Half of all boys will be obese in the UK by 2050, according to Sunday's Observer. There are similar worries in the US, where two-thirds of adults are overweight and one third are obese. So what makes someone put on weight? What contributes most to the rising tide of obesity?

Genes are the conventional answer - in particular, a single copy of a mutated FTO gene, the "fat gene", which raises the risk of obesity by 30%. People with two copies are 67% more at risk.

But this hereditary factor has now been blown out of the water by something much more potent: namely friends.

Researchers from Harvard Medical School and the University of California, San Diego, have found that obesity is not only a physiological matter. Reporting in the New England Journal of Medicine, Professor Nicholas Christakis and his co-author, James Fowler, found that obesity spreads through social ties.

An individual's chances of becoming obese increase by 57% if someone they consider a friend becomes obese. When an individual gains weight, it dramatically increases the chances that their friends, siblings, and spouses will likewise gain weight. The closer two people are in a social network, the stronger the effect.

"What we see here is that one person's obesity can influence numerous others to whom he or she is connected both directly and indirectly," Christakis explains. "In other words, it's not that obese or non-obese people simply find other similar people to hang out with. Rather, there is a direct, causal relationship. Most likely, the interpersonal, social network effects we observe arise not because friends and siblings adopt each other's lifestyles. It's more subtle that that. What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size. People come to think that it is OK to be bigger since those around them are bigger, and this sensibility spreads."

To put it another way, fat is a moral issue (though note, not in any simple sense of blame).

Interestingly, friendship is even more powerful a cause than relatedness. If someone becomes obese, their siblings have a 40% increased risk, and their spouse a 37% increased risk - less than the 57% of friends. In other words, correlations among siblings provide evidence for a biological, and possibly even a genetic component to obesity. But patterns seen among friends indicate that there's more than biology at work.

Who knows what other phenomena, commonly put down to genes, actually find more cause in relationships, causes that are routinely missed simply because researchers have not looked?

Gender apparently plays an important role in obesity too. In same-sex friendships, individuals experienced a 71% increased risk if a friend of theirs became obese. This pattern was also observed in siblings. Here, if a man's brother became obese, his chances of becoming obese increased by 44%. Among sisters, the risk was 67%. Friends and siblings of opposite genders showed no increased risk.

The research also shows that the friendship effect operates over distance. The impact on obesity is not lessened if the friends live near or far from each other.

Obesity is a serious problem. It threatens to undo, perhaps is already undoing, decades of health improvements in the west. This research suggests that to tackle the problem, medical interventions and education in regard to what we eat will not be enough and may not even touch the root of the problem. Education with regard to what we think is acceptable, even beautiful, might be required too - treacherous territory for anyone to enter, let alone politicians.

And there are important philosophical messages from this research too. First, again contrary to received wisdom, we are not just driven by our genes, no matter how "selfish" our DNA might be. Our relationships with others, particularly non-genetically-related friends, can count for more.

Second, morality - what we regard as norms - is a causal factor, even in matters of physiology, like weight. Morality is nothing if not about choice. In short, it seems we are not determined by our innate biology but can determine ourselves - with a little help from our friends.