The world's largest study of the human brain, involving over 200 scientists worldwide, found genes that affect brain size and may play a part in intelligence and memory function.

Dr Margie Wright from the Queensland Institute of Medical Research (QIMR), which contributed to the study, said brain size can not only have an effect on people's thoughts and behaviour, but also intelligence.

The study was put together by combining brain scans and genetic data from 21,000 people worldwide.

Dr Wright said one gene showed a strong correlation with overall brain size while another influenced the size of the brain's hippocampus, which is involved with memory.


Dr Wright said the gene involved with the hippocampus influences the rate at which this part of the brain shrinks with age.

People with dementia often show pronounced shrinkage in the hippocampus so further investigation to see if there are genetic links to dementia will be worthwhile, she said.

The hippocampus is also reduced in people with schizophrenia and major depression.

A separate study at QIMR showed those with larger brains scored slightly higher on a standardised IQ test.

Dr Wright said the global brain study, which has created the world's largest database of brain imaging results, could be a stepping stone for more work into the brain's genetics and disorders.

"The effects of the two genes on brain size are very small and the links to cognitive function are subtle," Dr Wright said.

"However, as we can lose up to 10 per cent of our brain volume in later life, these results are quite significant in people with the genetic variant that increases shrinkage."

These individuals could be more vulnerable to factors such as poor diet, excessive alcohol consumption, or little exercise, she said.