions.

Third, this intervention has been partly exposed. The number of interventions revealed prior to the elections in my data is too small for a statistical analysis. Nevertheless, I found that one reason that governments choose a covert intervention is out of the fear of a backlash, should their actions be revealed. Accordingly, the exposure of the possible ties between the Democratic National Committee leak or the electoral data heists and the Russian government is likely to weaken any impact of these moves. So, assuming that no new kinds of covert/overt acts take place between now and Election Day, the Russian intervention is unlikely to have a significant effect either way.

Dov H. Levin is a post-doctoral fellow in the Institute for Politics and Strategy at Carnegie-Mellon University. The views expressed in this article are those of the author and do not necessarily reflect the views of Carnegie-Mellon University or of the Institute for Politics and Strategy.