It is called the last bastion of prejudice. Nowadays everyone seems to claim the right not only to poke fun but also to pour scorn on fatness – even when this means pointing a pudgy finger at the majority of the adult population.

The routine stigmatising of obesity is on the increase, perhaps exemplified by the comical Ricky Gervais, who received a lot of attention when – far from joking – he suggested people should shout "fatty" at him in the street to make him feel ashamed of his weight. The enthusiasm of the media to give vent to this self-deprecating put-down reflects an unusual appetite for animosity towards obesity.

The blame culture that characterises the obese as "greedy fat people" – entirely at fault for their own plight – is based on a startling ignorance. Evidence has been amassing over more than a decade of the role of key genes linked to obesity. Gervais can't be sure he isn't among the one in six of us with a double dose of the FTO gene carrying a 70% higher risk of beefing up the BMI.

In trying to unravel the mystery of why some of us and not others are more likely to become fat, the finger also points towards epigenetic influences, which affect the way in which genes are switched or expressed. This effect can be passed on across generations, even though the genes themselves are unchanged. In theory the tendency to store fat – understood in any case as an evolutionary imperative – may be a legacy of epigenetics, with the survival mechanism to combat famine embedded in our genetic memory.

There are other factors such as the hazards of being a low birth-weight baby or a large baby – both risks for a future of being subjected to bad jokes and outright prejudice. More recently scientists have been pointing to the impact of the cocktail of chemicals to which all of us have been exposed to different degrees throughout the second half of the 20th century. DDT, PCBs, dioxins are a few of the more familiar ingredients in the alphabet soup of endocrine-disrupting toxins that have been abundant in the environment and whose residues are still affecting wildlife in even the remotest parts of the world. Our plastic-lined world may well be contributing imperceptibly to making us all fatter.

So the eagerness of some people to line up and shout "fatty" is ironic when there are a multiplicity of factors at work and the majority of us are not just overweight, but advancing rapidly to join the one in four adults who are already technically obese in the UK. Who speaks for them? Who can challenge the more insidious prejudice that denies the obese access to treatment?

Ironically many are working in the media, whose expense account meals and "lifestyle choices" have set them on the slippery slope towards obesity. Perhaps this last bastion of prejudice is tolerated because all of us feel guilty when we see the scales rising and our belts loosened by another notch or two. There but for fortune…