Your friends could be making you fatter: Obesity can spread like a ‘social contagion’

Your social circle may be the reason you are overweight, a study has suggested.

Experts have warned that obesity can spread through communities like a ‘social contagion’.

Researchers studied hundreds of military families – who can’t choose where they live – across the US.

Their results revealed if you move to an area with a high rate of obesity, it increases your risk of becoming obese, too.

For every percentage-point increase in the local obesity rate, the chances a person would be overweight or obese increased by up to six per cent.

Obesity can spread through communities like a ‘social contagion’, researchers at the University of Southern California have said after finding that if you move to an area with a high rate of obesity, it increases your risk of become obese, too

The University of Southern California researchers said people adopt behaviors of others subconsciously.  

Dr Ashlesha Datar and colleagues recruited families of US Army personnel at 38 military bases across the country.

A total of 1,314 parents and 1,111 children participated, whose rates of obesity were reflective of the national rates.

One in three adults in a typical US county is obese, and in the UK, around a quarter of adults and a fifth of children are obese.

Some of the bases were in counties with higher rates of obesity. Rates ranged from 21 to 38 per cent.

The study showed when assigned to bases in counties with higher rates of obesity, residents were more likely to become overweight or obese. 

For every single percentage-point increase in obesity rate of local people, the chances that a teenager would be overweight or obese went up by four to six per cent, while the odds that a parent would be obese went up by five per cent.

The risk of the parent becoming obese or overweight went up by five per cent for every single percentage increase of obesity in the area. 


Portion sizes are nearly 20 per cent bigger than they were 30 years ago, a new study finds.

Researchers say that – despite attempts to add healthier menu options – the number of calories and sodium have increased as the size of burgers, fries and ice cream in our meals have swelled. 

Over the last three decades, entrées have about 90 more calories than they once did while nearly 200 calories have been added to desserts.

Additionally, nearly 14 per cent of the daily recommended value of sodium has been added to our sanwiches and about 12 per cent has been added to our fries and chips.  

The team, from Boston and Tufts Universities, says the results show that the larger portions and calories have likely played a huge role in the obesity epidemic crippling the US.

The longer the families lived there, the more likely they were to see the weight pile on.

Dr Datar said: ‘Social contagion in obesity means that if more people around you are obese, then that may increase your own chances of becoming obese.

‘Subconsciously, you are affected by what people around you are doing. 

‘If you move to a community where a sedentary lifestyle is the norm, you join that. There is this social influence.’

The researchers accounted for environmental factors that could have explained the obesity rate, such as if there was a lack of gyms but lots of fast food restaurants.

Despite this, the obesity connections were still apparent, suggesting other factors were to blame, the study said.

‘We cannot say for sure that we accounted for everything that might influence eating and exercise behaviors,’ Dr Datar said.

‘But we did account for things that researchers in this field typically measure and found that shared environments did not play a critical role in explaining our results.’

Co-author Dr Nancy Nicosia, a senior economist at RAND Cooperation, said: ‘Although we could not measure social contagion directly, our findings support a role for social contagion in obesity.’  

Obesity can be caused by many variables, and previous research shows that living in certain communities carries a higher risk of obesity than living in other communities.

But the association has been challenging for scientists to explain.

One possibility is that people with similar interests and backgrounds tend to locate in similar areas. 

Another explanation may be people are all influenced by the shared environment, such as opportunities for exercising and healthy eating.

A third explanation may be that obesity is transmitted through social influence.

Dr Datar said: ‘Assessing the relative importance of these explanations has been a challenging task and yet is important for designing effective policies to address obesity. Our study sought to take on that challenge.’


Obesity is defined as an adult having a BMI of 30 or over.

A healthy person’s BMI – calculated by dividing weight in kg by height in metres, and the answer by the height again – is between 18.5 and 24.9. 

Among children, obesity is defined as being in the 95th percentile.

Percentiles compare youngsters to others their same age. 

For example, if a three-month-old is in the 40th percentile for weight, that means that 40 per cent of three-month-olds weigh the same or less than that baby.

Around 58 per cent of women and 68 per cent of men in the UK are overweight or obese. 

The condition costs the NHS around £6.1billion, out of its approximate £124.7 billion budget, every year.

This is due to obesity increasing a person’s risk of a number of life-threatening conditions.

Such conditions include type 2 diabetes, which can cause kidney disease, blindness and even limb amputations.

Research suggests that at least one in six hospital beds in the UK are taken up by a diabetes patient.

Obesity also raises the risk of heart disease, which kills 315,000 people every year in the UK – making it the number one cause of death.

Carrying dangerous amounts of weight has also been linked to 12 different cancers. 

This includes breast, which affects one in eight women at some point in their lives.

Among children, research suggests that 70 per cent of obese youngsters have high blood pressure or raised cholesterol, which puts them at risk of heart disease.

Obese children are also significantly more likely to become obese adults. 

And if children are overweight, their obesity in adulthood is often more severe.  

As many as one in five children start school in the UK being overweight or obese, which rises to one in three by the time they turn 10.