A social network is a network of people or groups of people, for example a group of people who know each other or organizations (companies, institutions) that work together.

Social networks are the subject of study in sociology and social psychology. There are different social networks to distinguish. In a family, school, street gangs or in an office where people see and speak to each other every day, the intensive social contacts have a different influence than, for example, among scientists who see each other once a year at a conference.

To investigate group processes, sociologists often study social networks. To map these networks in schools, for example, questionnaires list all of a student’s classmates, after which the question is asked: who are your best friends? Who do you not like at all? Who are you bullied by? Who is Popular? With these network questions, the relationships in a class can be mapped. Next to it is a social network diagram also called a “sociogram”. If, for example, the school performance or the problem behavior of students is also determined, it can be investigated whether students influence each other’s performance or behavior. The development of SIENA (Simulation Investigation for Empirical Network Analyzes) by Tom Snijders and colleagues at the University of Groningen in the Netherlands is important for studying social networks over a longer period of time. With this computer program, the dynamics in networks and behavior can be studied.

Scientific studies of social networks are also interesting for all kinds of organizations, such as the business community. If knowledge about the functioning of social networks is used, the right person can be put in the right place more often. This promotes cooperation and reduces conflicts.

The verb faboulous women “networking” (maintaining relationships and establishing new relationships during an occasion) is also derived from this.

Six degrees of separation

Six degrees of separation is the hypothesis that every living person on Earth is a maximum of six steps away from every other person in the graph of personal knowledge: to get from one person to another, you never need more than five intermediaries. Various research has been done to support this hypothesis empirically or theoretically.


Frigyes Karinthy, a popular Hungarian writer and translator, was the first to use the term in his short story collection Everything is Different (1929). He assumes that humanity is a network in which all people are connected to each other through a maximum of five intermediaries and six intermediate steps. The term became more widely known from the moment John Guare used it as the title for his play in 1990.


The hypothesis has led to diverse research, with the aim of determining whether it can be disproved or confirmed. A computer simulation performed on relatively simple computers in 1973 predicted that across the entire population of the U.S. all people could make contact with each other within 3 steps.

Small world experiment

A study known as Stanley Milgram’s Small World Experiment was conducted to empirically measure this putative connectedness. Milgram’s study also previously indicated a distance of three steps as an average within the US, with no speculation about the number of steps required throughout the world. Milgram never used the term “six degrees of separation.” Opponents also argue that Milgram’s experiment did not prove such a link.

Computer networks

In 2001, Duncan Watts, a professor at Columbia University, attempted to rerun Milgram’s experiment on the Internet. He used an e-mail message as the “package” to be delivered to 48,000 senders and 19 destinations in 157 countries. Watts determined that the average number of intermediate steps was about six. Another 2007 study by Jure Leskovec and Eric Horvitz examined a data set of messages consisting of 30 billion conversations between 240 million people. They concluded that the average path length among Microsoft Messenger users was six.

Facebook research

The Facebook data team published two articles in November 2011, which showed that among all Facebook users at the time of the survey (721 million users with 69 billion friendship links), the average distance was 4.74.

Mathematical research

In the 1950s, Ithiel de Sola Pool and Manfred Kochen wrote Contacts and Influences. In it they came up with mathematical models of the structure of social networks.

The concept of this network with up to five intermediaries is based on the idea that the number of knowledge circles increases exponentially with the number of links in the network chain, similar to what the friends of the Hungarian mathematician Paul Erdős invented: the Erdős number.