Peer pressure is a more important tool in recruiting young British Muslims to fight in Syria than propaganda videos by Islamic State (Isis), a study will find.

The report by the International Centre for the Study of Radicalisation and Political Science (ICSR) – due to be published in the next few weeks – will conclude that the role of the internet and social media is often exaggerated.

Instead, real-world social networks, friendships and small group dynamics are the decisive influence in radicalising young British Muslim men and making them go to Syria, researchers will say.

Peter Neumann, director of ICSR and professor at the department of war studies at Kings College in London, told the Guardian: “While online recruitment plays a role, people go because they know people who are in Syria. It’s all about networks in the real world. That’s why you get clusters in Portsmouth or Brighton or Cardiff.”

Last weekend, the Bangladeshi community in Portsmouth was mourning the death of a fourth young Muslim from the city to be killed in battle after joining Isis in Syria. The family of Muhammad Mehdi Hassan, 19, said they had been told he was killed in Kobani, a town on the border of Syria and Turkey which has been the centre of fierce fighting. He was one of six from the town who had travelled to fight for the terrorist group using the nickname Britani Brigade Bangladeshi Bad Boys.

Neumann said: “There is a belief that all that was facilitated by the internet. The role of the internet has been exaggerated.”

Over the last two years, ICSR has conducted the most comprehensive study so far of the online activity of foreign jihadis in Syria, who – evidence suggests – are increasingly crossing the Turkish border to join groups such as Isis and the al-Nusra front. Researchers have created a database of the social media profiles of almost 200 western and European fighters, engaging directly with some and building up an important insight into what is driving the flow of foreigners from countries including the Scandinavian nations, the UK, Belgium, France and Australia. Up to 500 Britons are estimated to have travelled to Syria.

Facebook Twitter Pinterest Fighting in Kobani, where Mehdi Hassan is said to have been killed. Photograph: Vincent Wartner/Sipa/Rex

The researchers have built up relationships with young men and a few women from cities including Portsmouth, Manchester, Cardiff, London, Bradford and Crawley, who are now in Syria with jihadi groups. They have learned to discern the fighters from those they describe as “fanboys”.

Neumann said that social media is full of “fanboys” who talk a lot about going. “They are very loud but they are not going to do anything about it.”

While Isis videos – slickly produced by its English language propaganda arm al-Hayat media centre and sometimes resembling computer games – can re-enforce beliefs, Neumann does not believe they are the primary driver of the flow of fighters to Syria.

“The Isis propaganda videos don’t make them go,” he said. “Typically there is a pre-existing friendship. What typically happens is that one or two go over, and that the friends who’ve stayed behind eventually follow them.”

Neumann said it is the result of small group dynamics and peer pressure, “although in many cases it’s simply social obligation and feeling that one wants to be with one’s friends”.

“They will watch those videos and their beliefs will be re-enforced, but what makes them go is that people are out there, egging them on and there’s a degree of social pressure.

“Police are removing the videos and it’s like whack-a-mole, another pops up. It’s not very effective.”

Facebook Twitter Pinterest Mashudur Choudhury is the first person in the UK to be convicted of a Syria-related terror offence. Photograph: Thames Valley police

In a statement issued last month, Mark Rowley, Britain’s most senior counter-terrorism officer, said officials were removing 1,000 pieces of illegal content from the internet each week, including videos of beheadings, murders and tortures, 80% of which is related to Syria and Iraq.

Rowley also said there are 100 Syria-related “preventative activities” per week, with half of those being referred to deradicalisation programmes. A further 66 missing persons have been reported to the police by families worried they may have gone abroad to fight.

So far, 16 people have been charged after returning to Britain from Syria.

Shiraz Maher, an ICSR researcher, underlined the importance of peer pressure and the ease with which would-be fighters can communicate with jihadis in the field through social media. Writing in the New Statesman on Thursday, Maher described the influence Ifthekar Jaman, 23, from Portsmouth – one of the initial wave of British jihadis – had on his peers in his home city and beyond. Before he left for Syria, Jaman had grown close to a circle of five friends from Portsmouth, all of Bangladeshi origin, who grew up within a few streets of each other. The five, including his cousin, Asad Uzzama, 25, travelled to Syria together, following Jaman’s path and joining Isis. Three of them are now dead and one, Mashudur Choudhury, 31, is in prison – the first person in the UK to be convicted of a Syria-related terror offence after he returned to Britain. Jaman was killed in December 2013.

Jaman’s influence extended further than his home town, according to Maher. Another group of friends, from Manchester, were “inspired by his accounts of life in Syria”, Maher said, and took a similar path. One has since died.

Overall, one British citizen dies every three weeks in Syria. Twenty-five Britons have been killed so far.

Earlier this week, Quilliam, a counter-extremism thinktank, said that women who travel to Syria to marry Isis fighters are using social media to shame Muslim men into travelling to the country to fight.

Erin Marie Saltman compared the phenomenon to the white feather girls in the first world war, when men would be shamed and branded cowards if they did not enlist.