Such interest in summits is understandable. They are the high-politics version of a well-scripted, yet, to the audience, unpredictable reality show in which two powerful men of very different backgrounds meet and discuss the interests of their countries. It is fascinating to speculate about the human drama that unfolds mostly in private between them: Can they establish a warm, working relationship akin to a (political) friendship the way Richard Nixon and Leonid Brezhnev or Ronald Reagan and Mikhail Gorbachev were able to? Or, as John F. Kennedy and Nikita Khrushchev or Barack Obama and Putin, will they fail to connect on a personal level and find common ground?

Indeed, there is no better way for leaders to size each other up than in person. Summits provide an excellent opportunity for them to interact as human beings, allowing them to feel and respond to each other in a way they cannot through other media—letters, messages, phone, or even video calls.

Such first impressions matter, because whether leaders see each other as worthy and capable partners whose company they enjoy and by whom they feel accepted can substantially create a sense of and desire for cooperation, positively influencing the conduct of their states during their tenure.

Current U.S.-Russia relations could certainly benefit from cooperative incentives from the top. Good relations between Trump and Putin could introduce much-needed stability in the tense relationship of their states, which is characterized by diabolically opposing interests in many ongoing conflicts around the world, including the ones in Ukraine and Syria.

Additionally, the development of a close, working relationship between Trump and Putin could help them to achieve breakthroughs the way mutual trust helped Nixon and Brezhnev to make the final compromises that were necessary for the successful conclusion of the SALT I negotiations, and that their subordinates had been unable to achieve during the negotiating rounds.

However, the media interest that surrounds these summits tends to overemphasize the effect that positive and negative first impressions have on policies and interstate relations. Leaders do not assess each other on the basis of their personal meetings only. They also rely on intelligence sources, other state leaders’ and diplomats’ personal accounts, and messages exchanged on confidential channels.

Depending on the behavior of their counterparts over time, leaders may also change their opinion and first impressions of each other.

Both President Dwight D. Eisenhower and Premier Khrushchev positively appraised their first meeting in 1959. Nonetheless, a U2 overflight that the USSR shot down in 1960 changed Khrushchev’s opinion about ‘his friend’ whom he felt betrayed him. This episode put an end to their relationship, and resulted in the collapse of the Geneva summit in 1960.

Contrarily, President Kennedy and Premier Khrushchev failed to impress each other during their ill-fated meetings in Vienna in 1961. Kennedy left the summit shocked by Khrushchev’s bellicosity. Meanwhile, Khrushchev found his belief in his own superiority over the young American president confirmed. Yet, once Kennedy was able to remove Khrushchev’s doubts about his resolve in the course of the Cuban Missile Crisis, their relationship swiftly moved toward mutual trust and respect and resulted in the limited Test Ban Treaty in 1963.

Sometimes even positive impressions at a summit fail to have a lasting impact on the relationship of two leaders. Even though President Jimmy Carter made a good impression on Brezhnev in 1979, it was too late to overturn the negative developments that had taken place between them earlier.

Finally, Russian and American leaders may cooperate even when they personally do not enjoy each other’s company, a case in point is the Putin-Obama relationship. Despite the physical discomfort they felt in each other’s company, they could still work together before the Russian annexation of Crimea and incursions in Ukraine. Putin played an integral role in the 2015 nuclear deal between Iran and the United States, which Obama acknowledged with gratitude.

Therefore, the positive chemistry between Trump and Putin in Hamburg is a welcome development, which should, however, be appraised with caution. The future of their relationship will much depend on whether they will be able to sustain they positive impressions about each other.

Eszter Simon is a research fellow at the Institute for Conflict, Cooperation and Security at the University of Birmingham.