Think you know how overweight your country is, or its immigration levels? Then you’re probably wrong, according to an Ipsos Mori survey

Britons think the nation is thinner than it is, overestimate immigration levels and believe the top 1% of earners hold far more of the country’s wealth than they actually do.

But they are not alone in these misconceptions, according to data from an Ipsos Mori survey measuring the gap between public perception and reality carried out in 33 countries.

The common perception among British people is that 44% of the adult population are overweight. However, the Institute for Health Metrics and Evaluation (IHME) at the University of Washington puts the actual proportion at 62%.

Of the 33 countries surveyed, 29 underestimate the proportion of the population that is overweight or obese. Americans think half the population is overweight where in reality two-thirds are. The greatest gap between perception and reality is in Saudi Arabia, where participants believe fewer than three in 10 people are overweight whereas more than seven in 10 are.

South Korea, where just under one-third of the population are overweight or obese, is the only country where perception matches reality. Participants in three countries – China, India and Japan – think there are more overweight people than there actually are.

While many tend to understate the proportion of overweight people nationwide, the opposite applies to immigration estimates, with the majority of nations surveyed overstating the level of foreign-born residents.

In 30 of 32 countries where this question was asked, participants believe there are more immigrants living in the country than there are. The greatest differences in public perception and reality are in Argentina, Brazil and South Africa, where the proportion of migrants was overstated by at least 25 percentage points.

In the US, public perception is that one-third of the population were born elsewhere, whereas only 14% actually are. In France, Germany, the Netherlands and the UK, people assume the proportion of immigrants is about twice the actual level. In Italy, where according to the UN 9% of the population were born elsewhere, participants estimate it to be almost three times that.

When it comes to household wealth held by the wealthiest 1%, the figures show that perceptions vary hugely depending on the country where the question is asked.

Britons are most likely to overstate the amount of wealth held by the wealthiest in society: they believe that 59% of the total household wealth is held by the top percentile, but Credit Suisse estimates it is 23%.

Conversely, Russians are most likely to underestimate the proportion of total household wealth in the hands of the country’s richest. Russians estimate that 53% of household wealth is held by the richest 1%, while the true figure is 70%.

People in most countries overestimate the proportion of people who are atheist, agnostic or do not identify with a religion. Indians think one-third of the population fit into this category, but Pew Research Centre puts the actual level at less than 0.1%.

There is a similar gap in perception among Mexicans, who estimate that 35% of the population are unaffiliated to any religion, when the real figure is 5%.

Britons think almost half of people (45%) consider themselves atheist, agnostic or unaffiliated to any religion. In reality, just a quarter of people identify as such.

Only two countries – Japan and South Korea – underestimate the proportion of people unaffiliated to a religion.

People in a smaller group of countries were asked to estimate to what scale young people aged 25 to 34 were still living with their parents. The mean answer given by Britons is 43%, which compares with the actual 14% who lived with their parents in 2013. This is the biggest perception gap in any of the 14 countries where this question was posed.

In Spain, where 40% of 25- to 34-year-olds still lived with their parents in 2014, the public believe almost two-thirds remain at home. Similarly, French people’s estimate is 36%, which is far higher than the actual 11%.

Participants were asked about the level of female representation in national parliaments. Countries with a relatively high proportion of female politicians tend to underestimate how many women are in their respective parliaments. In Sweden, where 44% of parliamentarians are women, the public put the figure at 38%. The perception gap is even wider in Mexico, where 42% are female but the public perception is 26%.

Russians, on the other hand, significantly overestimate the proportion of women in parliament: the public perception is 31%, whereas the true figure is 14%.

Overall, the country that was most accurate in its perceptions across the issues raised was South Korea, followed by Ireland and Poland. Mexico, India and Brazil were the most inaccurate nations. Britain ranked mid-table in 16th place, while the US was fifth.

Methodology: Ipsos Mori conducted 25,556 online interviews between 1-16 October in 33 countries: Argentina, Australia, Belgium, Brazil, Canada, Chile, China, Colombia, France, Germany, Great Britain, Hungary, India, Ireland, Israel, Italy, Japan, Mexico, Montenegro, the Netherlands, New Zealand, Norway, Peru, Poland, Russia, Saudi Arabia, Serbia, South Africa, South Korea, Spain, Sweden, Turkey and the US. About 1,000-plus individuals were surveyed in Australia, Brazil, Canada, China, France, Germany, Great Britain, Ireland, Italy, Japan, Serbia, Spain, Montenegro, the Netherlands, Norway, Sweden, and the US. About 500-plus individuals were surveyed in the remaining countries. Data is weighted to match the profile of the population. The “actual” data for each question is taken from a variety of verified sources for each question and country – a full list of sources/links can be found here.