Men on average have larger total brain volumes than women, according to a new Cambridge study which confirms that brain size and structure are different in males and females.
Scientists reviewed over 20 years of neuroscience research into sex differences in brain structure.
"For the first time we can look across the vast literature and confirm that brain size and structure are different in males and females," said researcher Amber Ruigrok.
The team, led by Ruigrok and Professors John Suckling and Simon Baron-Cohen in the Cambridge University's Department of Psychiatry, performed a quantitative review of the brain imaging literature testing overall sex differences in total and regional brain volumes.
They searched articles published between 1990 and 2013. A total of 126 articles were included in the study, covering brains from individuals as young as birth to 80 years old.
They found that males on average have larger total brain volumes than women (by 8-13 per cent).
On average, males had larger absolute volumes than females in the intracranial space, total brain, cerebrum, grey matter, white matter, regions filled with cerebrospinal fluid and cerebellum.
Looking more closely, differences in volume between the sexes were located in several regions. These included parts of the limbic system, and the language system.
Specifically, males on average had larger volumes and higher tissue densities in the left amygdala, hippocampus, insular cortex, putamen; higher densities in the right VI lobe of the cerebellum and in the left claustrum; and larger volumes in the bilateral anterior parahippocampal gyri, posterior cingulate gyri, precuneus, temporal poles, and cerebellum, areas in the left posterior and anterior cingulate gyri, and in the right amygdala, hippocampus, and putamen.
By contrast, females on average had higher density in the left frontal pole, and larger volumes in the right frontal pole, inferior and middle frontal gyri, pars triangularis, planum temporale/parietal operculum, anterior cingulate gyrus, insular cortex, and Heschl's gyrus; bilateral thalami and precuneus; the left parahippocampal gyrus, and lateral occipital cortex.
The results highlight an asymmetric effect of sex on the developing brain.
"The sex differences in the limbic system include areas often implicated in psychiatric conditions with biased sex ratios such as autism, schizophrenia, and depression," said Suckling.
"This new study may therefore help us understand not just typical sex differences but also sex-linked psychiatric conditions," Suckling said.
The study appears in the journal Neuroscience and Biobehavioral Reviews.