Old big head: A 3D image replica of a 28,000-year-old skull found in France shows it was 20 per cent larger than ours
It's not something we'd like to admit, but it seems the human race may actually be becoming increasingly dumb.

Man's brain has been gradually shrinking over the last 20,000 years, according to a new report.

This decrease in size follows two million years during which the human cranium steadily grew in size, and it's happened all over the world, to both sexes and every race.

'Over the past 20,000 years, the average volume of the human male brain has decreased from 1,500 cubic centimetres to 1,350 cubic centimetres, losing a chunk the size of a tennis ball,' Kathleen McAuliffe writes in Discover magazine.

'The female brain has shrunk by about the same proportion.'

She was reporting on comments made by Dr John Hawks, an anthropologist from the University of Wisconsin, who argues that the fact the size of the human brain is decreasing doesn't necessarily mean our intelligence is in decline as well.

Some paleontologists agree with this diagnosis, that our brains may have become smaller in size, but increasingly efficient.

But others believe that man has indeed become steadily more stupid as he has evolved.

Several theories have been advanced to explain the mystery of the shrinking brain. One is that big heads were necessary to survive Upper Paleolithic life, which involved cold, outdoor activities.

A second theory is that skulls developed to cope with a chewy diet of rabbits, reindeer, foxes and horses.

As our food has become easier to eat, so our heads have stopped growing, according to supporters of this theory.

Other experts say that with high infant mortality, only the toughest survived - and the toughest tended to have big heads. Therefore a gradually decreasing infant mortality rate has led to a proportionate decrease in the size of our brains.

As increasingly complex societies emerged, the brain grew smaller because people didn't have to be as smart to stay alive

A recent study conducted by David Geary and Drew Bailey, cognitive scientists at the University of Missouri, explored how cranial size changed as humans adapted to an increasingly complex social environment between 1.9 million and 10,000 years ago.

They found that when population density was low, such as during the majority of our evolution, the cranium increased in size. But when a certain area's population changed from sparse to dense, our cranium size decreased.

They concluded that as increasingly complex societies emerged, the brain grew smaller because people didn't have to be as smart to stay alive.

But Dr Geary warns against stereotyping our ancestors as being more intelligent than us.

He said: 'Practically speaking, our ancestors were not our intellectual or creative equals because they lacked the same kind of cultural support.'

'The rise of agriculture and modern cities based on economic specialisation has allowed the very brightest people to focus their efforts on the sciences, the arts and other fields.

'Their ancient counterparts didn't have that infrastructure to support them. It took all their efforts just to get through life.'

Dr Hawks, on the other hand, believes that the decrease in the size of our brains may actually show we are getting more intelligent.

The brain, he says, uses up to 20 per cent of all the fuel we consume. Therefore a bigger brain will require more energy and take longer to develop.

Dr Hawks notes that a boom in the human population between 20,000 and 10,000 years ago led to an unusual advantageous mutation to take place.

He believes this could have resulted in the brain becoming more streamlined, our neurochemistry shifting to boost the capacity of our brains.

But it seems the size of our brains could be on the increase again.

A recent study by anthropologist Richard Jantz of the University of Tennessee found that our brain size is on the increase again.

He measured and compared the craniums of Americans of African and European descent from late colonial times to the 20th century and found that our brain size is on the move again.