THE English are fat and getting fatter. The scales of the average adult clocked 77.5kg (171lb) in 2014, an increase since 1993 of 5.1kg—about the weight of a border terrier. Over the period the share of adults classed as obese rose from 14.9% to 25.6%, about twice the rate in France and Sweden.

And yet policy analysts have been trying to digest a podgy paradox. Data suggest people are heavier—but also eating less. According to the Living Costs and Food Survey (LCFS), a long-running study that tracks shopping, average daily calorie purchases fell from 2,534 in 1974 to 2,192 in 2013. Another official survey based on reported food consumption found a similar pattern.

People gain weight when they consume more calories than they burn off. Therefore the two official data sources suggest that England’s weight gains result from people exercising too little, not eating too much. Researchers using these data tend to agree. A forthcoming paper by the Institute for Fiscal Studies, a think-tank, argues that the rise in obesity in England reflects how jobs and housework have become less strenuous. They suggest sloth is more important than gluttony.

But a report published on August 8th rejects this idea. Hugo Harper and Michael Hallsworth of the Behavioural Insights Team, a research group spun out of government, compared official data with those from other sources. A private survey and, more damningly, measures of actual calorie consumption, suggest that the participants in the LCFS are dramatically under-reporting their intakes. The authors estimate that England consumes 30-50% more calories than is declared.

Such under-reporting would mean that sloth is not the main cause of England’s weight gain. For this to be the case the country would have to have seen a big drop in activity, according to Mr Harper and Mr Hallsworth. They estimate that it would need to have been akin to every adult jogging for 56 minutes less per day than in the 1970s. This is hard to believe.

There are two reasons why people may have been fibbing about their calories. The first is that such data are hard to track, especially given that snacking and eating out have become more common. The second reason is wishful thinking. People who say they want to lose weight are more likely to underestimate how much they eat. And since fat people are more likely to say they want to lose weight, the rise of obesity has led to a growing tendency to under-report.

Simon Stevens, head of the National Health Service, says obesity is “the new smoking”. It costs the NHS billions of pounds per year and cuts life expectancy by up to ten years (about the same as a lifelong cigarette habit). The report’s findings therefore have important implications.

The government is looking at new ways of measuring calorie consumption. And the report may encourage policies targeting gluttony rather than sloth: for example, an expansion of the sugar tax that was announced in March. Nearly one-third of English children are overweight or obese. There is no time for flabby thinking.