Science has achieved many wonderful things, but its gift to human wellbeing is the removal of guilt from gluttony at Christmas - the discovery that we are programmed to over-eat at this time of year.
We are not just greedy individuals with neither will-power nor minds of our own. Prof John de Castro, of the Department of Psychology in Georgia State University, Alabama, has looked at two aspects of our eating habits - how the amount we consume varies with the seasons and how our genetic make-up influences how much we eat.
Nutritionists have long known that we tend to eat about 10 per cent more - or 200 calories - a day in the last three months of the year than we do in the other nine. However, Prof de Castro's research suggests that we actually eat less in the colder months of January and early February than we do in the somewhat less bitter months of October and November. His research compared food diaries kept by individuals which were gathered over six years. This found that food intake in autumn, particularly of carbohydrates "was significantly higher than during the winter".
Prof de Castro wrote at the time how striking it was that these seasonal, or circannual, rhythms were expressed by humans, despite living in modern environments with artificial lighting and temperature control.
His subjects, he found, were hungrier in the autumn (and summer) months, and were less sated by similarly sized meals in the autumn than they would be in the spring and winter. While fat and protein intake remained fairly constant through the seasons, total carbohydrate and sugar intake was significantly higher (by about 14 per cent). The one exception, he says, was alcohol intake, which was lower during the autumn than during the winter.
"Of course it makes ecological and evolutionary sense," he told The Irish Times, "that humans should eat more in the fall when supplies are abundant, in preparation for the winter when supplies are scarce."
How humans instinctively know to do this, however, he has only begun to explain with his recent research, which had 250 sets of twins, who lived and ate separately, keeping detailed diaries of their food intake and the situations in which they ate. The twins had almost identical eating patterns. Genetics, he was forced to conclude, overwhelmingly affect not only how much we eat but our eating behaviour.
"It is in part speculation, but the diaries do seem to indicate that genes not only determine our physical make-up but have a big part to play in our psychological make-up. They affect how much body fat we have, but also how gregarious or social we are, how much we want to be in social situations and how we respond to social pressures to eat more."
We seem to be genetically driven as a species to schedule the big feasting events of Christmas and the US Thanksgiving holiday so as to build up fat reserves to cope with the hardships of winter, he said. And so, science tells us, we have no choice but to bow before the wisdom of our genes, for merrily 'tis there that the real explanation for Christmas stuffing may be found.