Eighteen months after the 2001 census results were first published, the Office for National Statistics is still trying to solve the mystery of vanishing men.
When the statisticians first added up the numbers, they found a big hole in the figures: there were a million fewer people than they had expected from the population estimates.
Harder to get a man?
Most of the missing were young men in their 20s, 30s and 40s. It meant that for the first time in nearly a century, there were more women than men throughout the population.
"Continuing to make sense of it all is another year or two's work for us yet," says Len Cook, director of the Office for National Statistics (ONS).
More boys are born than girls, and men usually continue to outnumber women until well into middle age, so it was a baffling result with serious implications - not least for women looking for a partner. So where had they gone?
At first the ONS said that they had all emigrated. So how come no one had noticed so many men leaving the country?
Part of the explanation is that there is no accurate method of measuring emigration and so the slow trickle of men who had been emigrating over the past 20 years had not been noticed.
People also move around a lot more than they used to within the UK and it's getting much harder to count them in a census. Statisticians say that young men, in particular, are notoriously difficult to survey.
The geographer Danny Dorling has made a close study of the census and thinks that we're witnessing a dramatic change in migration.
Easier to get a season ticket?
"This is a new way of thinking for us. This has been happening in other countries for years. Ireland is a classic case. The idea that, for a large number of people, Britain is a country not worth living in is quite hard to grasp, says Mr Dorling.
The areas of the UK where the absence of men is most acute are in the old industrial heartlands of the UK - and particularly around such rivers as the Mersey and the Tyne, and the Clyde.
A counting error?
"It's very hard to think that there was some great systematic error in the enumeration just in these places next to rivers which used to have great shipyards and coalfields," says Mr Dorling.
"Unless there was a conspiracy of enumerators to do a bad job here, it doesn't fit. But what does fit is the story of industrial change."
Part of the confusion over how many men there are in the country also goes back to the 1991 census. There were a million fewer people than expected in the population then as well, but the statisticians didn't trust the results.
It was a year after the poll tax riots and the thinking was that people were avoiding the census to dodge the tax. So the statisticians adjusted the figures.
As heavy industry disappeared, so did the men
In 2001, the ONS realised that in fact too many men had been added to the population - 300,000 too many. As a result, the statisticians have had to revise the population estimates for the past 20 years.
But the mystery is still being unravelled. Last year, the statisticians realised that they had in fact undercounted the male population and they added 190,000 men to the figures.
They also discovered that 14,000 addresses had not been counted in Manchester and they are now taking another look at other local authorities in the country where there is uncertainty about the census count.
The ONS has embarked on some major changes. It is altering the way it measures migration and is also planning to introduce an ongoing population survey. The hope is that will be no more mysteries about the population to solve in the next census.
Where Have All The Young Men Gone? was broadcast on BBC on Radio 4 at 2000 BST on 6 April 2004.