A research study at Rosalind Franklin University of Medicine and Science has debunked the widely-held belief that the hippocampus, a crucial part of the brain that consolidates new memories and helps connect emotions to the senses, is larger in females than in males.
Lise Eliot, PhD, associate professor of neuroscience at the university’s medical school, headed a team of students in a meta-analysis of structural MRI volumes that found no significant difference in hippocampal size between men and women. Meta-analysis is a statistical technique that allows researchers to combine the findings from many independent studies into a comprehensive review. The team examined findings from 76 published papers, involving more than 6,000 healthy individuals.
“Sex differences in the brain are irresistible to those looking to explain stereotypic differences between men and women,” said Dr. Eliot. “They often make a big splash, in spite of being based on small samples. But as we explore multiple datasets and are able to coalesce very large samples of males and females, we find these differences often disappear or are trivial.”
Hippocampi are located on both sides of the brain, under the cerebral cortex. The team’s findings challenge the common claim that a disproportionately larger hippocampus explains females’ tendency toward greater emotional expressiveness, stronger interpersonal skills, and better verbal memory.
“Many people believe there is such a thing as a ‘male brain’ and a ‘female brain,'” Dr. Eliot said. “But when you look beyond the popularized studies — at collections of all the data — you often find that the differences are minimal.”
The study appears in the journal NeuroImage.
Meta-analyses by other investigators have also disproved other purported sex differences in the brain, Dr. Eliot noted. There is no difference in the size of the corpus callosum, white matter that allows the two sides of the brain to communicate, nor do men and women differ in the way their left and right hemispheres process language.
The human hippocampus is not sexually-dimorphic: Meta-analysis of structural MRI volumes
Hippocampal atrophy is found in many psychiatric disorders that are more prevalent in women. Sex differences in memory and spatial skills further suggest that males and females differ in hippocampal structure and function. We conducted the first meta-analysis of male–female difference in hippocampal volume (HCV) based on published MRI studies of healthy participants of all ages, to test whether the structure is reliably sexually dimorphic. Using four search strategies, we collected 68 matched samples of males’ and females’ uncorrected HCVs (in 4418 total participants), and 36 samples of male and female HCVs (2183 participants) that were corrected for individual differences in total brain volume (TBV) or intracranial volume (ICV). Pooled effect sizes were calculated using a random-effects model for left, right, and bilateral uncorrected HCVs and for left and right HCVs corrected for TBV or ICV. We found that uncorrected HCV was reliably larger in males, with Hedges’ g values of 0.545 for left hippocampus, 0.526 for right hippocampus, and 0.557 for bilateral hippocampus. Meta-regression revealed no effect of age on the sex difference in left, right, or bilateral HCV. In the subset of studies that reported it, both TBV (g = 1.085) and ICV (g = 1.272) were considerably larger in males. Accordingly, studies reporting HCVs corrected for individual differences in TBV or ICV revealed no significant sex differences in left and right HCVs (Hedges’ g ranging from + 0.011 to − 0.206). In summary, we found that human males of all ages exhibit a larger HCV than females, but adjusting for individual differences in TBV or ICV results in no reliable sex difference. The frequent claim that women have a disproportionately larger hippocampus than men was not supported.
“The human hippocampus is not sexually-dimorphic: Meta-analysis of structural MRI volumes” by Anh Tan, Wenli Ma, Amit Vira, Dhruv Marwha, and Lise Eliot in NeuroImage. Published online August 31 2015 doi:10.1016/j.neuroimage.2015.08.050