According to fossil records, at the end of the last Ice Age, 20,000 years ago, the average male brain was 1,500 cubic cms.
Now it is 1,350, meaning that we have lost roughly a tennis ball's worth of neurons in volume, researchers said.
Professor Bruce Hood, a psychologist at the University of Bristol, compared the diminishing size of the human brain to differences between the brains of wild animals and their domestic counterparts, 'The Times' reported.
"It runs totally counter to the view that the brain has been getting larger. One by-product of domestication is that brains shrink," Hood said at The Times Cheltenham Science Festival.
The reduction in brain size of about 10 per cent may be linked to the shift from a hunter-gatherer lifestyle toward living in close-knit agricultural communities, where social skills may often have trumped aggression in terms of evolutionary fitness.
If aggression was being "selected against" this may have led to a reduction in testosterone and other hormones that regulate growth, said Hood.
A similar process occurs when wild animals are purposely bred to be companion animals.
"If you breed foxes to become pro-social, they become like puppy dogs. Every animal that's been domesticated has shrunk in brain size," Hood said.