A French-Swedish Professor, now based in Melbourne, Australia, has developed a data analysis technique that examines social networks to highlight connections and examine their impact on human behaviour. Originally developed for analysing criminal activity, the key player theory is proving useful across multiple settings.

Etymologists think the term ‘key player’ may have its origins in chess. For Professor Yves Zenou, one of the world’s leading scholars in the network economics field, it was economic game theory, a tool for studying conflict and co-operation between people, that inspired his adoption of the term.

Prof Zenou came up with his key player theory while studying African-Americans and segregation in the United States, and was looking for a new cohort to test it on. In 2014, when he heard that the Swedish police needed help to reduce juvenile crime, he was immediately interested.

He wondered, would targeting the key players in criminal activities, rather than those committing the most crimes, yield better results?

Monash_Yvez Zenou

Using data to delve into a criminal network

The Swedish police, like most others around the world, had a lot of data, but no scientific method to sort it with. They tended to concentrate on analyses of major social groups, or networks, such as immigrants, or people with existing criminal records, in their search for insights.

“But these are not always the best people to concentrate on. Some people have a very low record of crime,” says Prof Zenou, who was more interested in studying the importance of particular individuals – the key players – in those networks.

So, along with a colleague at Stockholm University, where he was a Professor of Economics from 2007-2015, Prof Zenou took a different approach. He asked for access to a register of all people suspected of committing a crime, and their associates, over a six-year period.

Each time two people committed a crime together, Prof Zenou linked them in a graph. The network they were able to create with this information charted the links and different connections between all of the people.

In doing so, Prof Zenou was able to arrive at the ‘key players’ the police should be targeting. He was also able to show that targeting key players, rather than the most active criminals, can reduce crime by up to 30%.

“If I remove someone from the network, people are going to react,” he says. “Some people are going to create new things, others are going to commit more or less crime. But by adjusting the network and removing a person and seeing how much crime reduction is indicated by the removal, and then doing that again and again – eventually we were able to identify the key players.”

A research tool with endless real-world applications

Since then, Prof Zenou has applied his theory to assess the vulnerabilities of the banking industry, the links between firms, research and development networks, friendship networks, and to forecast the outcomes of different economic development investment strategies in Africa. All of these topics fit within his broad research interests in urban economics, the segregation and discrimination of ethnic minorities, the identity and assimilation of immigrants, criminality and education.

The theory’s diversity lies in its simplicity. Network economic theory examines how people are linked to each other and how they interact. It provides a mathematical model for the testing and predicting of relationships, and the interpretation of behaviour. It looks not just only at direct interactions between two people, but the ripple effect of links between people they are connected to.

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Prof Zenou’s theory takes this one step further, enabling researchers to remove certain individuals from social networks and observe the impact of that removal on that network. This becomes possible when a data mapping algorithm that can be likened to a process of elimination is applied to a large data set. You have network data? Most likely the key player theory can be used to analyse it in new and interesting ways.

It was recently applied to studies of war and peace in the Democratic Republic of Congo by a team of researchers from the University of Zurich, the University of Lausanne and Yale University.

“The researchers used my approach to look at the networks between the various tribal groups in the Congo, and the fighting between them,” Prof Zenou says. “They wondered what impact adjusting the network in various ways would have on the network, and on the fighting, to help understand how conflict might be reduced. It was very interesting.”

The university with its feet firmly planted in the business world

Prof Zenou’s own network is also worth giving some attention to. He is preparing to host a Summer School and Symposium on Social and Economic Networks, in Melbourne, in June. More than 20 presenters from academic institutions from all around the world, including Stanford and Harvard in the United States and Cambridge University in the United Kingdom, will share their expertise, and 100 attendees – mostly academics – are expected.

The Summer School programme, which runs across two days, will give attendees an introduction to networks, from network formation and games on networks to the econometrics of networks and its applications.

Prof Zenou has been located at Monash Business School in Melbourne, Australia, since 2016, where he holds the Richard Snape Chair in Business and Economics.

It was his expertise in network economics and his work on the key player theory that drew the School to him – and vice versa. The key player theory is highly innovative and transferable to the real world, and is typical of the school’s approach to research, learning, teaching in and about the world of business.

Monash Business School encompasses business and economic disciplines and regularly ranks as one the world’s top business schools. Its 720-strong faculty is spread across 11 departments and centres that students – both undergraduate and postgraduate – might experience when they study at the School.

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