Brains get slacker with age in much the same way as the skin, new research has shown.
Sagging skin caused by a loss of elasticity is one of the most obvious outward signs of ageing, but something similar happens to the brain, too, scientists have discovered.
Furthermore, the way it influences folding of the brain surface may be linked to Alzheimer's disease.
Scientists at the University of Newcastle, working with colleagues in Brazil, found that as we age, tension on the cerebral cortex - the brain's outermost layer - appears to decrease.
The effect is more pronounced in people suffering from Alzheimer's.
Lead researcher Dr Yujiang Wang, from the University of Newcastle, said: "One of the key features of a mammalian brain is the grooves and folds all over the surface - a bit like a walnut - but until now no-one has been able to measure this folding in a consistent way.
"By mapping the brain folding of over 1,000 people, we have shown that our brains fold according to a simple universal law.
"We also show that a parameter of the law, which is interpreted as the tension on the inside of the cortex, decreases with age.
"In Alzheimer's disease, this effect is observed at an earlier age and is more pronounced. The next step will be to see if there is a way to use the changes in folding as an early indicator of disease."
Mammalian brain evolution has been accompanied by increasing degrees of folding of the cerebral cortex, providing a greater surface area of "grey matter" - the cell bodies of neurons.
If one half of the average adult human brain was unfolded and flattened out it would have a surface area of about 10,000 square centimetres (10.7 sq ft), roughly one and a half times the size of a piece of A4 paper.
Previous research has shown that regardless of size and shape, cortical folding occurs the same way across all mammalian species according to a universal mathematical law.
Dr Wang, a computer scientist, added: "Our study has shown that we can use this same law to study changes in the human brain.
"From this, we identified a parameter that decreases with age, which we interpret as changing the tension on the cortical surface. It would be similar to the skin. As we age, the tension drops and the skin starts to slacken."
The team also found that female brains tended to be slightly less folded than male brains of the same age. Despite this, cortical folding adhered to exactly the same law in men and women.
The findings are published in the journal Proceedings of the National Academy of Sciences.