Last Thursday Donald Trump authorised the release of almost 3,000 previously unseen documents relating to JFK’s assassination in 1963. What did we learn? In truth, not much.

But this was America’s greatest conspiracy theorist sharing a moment with the event that has given rise to America’s greatest conspiracy theories. Trump, remember, launched his political career with the so-called “birther” movement, which alleged that Barack Obama was not born in Hawaii but in Kenya, and therefore was not eligible for office (and was also secretly a Muslim).

You can find conspiracy theorists in all walks of life, but they are characterised by sense of exclusion

But conspiracy theories are not just an American phenomenon. Two studies – the first of their kind – conducted by YouGov in collaboration with the University of Cambridge Conspiracy and Democracy project, one in February 2015 and the other in March 2016, show that Brits are just as likely to believe in conspiracy theories as Americans. While 19% of Americans believe that 9/11 was an inside job, for example, we found that 11% of Brits thought it too – and 18% of British people believe that climate change is a hoax, compared to 13% in the US.

In fact, over half the population on either side of the Atlantic believes in at least one conspiracy theory. Did aliens make contact with us at Roswell? Were the moon landings filmed in a Hollywood studio? Why did Building 7 of the World Trade Center collapse on itself? Are we ruled by lizards? And what really happened to Princess Diana? Think about it: at least once in your life you’ve asked yourself one of these questions.

Much has been made of the role fake news and conspiracy theories have played in Brexit and the election of Trump, but these two surveys help us better understand what is going on. Here’s something to consider. In our survey of 2015, 55% of respondents agreed with the statement: “The government is deliberately hiding the truth about how many immigrants really live in this country.” And 52% agreed with the view: “Officials of the European Union are gradually seeking to take over all law-making powers in this country.”

Think about that: one year before the referendum, over half the British population believed that there were too many immigrants in the country and that the EU were plotting to take over the UK – the two arguments made in favour of leaving. It seems that some sections of the rightwing press have made their anti-Brussels agenda heard. And it won’t have escaped your attention that 52% was the final share of the vote in favour of Brexit.

On the question of immigration, the party-by-party breakdown was telling: 87% of Ukip voters agreed, compared with 52% of Conservative, 57% of Labour, and only 37% of Liberal Democrat voters. On the EU takeover, again 87% of Ukip voters agreed, together with 67% of Tory, 42% of Labour and 38% of Lib Dems.

So rightwing voters have been against the EU for some time, which is why Theresa May seems to have no option but to pursue a hard Brexit. Labour has its difficulties, too: a significant number, although not a majority, of its voters are opposed to the EU, but immigration is a real concern, which is why Jeremy Corbyn and Keir Starmer are trying to square single-market membership with control over immigration.

In 2016, a few months before the vote itself, anti-immigration sentiment was again high, but it was higher in one country: Germany. This was partly in response to Angela Merkel’s decision to allow in 1 million Syrian refugees during the summer of 2015. In that context, the rise of the anti-immigration far-right Alternative for Germany party, which made it into the German parliament for the first time last month, could have been predicted.

You can find conspiracy theorists across all walks of life – and yet certain stereotypes remain, and refuse to be disproved: white middle-aged men who have lower educational qualifications and are unemployed are still most likely to be conspiracy theorists. Indeed, it is a sense of exclusion that characterises conspiracy theorists: those who reject the political system as a whole, who have a complete distrust of all political institutions, and those who don’t vote. Or if they do vote, they vote for extremes – in this case, Ukip.

It is not conspiracy theories that are the cause of disenchantment with democracy, but rather disenchantment with democracy that leads to conspiracy theories. Conspiracy theories are the symptom of a deeper malaise at the heart of our societies: political and economic exclusion. In our 2016 survey, which also included five countries from mainland Europe (Germany, Italy, Poland, Portugal and Sweden), we found that countries that are more democratic and less unequal (Sweden) have up to four times less belief in conspiracy theories than those that are less democratic and more unequal (Portugal).

Brexit has revealed that malaise, exposing the divisions in this country. Sadly, if the economic consequences of the vote are really going to be as severe as some say, then Brexit will only increase divisions – and lead to even more conspiracy theories.

• Dr Hugo Drochon teaches politics at the University of Cambridge. He is the author of Nietzsche’s Great Politics (Princeton 2016) and is currently working on the history of elites and democracy in the 20th century