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Mounting Challenge to Brain Sex Differences

How different are men and women's brains?

The latest evidence to address this controversy comes from a meta-analysis of human amygdala volumes conducted by two Rosalind Franklin University students, Dhruv Marwha, CMS ‘18 and Meha Halari, CMS ‘18, working with Lise Eliot, PhD, CMS associate professor of neuroscience.

In their study, Dhruv and Meha systematically identified all MRI studies comparing amygdala volume in men and women over the past 30 years. Using meta-analysis, a statistical approach for combining the results of multiple studies, the team found no significant difference between the sexes.

“Despite the common impression that men and women are profoundly different, large analyses of brain measures are finding far more similarity than difference," Dr. Eliot said. "There is no categorically 'male brain' or 'female brain,' and much more overlap than difference between genders for nearly all brain measures."

It took six months to examine thousands of papers and extract data for the study, according to Dhruv, a self-described “math and computer geek” who ran the statistical meta-analysis.

“Going through the papers was grueling work,” he said. “The meta-analysis was the fun part; finding trends, comparing amygdala volume to see if it changed by subgroups within the data.”

The olive-sized amygdala is a key brain structure involved in all types of emotion and in social behaviors such as aggression and sexual arousal. Animal studies and early MRI reports indicated that the amygdala is disproportionately larger in males' brains. Such a size difference has been suggested to contribute to sex differences in emotionality and in the prevalence of disorders such as anxiety and depression.

Biologists use the term "sexually dimorphic" (literally, "two different forms") to describe male-female differences. This new study shows that the term does not apply to human amygdala volume. It joins other recent research that challenges the concept of binary "male" and "female" human brains, and may have relevance to understanding disorders including depression, substance abuse, and gender dysphoria.

The CMS study identified 58 published comparisons of amygdala volume in matched groups of healthy men and women (or boys and girls) that included 6,726 total participants. Studies reporting raw amygdala volume show that the structure is indeed about 10 percent larger in male brains. However, this difference is comparable to males' larger body size, including the 11-12 percent larger volume of males' brains overall. Among studies that reported amygdala volumes corrected for overall brain size, the volume difference was negligible (<0.1 percent in the right amygdala, 2.5 percent in the left amygdala) and not statistically significant.

The study appeared in the December volume of the journal NeuroImage.

“I'm really excited and impressed by the resources made available by RFU for this study and Dr. Eliot’s willingness to help,” said Dhruv, first author on the paper.

“There’s a long-held idea that males and females are really different and there must be a [neurological] reason for it,” he said. “We’re working to decipher if that’s truly the case.”

In another 2015 meta-analysis, Dhruv also worked with Dr. Eliot and first author Anh Tan, MD ’16, to debunk the widely-held belief that the brain's hippocampus, which consolidates new memories, is larger in women than men.

"There are behavioral reasons to suspect a sex difference in the amygdala," Dr. Eliot said. "Emotion, empathy, aggression, and sexual arousal all depend on it. Also, the evidence from animal studies suggesting a sex difference in amygdala volume is stronger than it is for the hippocampus. So this finding is more surprising than our hippocampal result and suggests that human brains are not as sexually dimorphic as rats."

This study strengthens the case for gender similarity in the human brain and psychological abilities and has implications for efforts to understand the transgender brain.