Scientists know that men's and women's brains contain varying amounts of gray and white matter. Photo: AP

Male brains are about 10 per cent bigger than female brains, and now a group of Chinese scientists think they have found the reason why, thus solving a riddle that has confounded researchers for decades.

Oestrogen, the female sex hormone, was found capable of suppressing key genes that regulate brain size, they reported in their paper, which was published in the latest issue of the journal BMC Evolutionary Biology.

The team analysed seven genes that regulate the brain size of both humans and non-human primates, and found that four were able to chemically bind with oestrogen.

The genes were then cloned and inserted into human embryonic kidney cells, where they bonded with oestrogen in a way that impeded the way in which they function.

"Puberty plays an important role in shaping up the difference in brain development between males and females," the team said, adding that the underlying mechanism behind this remained “unclear".

The study was led by Professor Su Bing, who works for the State Key laboratory of Genetic Resources and Evolution at the Kunming Institute of Zoology.

The institute operates under the Chinese Academy of Sciences. Kunming is the capital city of southwest China’s subtropical Yunnan province.

To confirm the results, the researchers conducted extensive experiments on animals.

The impact of oestrogen on the size of the brain was observed in three primate species: chimpanzees, rhesus macaques and marmosets.

The researchers did not relate the brain size difference between men and women to their intelligence.

"Why natural selection would favour a relatively smaller brain size in females is not clear," they wrote in the paper.

"One possible explanation is the different social roles of males and females, which would lead to different selective pressures during evolution." (South China Morning Post)