"Ignorance is bliss", a proverb says. In other words, if you don't think too much or better yet, if you don't use your brain too much, you will go through life much easier.

Although this is undoubtedly true, this type of approach might also lead to dementia, a new study by New South Wales University researchers suggests.

They come with the most convincing evidence yet that complex mental activity during the lifetime halves the dementia risk.

"Until now there have been mixed messages about the role of education, occupation, IQ and mentally stimulating leisure activities, in preventing cognitive decline. Now the results are much clearer. It is a case of 'use it or lose it'. If you increase your brain reserve over your lifetime, you lessen the risk of Alzheimer's and other neurodegenerative diseases," said the lead author, Dr Michael Valenzuela.

Brain reserve refers to the amount of damage that can be sustained before a threshold is reached for clinical expression and reflects the brain's physical properties such as size, synapse count, dendritic interactions, etc.

The key conclusion is that individuals with high brain reserve have a 46 percent decreased risk of dementia, compared to those with low brain reserve. All the studies assessed agreed that mentally stimulating leisure activities, even in late life, are associated with a protective effect.

"This suggests that brain reserve is not a static property, nor that it is determined by early life experiences such as level of education, socio-economic deprivation or poor nutrition. It is never too late to build brain reserve," Dr Valenzuela concluded.