The attacks in Paris in January and November 2015 and more recently in Brussels in March 2016 that involved members of the same family, raise the question of why so many jihadists turn out to be siblings and/or friends.

The first attack in Paris against the satirical magazine Charlie Hebdoon January 7 2015, was carried out by brothers Saïd and Chérif Kouachi. They killed 12 people. That same day, a friend of the Kouachi brothers, Amedy Coulibaly attacked a Jewish supermarket taking 17 hostages and killing four individuals.

The second attack in Paris on November 13 2015, which targeted the Stade de France, the bars and restaurants in the 10th and 11th district and the Bataclan concert hall, killed 130 people, and was carried out by11 individuals, among them friends and two brothers, Salah and Brahim Abdesalam.

More recently, on March 22 2016, two brothers - Khalid and Ibrahim El Bakraoui - carried out the suicide attacks in Maelbeek metro station in the city center of Brussels and Zaventem international airport, respectively.

History is full of examples of siblings and friends operating together. Research shows that being related in one way or another to a person engaged in political activism can boost one's predisposition and encourage action.

study on the Italian armed groups, the Red Brigades, showed that out of 1,214 fighters, 843 had a friend within the organisation prior to their commitment. In 74 percent of cases, the new recruit had more than one friend within the organisation and 42 percent had more than seven friends.

Family, friends and acquaintances act as the 'sponsor's integrator'

The same dynamics are at work in jihadist cells. Marc Sageman showed in his study on cells in Montreal, Hamburg, Khamis Mushayt (Saudi Arabia) and Lackawanna (New York) that out of 174 terrorists, 75 percent of them had pre-existing relationships with individuals already enrolled in jihadist groups.

In the case of the Palestinian militant group Hamas, for instance, Agnès Pavlowsky points out that family ties play a crucial role in the commitment of many young people: It is a mode of 'conviction by impregnation' where the elder sibling convinces the rest of his siblings, sometimes the whole family, to join the cause. Sometimes the influence is ascendant coming from a teacher to his student, an imam to his followers and so on.

During the civil war in Algeria (1991-2001) personal connections served as recruitment channels through which brothers, friends, cousins, classmates and neighbors joined first the Islamist movement with the Islamic Salvation Front (FIS) and later on the armed Islamist groups such as the Armed Islamic Group (GIA), among others.

In some hamlets, villages and neighborhoods in Algeria, entire groups of siblings or cliques of friends joined the armed struggle. This process is called "block recruitment".

Perhaps the most famous example is that of the Zouabri family, whose well-known son Antar, was emir of the GIA from August 1996 to February 2002. His older brother Ali was the first to join a jihadist group, followed by his younger brothers Toufik and Ahmed. Their sister Zohra was also implicated in the GIA.

Siblings within the group serve different purposes: they become emotional sustainers, lift each other's morale, bring comfort and help stand the difficulties and risks of underground life

There was also the case of Mansouri Meliani's family, from which four sons where involved in jihadism. It is worth mentioning that Mourad Si Ahmed, nicknamed Ja'far al-Afghani and one of the GIA leaders, was the son-in-law of Mansouri Meliani, who was himself the father-in-law of Mustapha Bouyali, the leader of the Islamic Armed Movement (MIA), the first jihadist group in Algeria.

In the case of the so-called Islamic State, the same dynamics operate. Family, friends and acquaintances act as the 'sponsor's integrator', in other words, they are facilitators who help foster the appeal of the cause.

As explained by a female former Syrian IS recruit to the al-Khansa brigade: "Since my relatives had all joined, it didn't change a great deal to join".

Examples abound of individuals whose involvement was facilitated by their siblings or friends: In France,Mohamed Merah - responsible for the killing of three French soldiers and three children and a Rabbi in Toulouse, 2012 - was influenced by his older brother Abdelkader and his half-brother Sabri Essid whose father was married to the mother of Merah and the Clain brothers respectively.

The latter were behind the French jihadist network of Artigat. Fabien and Jean Michel Clain left France for IS-held territories in 2014. Fabien Clain took his wife and three children with him as well as several friends. He is believed to be the person speaking in IS propaganda mentioning the November attacks in the French capital.

Similarly, the 16-year-old British twins Salma and Zahra Halane, nicknamed the "terror twins" by media outlets were influenced by their brother Ahmed Ibrahim Mohammed, a 21-year-old who left the UK to join IS-held territories in Syria in 2013.

It is more difficult for an individual to leave or betray to the authorities a group in which his sibling or friend is committed

Several cliques of friends joined IS-held territories together such as the three schoolmates from Bethnal Green in London, Amira Abase, Shamima Begum and Khadiza Sultana.

Siblings within the group serve different purposes: they become emotional sustainers, lift each other's morale, bring comfort and help stand the difficulties and risks of underground life. They encourage each other in case of fear, strengthen each other's beliefs where doubt may set in, and so on. Personal connections are also 'antidotes' to defection and 'pillars' to the continuation of the struggle.

It is more difficult for an individual to leave or betray to the authorities a group in which his sibling or friend is committed. These connections play a role in sustaining participation. Even if the allegiance to the cause is no longer effective, commitment to the siblings and brothers in arms remains strong. As explained by a former Algerian member of the Islamic Salvation Army (AIS):

"I wanted to leave the organization because I did not want to continue the fight anymore, we were becoming animals…but I had two cousins with me... I would not give up... they could pay with their lives... and then I have known people in the hideouts, and we were on the same boat, we became like brothers and I do not give up brothers…"

The fear of leaving behind loved ones, or subjecting them to reprisals in case of departure, forces people in many cases to remain faithful to the organisation

The fear of leaving behind loved ones, or subjecting them to reprisals in case of departure, forces people in many cases to remain faithful to the organisation. In addition, the participation of siblings forges new links by building and consolidating alliances through marriage.

Khaled Sharrouf for instance, who traveled with his wife and five children from Sydney to Raqqa, gave one of his daughters, a 14-year-old, in marriage to one of his friends, an IS fighter called Mohammed Elomar.

While there may be cases in which personal links are present but did not lead to participation, it is likely that jihadist organisations such as IS will continue to use personal networks whenever it is feasible.

Connections between family and friends then become a significant part of the answer as to why some people join jihadist groups. The better our comprehension of this phenomenon, the better our response will be to violent extremism and our ability to draw practical preventive measures.

This article was originally published on The New Arab